The Hacker's Encyclopedia 1998/hackers_encyclopedia.iso/phreak/sysinfo/6_4_04.txt

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                                  SECTION 4

                           INTERREGISTER SIGNALLING
                             ____________________
                             Recommendation Q.440
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 * 4.1 :  GENERAL

  Interregister signals are of the multifrequency type using a 2-out-of-6
in-band code in both directions.  The multifrequency combinations are sent and
received by multifrequency signalling equipment which is assumed to be
associated with the registers used to control the switching equipment at both
ends of the inter-exchange link.


4.1.1 End-to-end signalling method
~~~~~
   In System R2 interregister signalling is generally performed end-to-end by
a compelled procedure between the outgoing register and the incoming registers
which come into action one after another.  Signalling takes place over one or
more links in tandem without signal regeneration in intermediate exchanges.
With this signalling method only the address information needed for routing
the call through an intermediate exchange is transferred from the outgoing
register to the incoming registers.  In an intermediate exchange the speech
path is immediately through-connected and the incoming register released, then
the outgoing register can exchange information directly with the incoming
register of the next exchange (see Figure 11/Q.440).  If any intermediate
exchange has to carry out charging operations, additional address information
can be transferred from the outgoing register to the incoming register of that
exchange as required.

                                                       FIGURE 11/Q.440, p.

   In circumstances where transmission conditions do not comply with the
requirements specified for System R2 (see Recommendation Q.457) or where
System R2 is used in a satellite link, and thus the exchange of the
interregister signals over the complete System R2 connection is not permitted,
the overall multi-link connection is divided into end-to-end signalling
sections.  When the number of multifrequency combinations available is not the
same on all parts of the connection, the division should be made at an
exchange between links having different numbers of multifrequency combinations
to improve the transfer of information.

   The outgoing R2 register is situated at the outgoing end of a signalling
section on which System R2 interregister signalling according to the present
specifications is used.  It controls the call set-up over the whole signalling
section.  It sends forward interregister signals and receives backward
interregister signals.

   The outgoing R2 register receives information via the preceding links of
the connection in a form used by the signalling system applied over the last
of these links; this system may be System R2, a decadic pulse system, or any
other system.  The preceding link may also be a subscriber's line.  When an R2
register in a transit exchange is working according to this definition, it is
also called an outgoing R2 register.

   Special cases of the outgoing R2 register are the outgoing international R2
register (see S 4.1.2) and the outgoing R2 register at the incoming end of a
satellite link.

   The incoming R2 register is situated at the incoming end of a link on which
System R2 multifrequency interregister signalling according to the present
specification is used.  It receives forward interregister signals via the
preceding link(s) and sends backward interregister signals.  The information
received is used completely or in part for the control of selection stages and
may be sent in its entirety or in part to the succeeding equipment, in which
case the signalling used for retransmission is never System R2.  Inter-
working then takes place between System R2 and the other system.  Thus every
register not situated at the outgoing end of a System R2 signalling section is
called an incoming R2 register independently of the exchange type.

   The term incoming R2 register | ay be applied also to devices controlling
one or more selection stages, for example markers etc.  It should be noted
that in such equipment no provision is made for further transfer of the
information received.


4.1.2 International end-to-end working
~~~~~
   The inclusion of a special outgoing international R2 register | s shown in
Figure 11/Q.440 is dictated by a number of considerations.  An outgoing
international R2 register is an outgoing R2 register which controls the call
set-up on a signalling section which comprises at least one international link
and possibly national links in tandem.

   Even when System R2 is used in the national network of the originating
country an outgoing international R2 register must always be provided in the
country of origin to divide the connection into two signalling sections for
the following reasons:

   o To prevent the maximum number of links in a signalling section over which
     System R2 can function from being exceeded on international calls;

   o To permit the use of fewer signalling frequencies in the country of
     origin (e.g.  5 forward and 5 or 4 backward signalling frequencies);

   o To enable non-standardized meanings to be allocated to the spare national
     multifrequency combinations for use solely in the country of origin;

   o To provide criteria for distinction, at the incoming end, between
     national and international calls;

   o To have the facilities for routing and charging of international calls
     concentrated in one place if it is more economical than to equip every
     normal outgoing R2 register to perform these functions.

   In general the outgoing international R2 register is situated at the
outgoing international exchange.  However, it may be situated in a national
exchange preceding the international exchange provided that the transmission
requirements specified are fulfilled.

   End-to-end signalling from the outgoing international R2 register can be
extended beyond the incoming international exchange even if a reduced number
of signalling frequencies is used.  How-ever, such international/national
end-to-end signalling requires:

   o that the transmission conditions in the national destination network
     comply with the System R2 specifications, and

   o that the routing principles employed in the national destination network
     are compatible with the System R2 signalling procedures for end-to-end
     transmission of address digits from the outgoing international R2
     register to an incoming R2 register in a national exchange.


4.1.3 Register requirements
~~~~~
   Apart from signalling procedures specified in Section 5 the following
characteristics of R2 registers are specified.

      4.1.3.1 Outgoing R2 registers

      The multifrequency signalling equipment associated with outgoing R2
      registers in national exchanges may be of the 2-wire or 4-wire type (see
      Recommendation Q.451).

      The multifrequency signalling equipment associated with outgoing
      international R2 registers must be of the 4-wire type.  This is dictated
      by transmission constraints (see Recommendation Q.452).

      The outgoing R2 register controlling a given multi-link signalling
      section must be able to recognize at least all the backward signals used
      on that section.

      The outgoing international R2 register must be able to send all 15
      forward multifrequency combinations with the meanings specified for
      international working.  It must be able to receive all 15 backward
      multifrequency combinations and act upon the received information in the
      specified manner.  In the case when System R2 is used on the preceding
      link the calling party's category signals have to be translated by the
      outgoing international R2 register as specified (see Recommendation
      Q.480).

      The outgoing R2 register must start call set-up as soon as it has
      received the minimum requisite information.  Therefore signal transfer
      starts before the complete address information is received, i.e.  before
      the caller finishes dialling.  This overlap- ping interregister
      signalling particularly applies at an outgoing R2 register where the
      complete address information from a subscriber or operator is stored
      (e.g.  local registers).  This is in contrast to en bloc register
      signalling i.e.  the transmission of all the address information as a
      whole in one sequence starting only after the complete reception of the
      address information.


      4.1.3.2 Incoming R2 registers

      The multifrequency signalling equipment associated with incoming R2
      registers may be of the 2-wire or 4-wire type (see Recommendation
      Q.451).

      The multifrequency signalling equipment associated with the incoming R2
      register in an international exchange must be of the 4-wire type and
      equipped to send and receive all 15 multifrequency combinations.

      Every incoming R2 register in a multi-link signalling section must be
      able to recognize at least the forward signals used on that section and
      directed to that register.


4.1.4 System R2 compelled signalling method
~~~~~
   The compelled signalling operates as follows (see Figure 12/Q.440):

   o on seizure of a link, the outgoing R2 register automatically starts
     sending the first forward interregister signal;

   o as soon as the incoming R2 register recognizes this signal, it starts
     sending a backward interregister signal which has its own meaning and at
     the same time serves as an acknowledgement signal;

   o as soon as the outgoing R2 register recognizes the acknowledging signal,
     it stops sending the forward interregister signal;

   o as soon as the incoming R2 register recognizes the cessation of the
     forward interregister signal, it stops sending the backward interregister
     signal;

   o as soon as the outgoing R2 register recognizes the cessation of the
     acknowledging backward interregister signal it may, if necessary, start
     sending the appropriate next forward interregister signal.

   When the duration of the forward and backward signals is not controlled by
the compelling mechanism described above, it is either limited by the time-out
delay for release of the register or determined by the nature of the pulse
imposed on them (see Recommendation Q.442).

   Figure 12/Q.440 shows a basic compelled signalling cycle.

   Figure 18/Q.457 shows in detail the build-up and time sequence of a
compelled signalling cycle, including the operating and release times of
multifrequency signalling equipment and other internal operating times of
exchange equipment.

   Besides being a functional part of the compelled procedure, the
acknowledging backward signals serve to convey special information concerning
the required forward signals, to indicate certain conditions encountered
during call set-up or to announce switchover to changed meanings of subsequent
backward signals.  Changeover to secondary meanings allows information
concerning the state of the called subscriber's line to be transferred (see
Recommendation Q.441, S 4.2.4).  Therefore a choice of acknowledging backward
signals is provided.

                                                       Figure 12/Q.440 p.2

                             ____________________
                             Recommendation Q.441
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 * 4.2 :  SIGNALLING CODE

4.2.1 Multifrequency combinations
~~~~~
   Each interregister signal consists of the simultaneous sending of 2 out of
a range of 6, 5 or 4 in-band frequencies (multifrequency combination).  The
band of the interregister signalling frequencies does not overlap with the
frequency band generally used for line-signalling.

   This 2-out-of-n | ode allows erroneous signals consisting of less or more
than two frequencies to be detected and identified as faulty.

   To make the system suitable for application on 2-wire links, two different
sets of 6 frequencies are defined for the composition of the forward and the
backward signals.

   Table 5/Q.441 shows all multifrequency combinations which can be derived
from the maximum of 6 signalling frequencies per direction provided by the
system.  For reference purposes each multifrequency combination of a given
direction is identified by a serial number.  The numerical value of this
serial number can be calculated by adding the respective index and weight
allocated to the two frequencies making up the combination.

   The number of multifrequency combinations depends upon the number of
signalling frequencies used.  When the maximum of 6 signalling frequencies are
used, 15 multifrequency combinations are available.

   System R2 is designed for operation on international links with 15
multifrequency combinations in each direction.  However, it can be used in
national networks with a reduced number of signalling frequencies and still
permit international/national end-to-end working of System R2 signalling in
the case of incoming international traffic (see Figure 13/Q.441).

   The reduction naturally cuts the number of multifrequency combinations
available but has the economic advantage of effecting savings in equipment.
The resulting reduction in facilities is of less importance in automatic than
semi-automatic service.

                                                       Tableau 5/Q.441 p.3

                                                       Figure 13/Q.441 p.4
4.2.2 Allocation of interregister signals
~~~~~
   The signal coding consists of the association of the defined meaning of
interregister telephone signals to the multifrequency combinations transmitted
over the links.  Certain combinations are left spare for the allocation of
national or international signals.  (For signalling procedures, see
Recommendations Q.460 to Q.480.)

      4.2.2.1 Multiple meaning

      The meaning of both the forward and backward multifrequency combinations
      can change after transmission of certain backward signals.  The changed
      meaning is specific to the signal which caused or announced the change.
      In certain cases a change back to the original meaning is possible.  The
      meaning of certain forward multifrequency combinations may also vary
      depending on their position in the signalling sequence.


      4.2.2.2 Meanings of the forward multifrequency combinations

      There are two groups of meanings allocated to the forward multifrequency
      combinations.  The Group I meanings are indicated in Table 6/Q.441 and
      the Group II meanings in Table 7/Q.441 .  The change from Group I to
      Group II meanings takes place when requested by the backward signals A-3
      or A-5.  Change back to Group I meanings is only possible when the
      change to Group II meanings was in response to signal A-5.

      The first forward signal transmitted in international working is used
      for additional routing information.  It enables a distinction to be made
      between terminal and transit calls.  In the case of terminal calls it
      carries the language or discriminating digit whereas, for transit calls,
      it serves the dual purpose of providing a country code indicator and to
      indicate whether an echo suppressor is required or not.

      The above arrangements obviate the need for two different seizing
      signals (line signals) to distinguish between transit and terminal
      traffic on links terminated in a transit exchange.


      4.2.2.3 Meanings of the backward multifrequency combinations

      There are two groups of meanings allocated to the backward
      multifrequency combinations.  The Group A meanings are indicated in
      Table 8/Q.441 and the Group B meanings in Table 9/Q.441.  The change to
      Group B meanings is announced by the backward signal A-3.  No change
      back is possible once a change to the Group B meanings of the backward
      multifrequency combinations has been indicated.


      4.2.2.4 Integration of national and international signalling codes

      The utilization of Signalling System R2 in national networks is
      facilitated by the provision, in the specified signalling code, of
      signals specially assigned for national use.  Under the specified code,
      specific national meanings are allocated to some of these signals,
      others are available for the allocation of national meanings at the
      discretion of each Administration.

      National allocations must not conflict with the present specifications
      in order to ensure end-to-end interregister signalling i.e.  the direct
      dialogue between the outgoing international R2 register (in the
      originating country) and incoming R2 registers, in the national network
      of the destination country.

      The specified signalling code allows for a reduction of signalling
      frequencies in national networks (see S 4.2.1 above).


4.2.3 Forward signals
~~~~~

      4.2.3.1 Group I forward signals

      Compelled interregister signalling must always begin with a Group I
      forward signal.  For the signal codes see Table 6/Q.441.

                                                         TABLE 6/Q.441, p.

      The signals I-1 to I-10 | re numerical signals indicating:

      a) The address | equired for setting up the call (country code, national
         significant number); such address signals are sent by an outgoing R2
         register or an outgoing international R2 regis- ter, either
         spontaneously and immediately after the seizure of the link or in
         response to one of the backward signals A-1, A-2, A-7 or A-8;

      b) The country code (and possibly the area code as well) of the location
         of the outgoing international R2 register , in response to signals
         requesting the origin of the call.  For national traffic, the
         telephone number of the calling subscriber's line (see Recommendation
         Q.480);

      c) For automatic working the discriminating digit | r, in the case of
         semi-automatic working, the service language to be used by the
         operator (i.e.  language digit ).


      The signal I-11 | s a non-numerical address signal.  The meaning of the
      signal is dependent on its position within the sequence of address
      signals specified in Recommendation Q.107.

      a) Country code indicator, outgoing half-echo suppressor required

         When signal I-11 is transmitted as the first forward signal it
         indicates that:

         i) a country code will follow (international transit);

        ii) the call requires echo suppressors;

       iii) the outgoing half-echo suppressor has to be inserted.

      The use of this signal in international working is subject to bilateral
      agreement and is made in conformity with Recommendation Q.479.

      b) Access to incoming operator (code 11)

         When signal I-11 is preceded by the language digit (and possibly by
         one further address digit) it indicates the address of the incoming
         operator's position and is then always followed by signal I-15 alone.

         For international working this signal is to be used only in
         conformity with Recommendation Q.107 | fIbis.  It can be used in
         national traffic only if the incoming R2 registers are equipped to
         receive all six forward frequencies.  The necessary specifications
         are then the responsibility of the Administration concerned.

         The signal I-12 | s a non-numerical address signal.  The meaning of
         the signal is dependent on its position in the sequence of address
         signals specified in Recommendation Q.107.

      a) Country code indicator, no echo suppressor required

         When signal I-12 is transmitted as the first forward signal it
         indicates that:

         i) a country code will follow (international transit);

        ii) the call may not require any echo suppressor (see Recommendation
            Q.479).

      b) Access to delay operator (code 12)

         When signal I-12 is preceded by the language digit (and possibly by
         one further address digit) it indicates that the call must be routed
         to the delay operator's position, either to a particular operator, or
         one of those operating a particular group of positions.  It is then
         followed by further digits and the signal I-15 or by the signal I-15
         alone.

         In international working this signal is to be used in conformity with
         Recommendation Q.107 | fIbis .  It can be used in national traffic
         only if the incoming R2 registers are equipped to receive all six
         forward frequencies.  The necessary specifications are then the
         responsibility of the Administrations concerned.

      c) Request not accepted

         An outgoing international R2 register which receives a signal A-9 or
         A-10, the use of which is exclusively national, or which receives by
         signal A-13 a request to which it is unable to reply, should indicate
         by transmitting signal I-12 that it cannot answer the query (see
         Recommendation Q.480).  This signal may be similarly used in national
         traffic to indicate that response to signal A-9 or A-10 is not
         possible.

         The meaning of the non-numerical signal I-13 is dependent on its
         position in the sequence of address signals specified in
         Recommendation Q.107.

      a) Test call indicator

         When in international working the signal I-13 is transmitted as the
         first forward signal it occupies the position of the language or
         discriminating digit.  It serves then as a test call indicator and
         must be followed by the complete test equipment address information
         as specified in b) below.

      b) Access to test equipment (code 13)

         To access automatic test equipment the second signal I-13 (the
         address digit) must be followed by two digits xy and the signal I-15.

      c) Satellite link not included

         In response to signal A-13, the meaning of the signal I-13 is that up
         to the outgoing R2 register no satellite link is included.

         The meaning of the non-numerical signal I-14 | s dependent on its
         position in the sequence of address signals specified in
         Recommendation Q.107.

      a) Country code indicator, outgoing half-echo suppressor inserted

         When signal I-14 is transmitted as the first forward signal it
         indicates that:

         i) a country code will follow (international transit);

        ii) the call requires echo suppressors;

       iii) the outgoing half-echo suppressor has already been inserted.

         This signal is to be used in international working and only in
         conformity with Recommendation Q.479.

      b) Incoming half-echo suppressor required

         In response to signal A-14, the meaning of the signal I-14 is that an
         incoming half-echo suppressor is necessary.

      c) Satellite link included

         In response to signal A-13, the meaning of the signal I-14 is that up
         to the outgoing R2 register a satellite link is included in the
         connection.

         The non-numerical signal I-15 | ndicates the end of a sequence of
         forward interregister signals.  It is never sent as the first signal
         on an international link.

      a) End-of-pulsing

         In international working, signal I-15 is used to indicate that there
         are no more address signals to follow (see Recommendations Q.107 and
         Q.473).

      b) End of identification

         In national traffic, signal I-15 may be used to indicate that
         transmission of the sequence identifying the calling subscribers line
         is terminated (see Recommendation Q.480, S 5.8.2).


      4.2.3.2 Group II forward signals

      The Group II forward signals are calling party's category signals sent
      by outgoing R2 registers or by outgoing international R2 registers in
      reply to the backward signals A-3 or A-5 and give information whether
      national or international working applies.  For the signal codes see
      Table 5/Q.441.

      It is useful to identify calls according to type or function:

      i) to indicate whether the forward transfer facility is required in
         international traffic;

     ii) for adequate control of switching operations;

    iii) to enable any additional meanings for the signals A-5 used in a
         national network but not internationally accepted (for example, to
         change the meanings of one or more following signals, forward or
         backward) to be made inoperative for incoming international calls;

     iv) for maintenance purposes.

      The meanings of the calling party's category signals are detailed below:

      a) The signal II-1, subscriber without priority Indicates that the call
         is set up from a subscriber's line and is non-priority.

      b) The signals II-2 and II-9, subscriber with priority | ndicate that
         the call is set up from a subscriber's line to which priority
         treatment of calls has been accorded.  Signal II-2 is specified for
         national working only since there are no Recommendations regarding
         priority calls in automatic international working (see Recommendation
         Q.480).

      c) The signal II-3, maintenance equipment | ndicates that the call comes
         from maintenance equipment.

      d) The signal II-5, operator | ndicates that the call is set up from an
         operator's position.

      e) The signals II-6 and II-8, data transmission Indicate that the call
         will be used for data transmission.

      f) The signal II-7, subscriber Indicates that the call is set up from a
         subscriber's line, an operator's position or from maintenance
         equipment and that no forward transfer signal will be used.

      g) The signal II-10, operator with forward transfer facility | ndicates
         that the call is set up from an operator's position with possibility
         of recourse to the forward transfer facility.  Its use must be
         subject to bilateral agreement (see Annex A to the present
         Specifications).

         The signal II-4 and the signals II-11 to II-15 are spare.  The
         meaning of signal II-4 will be decided later by international
         agreement.

                                                         TABLE 7/Q.441, p.

4.2.4 Backward signals
~~~~~
      4.2.4.1 Group A backward signals

      Group A backward signals (for signal codes see Table 8/Q.441) are
      required to acknowledge Group I forward signals and under certain
      conditions, Group II forward signals.  Besides that functional part of
      the compelled procedure, the Group A signals convey signalling
      information as detailed below:

      a) The signal A-1, send next digit (n + 1) requests transmission of the
         next digit (n + 1) after reception of digit n .  The latest address
         signal sent is assumed to have the rank n within the signal sequence
         specified in Recommendation Q.107.

      b) The signal A-2, send last but one digit (n - 1) requests the
         transmission of digit (n - 1) after reception of digit n .  The
         latest address signal sent is assumed to have the rank n within the
         signal sequence specified in Recommendation Q.107.  This signal must
         not be used on a satellite link.

                                                         TABLE 8/Q.441, p.

      c) The signal A-3, address-complete, changeover to reception of Group B
         signals Indicates that the incoming R2 register at the incoming end
         needs no additional address digit and is about to go over to
         transmission of a Group B signal conveying information about the
         condition of the equipment at the incoming exchange or the condition
         of the called subscriber's line (see Recommendation Q.442).

      d) The signal A-4, congestion in the national network Indicates:

         i) congestion of national links;

        ii) congestion in selection stages of terminal international or
            national exchanges;

       iii) occurrence of time-out or abnormal release of a System R2 register
            produced for any reason.

            For exception to these rules see n) below.  See also signal B-4
            and Recommendation Q.442.

      e) The signal A-5, send calling party's category requests transmission
         of a Group II signal.

      f) The signal A-6, address-complete, charge - set-up speech conditions
         Indicates that the R2 register at the incoming end needs no
         additional digit, but will not send Group B signals.  The call has to
         be charged on answer (see Recommendation Q.442).

      g) The signal A-7, send last but two digit (n - 2) requests the sending
         of digit (n - 2) after reception of digit n.  The latest address
         signal sent is to have the rank n within the signal sequence
         specified in Recommendation Q.107.  This signal must not be used on a
         satellite link.

      h) The signal A-8, send last but three digit (n - 3) requests the
         sending of digit (n - 3) after reception of digit n.  The latest
         address signal sent is to have the rank n within the signal sequence
         specified in Recommendation Q.107.  This signal must not be used on a
         satellite link.

      i) The signals A-9 to A-10 | re spares available for allocation of
         national meanings.  The use of signals A-9 and A-10 in national
         networks can be decided by each Administration.  These signals must
         not be used on international satellite links.

      j) The signal A-11, send country code indicator requests the country
         code indicator (transit indication) in acknowledgement of any forward
         signal.  The signal is used in international transit calls only (see
         Recommendation Q.462).  This signal must not be used on a satellite
         link.

      k) The signal A-12, send language or discriminating digit requests the
         language digit or the discriminating digit in acknowledgement of any
         forward signal.  This signal must not be used on a satellite link.

      l) The signal A-13, send nature of circuit requests information
         regarding the nature of the circuits involved in the connection so
         far, i.e.  satellite link (see Recommendation Q.480).  This signal
         should only be used on a satellite link by bilateral agreement.

      m) The signal A-14, request for information on use of echo suppressor |
         is an incoming half-echo suppressor required?) indicates that an
         incoming international exchange acknowledges the discrimination digit
         or the language digit and that it is possible to insert an incoming
         half-echo suppressor in that incoming international exchange if
         required.  This signal must not be used on a satellite link.

      n) The signal A-15, congestion in an international exchange or at its
         output Indicates:

         i) congestion on international links;

        ii) congestion in selection stages at an international transit
            exchange or at a terminal international exchange and/or its
            outgoing links;

       iii) occurrence of time-out or abnormal release of a System R2 register
            produced for any reason (see Recommendation Q.442).


      4.2.4.2 Group B backward signals

      Any Group B backward signal (for signal codes see Table 9/Q.441)
      acknowledges a Group II forward signal and is always preceded by the
      address-complete signal A-3 which indicates that the incoming R2
      register has received all the Group I forward signals it requires from
      the outgoing international R2 register.  Besides that functional part of
      the compelled procedure the Group B signals convey information about the
      condition of switching equipment in the incoming exchange, or about the
      condition of the called subscriber's line, to the outgoing international
      R2 register which can then take the necessary action as specified in
      Recommendation Q.474.

                                                         TABLE 9/Q.441, p.

     The following Group B signals are specified:

     a) The signal B-1 is spare or national use and its meaning must be
        compatible with that of signal B-6 (see Recom- mendation Q.474).

     b) The signal B-2, send special information tone | ndicates that the
        special information tone should be returned to the calling party.
        This tone indicates that the called number cannot be reached for
        reasons not covered by other specific signals and that the
        unavailability is of a long term nature (see also Recommendation
        Q.35).

     c) The signal B-3, subscriber's line busy Indicates that the line or
        lines connecting the called subscriber to the exchange are engaged.

     d) The signal B-4, congestion | ndicates that congestion condition is
        encountered after the changeover from Group A signals to Group B
        signals.  The signal B-4 shall be sent in the conditions specified for
        signal A-4 [see S 4.2.4.1, d) and Recommendation Q.474, S 5.3.5.1].

     e) The signal B-5, unallocated number Indicates that the number received
        is not in use (e.g.  an unused country code, an unused trunk code or
        subscriber number that has not been allocated).

     f) The signal B-6, subscriber's line free, charge Indicates that the
        called party's line is free and that the call has to be charged on
        answer.

     g) The signal B-7, subscriber's line free, no charge | ndicates that the
        called party's line is free but is not to be charged on answer.  This
        signal permits non-chargeable calls without the need for transferring
        "no charge" information by line signals.

     h) The signal B-8, subscriber's line out of order | ndicates that the
        subscriber's line is out-of-service or faulty.

        i) Signals B-9 and B-10 | re spare for national use.  Their meaning
           must not be incompatible with the sending of special information
           tone to the calling party (see Recommendation Q.474).

     j) Signals B-11 to B-15 | re spare for national use.  Their meaning can
        be allocated as required.  Sending of these signals into the
        international network must be prevented.

        To deviate from this rule will result in the actions defined in
        Recommendation Q.474.


                             ____________________
                             Recommendation Q.442
                             »»»»»»»»»»»»»»»»»»»»
 * 4.3 :  PULSE TRANSMISSION OF BACKWARD SIGNALS A-3, A-4, A-6 OR A-15

   Under certain conditions it may prove necessary or desirable to send one of
the signals A-3, A-4, A-6 or A-15 without prior reception of a forward signal.
This can occur when the incoming R2 register, after acknowledging a recognized
forward signal, is unable to complete the call (for example during congestion)
and the next forward signal does not appear on the line; or when the
address-complete signal must be sent after the last forward address signal has
been acknowledged.  It may be desirable to deliberately suspend compelled
signalling by acknowledging the last address digit, and signal I-15 if
received, with signal A-1 to avoid prolonging the transmission time of certain
interregister signals.  Such a course should certainly be considered when
there is a possibility that a relatively long period may elapse between
reception of the last digit and detection of the condition of the called
subscriber's line.  The average duration of such periods during the busy hour
must be limited to 3 seconds in view of the load on the carrier systems in the
case of international calls.

   The following conditions must be observed in transmitting pulsed
interregister signals (see Figure 14/Q.442):

   o the minimum delay between the end of transmission of the last signal of
     the compelled cycle and the start of transmission of the pulse signal
     must be 100 ms;

   o the pulse duration must be 150 _ 50 ms.

   Reception of a pulse signal must cause interruption of any forward signal
in course of transmission at the outgoing R2 register.  It is sometimes
impossible, however, to prevent a forward signal from being sent by the
outgoing R2 register at the very moment when one of the backward signals A-3,
A-4, A-6 or A-15 is sent in pulse form by the register at the incoming end.

    To reduce the operating difficulties which may result, the incoming R2
register must be so designed that no forward multifrequency combination can be
recognized during and after the transmission of signals A-4, A-6 or A-15 in
pulse form or during 300 _ 100 ms from the start of transmission of the
address-complete signal A-3 in pulse form (900 _ 180 ms when signal A-3 is
transmitted over a satellite link) (see Figures 14/Q.442 and 15/Q.442).  When
the end of a pulse signal A-3 has been recognized in the outgoing R2 register,
a Group II signal must be sent forward.  The incoming R2 register will
acknowledge this signal by a Group B signal.

   On recognizing signal A-4, A-6 or A-15 no forward signal is sent by the
outgoing R2 register.  The end of these backward signals must cause the
dismissal of the outgoing and incoming R2 registers       in accordance with Recommendation Q.475.

   The conditions under which pulse transmission of the backward signals A-3,
A-4, A-6 or A-15 apply are specified in Section 5.

                                                       Figure 14/Q.442, p.

 * 4.4 :  MULTIFREQUENCY SIGNALLING EQUIPMENT
                             ____________________
                             Recommendation Q.450
                             »»»»»»»»»»»»»»»»»»»»
4.4.1 GENERAL
~~~~~
   Since System R2 can provide, in international working, end-to-end
signalling from the outgoing international R2 register to an incoming R2
register at the called subscriber's local exchange (see Recommendation Q.440),
the specifications for multifrequency signalling equipment take account of
transmission conditions in both the international and national networks.  The
incoming national network may include both 4-wire and 2-wire links.

   However, it is assumed in the following specifications for multifrequency
signalling equipment for outgoing international R2 registers and incoming R2
registers in international exchanges including the incoming international
exchange that the registers are directly connected by four wires to the
virtual switching points of the links.  The registers thus contain a
multifrequency signalling equipment with a transmitting part and a receiving
part, each separately connected to the GO and RETURN path of the 4-wire
circuit respectively (see Figure 16/Q.451).

   When the outgoing international R2 register is situated in a national
exchange preceding the outgoing international exchange, or when the incoming
R2 register is situated in a national exchange following the incoming
international exchange, special conditions apply (see Recommendation Q.457).

   The upper and lower limits specified for the sending level and for the
national extension attenuation leave a degree of freedom, thus simplifying the
problem of interworking among different networks.  The outgoing international
R2 register relays the interregister signals by the method described in
Recommendation Q.478.  The present specification ensures that the system has
an adequate range.


                             ____________________
                             Recommendation Q.451
                             »»»»»»»»»»»»»»»»»»»»
4.4.2 DEFINITIONS
~~~~~
      4.4.2.1 Multifrequency signalling equipment

      During the exchange of multifrequency combinations the link or
      multi-link section is terminated at both ends by equipment allowing
      multifrequency combinations to be sent and received under mutual,
      compelled control, as indicated in Recommendation Q.440.  Generally
      speaking, the equipment involved in the signal transfer at each end of
      the multi-link section has the following functions:

      o reception of multifrequency combinations;

      o protection against disturbances (e.g.  2-out-of-n-check, 0-out-of-n
        -check, see Recommendation Q.458);

      o transfer of signals to and from the registers or equivalent equipment;

      o sending of multifrequency combinations.

      This equipment as a whole can be considered as a single functional unit,
      henceforth called the multifrequency signalling equipment.

      The functions of the multifrequency signalling equipment may be
      distributed among a number of sub-units, according to the design
      principles adopted in each particular case; these, within certain
      limits, are open to choice.

      For the purpose of the present specification, the multifrequency
      signalling equipment is divided into a sending part and a receiving
      part.

      a) Four-wire multifrequency signalling equipment

         Multifrequency signalling equipment connected to the speech path by
         two pairs of wires is called 4-wire multifrequency signalling
         equipment (see Figure 16/Q.451).  In such cases, the speech path
         itself is generally 4-wire too.

         In Figure 16/Q.451, point B is the output of the sending part and
         point C the input to the receiving part of the 4-wire multifrequency
         signalling equipment.  The latter includes the devices for protection
         against disturbances and the equipment for transferring signals to
         the register or equivalent equipment.


      b) Two-wire multifrequency signalling equipment

         Because different frequencies are used for interregister signalling
         in the two directions of transmission, a single pair of wires
         connecting the speech path to the multifrequency signalling equipment
         can be used for simultaneous sending and reception of multifrequency
         combinations.

         Multifrequency signalling equipment connected to the speech path by a
         single pair of wires is called 2-wire multifrequency signalling
         equipment (see Figure 17/Q.451).

         In Figure 17/Q.451, point A is the output of the sending part and, at
         the same time, the input to the receiving part.  The latter includes
         the devices for protection against disturbances and for transferring
         signals to the register or equivalent equipment.


      4.4.2.2 Operate and release times of the receiving part of the
              signalling equipment

      The speed and reliability of the compelled interregister signal transfer
      is dependent on the operation and release times T0, TR, T `0and T `
      Redefined below.  These times include the minimum recognition times
      referred to in S 4.4.5.3 below.

                                                       FIGURE 16/Q.451, p.
      a) Operation time

         If the two frequencies making up a multifrequency combination are
         applied simultaneously to the input of the receiving part of the
         multifrequency signalling equipment, the time interval between the
         application of both frequencies and recognition of the multifrequency
         combination is called operation time T0.

         If one of the two frequencies making up a multifrequency combination
         is applied to the input of the receiving part of the multifrequency
         signalling equipment somewhat after the other frequency, the time
         interval between the application of the second frequency and
         recognition of the multifrequency combination is called operation
         time T `0.


      b) Release time

         If the two frequencies making up a multifrequency combination are
         simultaneously cut off from the input of the receiving part of the
         multifrequency signalling equipment, the time interval between the
         cut-off and recognition of the end of the multifrequency combination
         is called release time TR.

         If one of the two frequencies making up a multifrequency combination
         is cut off from the input of the receiving part of the multifrequency
         signalling equipment somewhat after the other frequency, the time
         interval between the cut-off of the second frequency and recognition
         of the end of the multifrequency combination is called release time T
         ` R.


      4.4.2.3 Internal operation time

      a) If the equipment at a switching centre has to analyse one or more of
         the signals received and if it has to carry out routing and switching
         operations before being able to determine which backward signal
         should be sent, it can delay completion of the compelled signalling
         cycle.  Such delay depends on the function the centre has and on the
         design of the switching equipment used in it.  The delay cannot
         therefore be specified.

         The time required by an incoming R2 register to determine which
         backward signal to send in reply to a forward signal is denoted as T
         int 1 must therefore be kept to a minimum.  Its value will be zero if
         the signal to be sent is already determined.

      b) An outgoing R2 register sends a forward signal after interpreting the
         backward signal it has received.  The time required to determine the
         appropriate signal is denoted as T int 2

      c) At the end of a forward signal the incoming R2 register may have to
         perform certain functions in order to be ready to receive the next
         signal.  The time required for these functions is denoted as T int 3



                             ____________________
                             Recommendation Q.452
                             »»»»»»»»»»»»»»»»»»»»

4.4.3 REQUIREMENTS RELATING TO TRANSMISSION CONDITIONS
~~~~~
      4.4.3.1 Impedances

      The impedance ZAmeasured at the terminals A (see Figure 17/Q.451) of a
      2-wire multifrequency signalling equipment will have a nominal value
      equal to the nominal terminating impedance ZTof the links at the
      exchange under consideration and will be balanced to earth.  In most
      cases this value ZT will be 600 ohms non-reactive.  The impedance ZAwill
      then satisfy the conditions:

                                     20 log
                             |
                             |00 - Z AfR
                              __________|
                                        |  _" 10 dB
       (1)



       in the 300-3400 Hz band, and
                                     20 log
                             |
                             |00 - Z AfR
                              __________|
                                        |  _" 16 dB
       (2)



      in the 520-1160 Hz and 1360-2000 Hz bands.

      The impedances ZBand ZCmeasured at the terminals B and C (see Figure
      16/Q.451) of a 4-wire multifrequency signalling equipmen will have a
      nominal value equal to the nominal terminating impedance ZTat the
      exchange under consideration and will be balanced to earth.  In most
      cases this value ZTwill be 600 ohms non-reactive.  The impedances ZB and
      ZC will then satisfy condi- tion (1) above in the 300-3400 Hz band and
      condition (2) above in the 520-1160 Hz or 1360-2000 Hz bands, according
      to the set of frequencies generated by the equipment concerned.

      All the above requirements must be met, whether signalling frequencies
      are being transmitted or not.


      4.4.3.2 Echoes

      The balance return loss presented to the international link by different
      national networks varies considerably.

      Recommendation G.122 gives the desirable values based on transmission
      requirements and these values could hardly be improved on simply in
      order to facilitate signalling, especially as the balance return loss
      would then have to be very high.

      The multifrequency signalling equipment must therefore function with an
      echo signal.  Allowance is made for this in the test conditions (see
      Recommendation Q.455).

      In order to eliminate disturbing and unwanted double-echo effects,
      outgoing international R2 registers must be provided with 4-wire
      signalling equipment; the 4-wire loop will then remain open at the
      outgoing end of the multi-link section during signalling.

      For the same reason, it is recommended that 4-wire signalling equipment
      be provided whenever 4-wire switching equipment is used.


      4.4.3.3 Location and connection of mutifrequency signalling equipment
              associated with outgoing international R2 registers

      The multifrequency signalling equipment of the outgoing international R2
      registers is assumed to be directly connected to the virtual switching
      point of the links in an international exchange.  This equipment may be
      situated in a national switching centre of the country of origin on the
      national network side of the outgoing international exchange provided
      that the 4-wire extension links between this centre and the outgoing
      international exchange have the same upper limit for the standard
      deviation of transmission loss variations with time (1 dB) as the
      international links.

      Should the national extension links not have a nominal transmission loss
      of 0.5 dB between the national exchange and the virtual switching point
      in the outgoing international exchange, there must be suitable
      compensation of the power level of the multifrequency combinations.



                             ____________________
                             Recommendation Q.454
                             »»»»»»»»»»»»»»»»»»»»
4.4.4 THE SENDING PART OF THE MULTIFREQUENCY SIGNALLING EQUIPMENT
~~~~~
      4.4.4.1 Signalling frequencies

      The composition of the multifrequency combinations is specified in
      Recommendation Q.441.

      The frequencies in the forward direction are:

            f0 = 1380,  f1 = 1500,  f2 = 1620,  f3    = 1740,   f4 = 1860,
       f5 = 1980 Hz.

      The frequencies in the backward direction are:

            f0 = 1140,   f1 = 1020,   f2 = 900,   f3    = 780,   f4 = 660,
       f5 = 540 Hz.

      The frequency variation at the sending point must not exceed _ | Hz
      relative to the nominal value.


       4.4.4.2 Absolute power level transmitted

       a) Four-wire multifrequency signalling equipment for outgoing
          international R2 and incoming R2 registers assumed to be directly
          connected to the virtual switching point in an international
          exchange:

          i) The absolute power level of each non-modulated signalling
             frequency transmitted by the sending part of the multifrequency
             signalling equipment in the international exchange concerned will
             have a nominal value of -8 dBm0 with a tolerance _ | dB.  At an
             analogue input of the exchange, this corresponds to -11.5 dBm _ 1
             dB.

         ii) The difference in level between the two signalling frequencies
             making up a multifrequency combination must be less than 1 dB.

             The tolerances specified apply to the sending point itself, i.e.
             terminal B in Figure 16/Q.451.


       b) Four-wire multifrequency signalling equipment for outgoing
          international R2 registers situated in a national exchange further
          down in the network of the country of origin:

          i) The sending-level conditions mentioned above under a) are
             applicable in such national exchanges, provided that the national
             4-wire extension links between the national exchange and the
             outgoing international exchange have the same nominal
             transmission loss (0.5 dB) and the same upper limit of standard
             deviation of transmission loss variations with time (1 dB) as the
             international links.

         ii) If these national links do not have a nominal transmission loss
             of 0.5 dB, the level of interregister frequencies must be
             compensated as required.

      c) Multifrequency signalling equipment for incoming R2 registers in
         national exchanges:

         The nominal absolute power level N | f a single signalling frequency
         transmitted by the sending part of the multifrequency signalling
         equipment (2-wire or 4-wire) in any national exchange must be chosen
         within the limits:  N _" A b + 0,5 m + 2.3

       \|
         _______________________
         (m + k)+(m + k + 1)0.04  - 31 dBm (1)

               .sp 1 and          N  A b - 11.5 dBm (2)

                                    .sp 1 or          N  A b + 0.5 m - 2.3
       \|
         _______________________
         (m + k)+(m + k + 1)0.04  - 9 dBm (3)

               .sp 1

      whichever of (2) or (3) yields the lower value.

      In order to avoid marginal operation, it is recommended that levels
      higher than the minimum levels given by formula (1) be specified.

      In these formulae:

      o m = the number of 4-wire links switched in tandem between the outgoing
        international R2 register and the incoming international exchange.
        The number m thus includes the national 4-wire extension links in the
        country of origin (see Recommendation Q.457) and the international
        links.  In formula (1) m should always be given the maximum value 4.
        In formula (3), m should be given all its values, from minimum to
        maximum, and the lowest value so obtained or derived from formula (2)
        is to be taken as the upper limit of N .

      o k = the number of national 4-wire extension links switched in tandem
        between the incoming international terminal exchange and the national
        exchange.  The maximum value of k will be 4.

      o Ab = the nominal transmission loss at 800 Hz in the backward direction
        between the output terminals of the sending part of the multifrequency
        signalling equipment of the incoming R2 register in the exchange under
        consideration and the send side of the virtual switching point of the
        backward speech path in the incoming international exchange.

        Formulae (1), (2) and (3) were derived as shown in Annex A to Section
        4.

        The variation in level relative to the nominal value N | hosen for a
        given exchange must not exceed _ | dB.

        However, the difference in level between the two signalling
        frequencies making up a multifrequency combination must not exceed 1
        dB.


      4.4.4.3 Signalling frequency leak level

      The total power level of the leak current transmitted to line must be:

      a) at least 50 dB below the nominal level of one signalling frequency
         when no multifrequency combination is being sent;

      b) at least 30 dB below the level of either of the signalling
         frequencies when a multifrequency combination is being sent.
         Furthermore, any single leak current must be at least 34 dB below the
         level of either of the signalling frequencies when a multifrequency
         combination is being sent.


      4.4.4.4 Harmonic distortion and intermodulation

      The total power level of all frequencies due to harmonic distortion and
      intermodulation within the frequency band 300-3400 Hz must be at least
      37 dB below the level of one signalling frequency.


      4.4.4.5 Time tolerance for multifrequency combinations

      The time interval between the start of sending of each of the two
      frequencies constituting a multifrequency combination must not exceed 1
      ms.

      The time interval between the cessation of sending of each of the two
      frequencies must not exceed 1 ms.



                             ____________________
                             Recommendation Q.455
                             »»»»»»»»»»»»»»»»»»»»
4.4.5 THE RECEIVING PART OF THE MULTIFREQUENCY EQUIPMENT
~~~~~
      4.4.5.1 Sensitivity range

      The power levels given below relate to the nominal impedance of the
      receiving part of the multifrequency signalling equipment.

      The receiving part of the multifrequency signalling equipment shall have
      a sensitivity range of -31.5 dBm0 to -5 dBm0

                  [ For equipments already in service the sensitivity range is
                  [ -35 dBm to -5 dBm.


      4.4.5.2 Operate and release time requirements

      The operating and release times of the receiving part of the
      multifrequency signalling equipment depend on the design and, for a
      given design, on:

      o the time difference between the instants of reception of each of the
        two frequencies making up a multifrequency combination;

      o the level of each of the two frequencies;

      o the difference in level between the two frequencies;

      o the level, spectrum and instant of onset of the noise.

      The factors vary with transmission conditions.  With certain types of
      switching equipment it may prove advisable to embody devices to
      counteract low-frequency disturbances in the multifrequency signalling
      equipment.

      Time requirements have been specified for two types of multifrequency
      test combinations , A and B, applied to the input of the receiving part
      of the multifrequency signalling equipment in the presence of disturbing
      signals as specified below.

      When test combinations and disturbing frequencies as specified under a)
      to c) below are applied to the terminals A (see Figure 17/Q.451) of
      2-wire multifrequency signalling equipment or to the terminals C (see
      Figure 16/Q.451) of 4-wire multifrequency signalling equipment, the
      following time requirements must be met:

               -         for type A test combinations:

                                     T 0 + T
                                    R  70 ms



               -         for type B test combinations:

                                     T 0 + T
                                    R  80 ms



               -         for test combinations of types A and B:

                                      (T `
                                     0 + T `

                                   R )  (T 0 +
                                        T
                                   R ) + 5 ms









      For definitions of T0, T `0, TR, and T `

      R see Recommendation Q.451.

      For the third requirement, the only cases to be considered are those in
      which the frequency first appearing is also the first to disappear
      (transmission delay effect).  The T ` 0 and T `

      R time requirements are specified to ensure proper functioning of the
      multifrequency signalling equipment when the received multifrequency
      combination is affected by group delay distortion, for example.  The
      appropriate procedure for this test depends on the design of the
      equipment to be tested; in many cases it will be convenient to assume
      that a delay of one second or more for the second frequency constitutes
      the most unfavourable case.

      When a multifrequency combination has caused the receiving part of the
      multifrequency signalling equipment to operate, the latter cannot
      release if the signal frequencies are interrupted for not more than 7
      ms.  A method of improving the system reliability in case of interrupted
      signals is described in Annex B of Section 4.

      a) Multifrequency test combinations type A

         o The multifrequency test combination consists of any 2-out-of-n
           combination of the n signalling frequencies;

         o each frequency differs from the nominal frequency by not more than
           _ | Hz;

         o the absolute power level of each of the two frequencies of the
           multifrequency combination lies between -5 dBm and -20 dBm;

         o the difference in level between the two frequencies is not greater
           than 3 dB.

      b) Multifrequency test combinations type B

         o The multifrequency test combination consists of any 2-out-of-n
           combination of the n signalling frequencies;

         o each frequency differs from the nominal frequency by not more than
           _ | 0 Hz;

         o the absolute power level of each of the two frequencies of the
           multifrequency combination lies between -5 dBm and -35 dBm;

         o the difference in level between the two frequencies is not greater
           than 5 dB for adjacent frequencies and 7 dB for non-adjacent
           frequencies.

      c) Disturbing frequencies

         The disturbing frequencies to be applied are:

         o in all cases one or more of the n | requencies for which the
           receiving part under test is designed, with a total power level of
           -55 dBm or less, when no multifrequency test combination is being
           applied;

         o when a multifrequency test combination is applied, one or more of
           the (n - 2) remaining frequencies with a total power level 20 dB
           below the highest test combination frequency level during
           application of the test combination;

         o for testing the receiving part of 4-wire multifrequency signalling
           equipment in an outgoing international R2 register:

      any multifrequency combination consisting of two frequencies out of the
      forward group of frequencies, each of these two frequencies having a
      level of 13.5 dB above the lowest test-combination frequency level in
      the backward direction; an upper limit of -12.5 dBm is nevertheless
      specified for the level of the disturbing signal

      System R2 interregister signalling may be used in the country of origin
      on the national links preceding the out-going international R2 register.
      In that case, the receiving part of 4-wire multifrequency signalling
      equipment connected to the forward speech path of the national 4-wire
      links may have to function in the presence of frequencies used in the
      backward direction.  No general specification can be given for the level
      of these disturbing frequencies; it is recommended that Administrations
      formulate their own specifications.

      o for testing the receiving part of 2-wire multifrequency signalling
        equipment:

        any multifrequency combination at the highest level used in
        operational signalling (measured at point A, Figure 17/Q.451) by the
        sending part of the 2-wire multifrequency signalling equipment
        concerned.


      4.4.5.3 Non-operate and non-recognition requirements

      The receiving part of the multifrequency signalling equipment must
      remain in the non-operate state when the following disturbances, singly
      or together, are the only signals that are applied to the terminals A or
      C (see Figures 17/Q.451 and 16/Q.451):

      o any single pure sine wave or any combination of two pure sine waves,
        each with a power level of -38.5 dBm0 within the 300-3400 Hz band;

      o any single pure sine wave or any combination of two pure sine waves,
        each with a power level of -42 dBm within the 300-3400 Hz band;

      o any combination of two pure sine waves, each with a power level of -5
        dBm within the 1300-3400 Hz band for the set of frequency receivers
        used in the backward direction; and within the 330-1150 Hz and
        2130-3400 Hz bands for the set of frequency receivers used in the
        forward direction.

      Furthermore, when signalling tones have activated the receiving part of
      the multifrequency signalling equipment, this must assume the
      non-operate state when the signalling tones are removed in the presence
      of these same disturbances, singly or together, at the above-mentioned
      terminals.

      The receiving part of the multifrequency signalling equipment must not
      recognize a combination consisting of two signalling frequencies out of
      the set of frequencies normally used in the transmission direction
      considered, each having a level not exceeding -5 dBm and a duration of
      less than 7 ms.

      The receiving part of the multifrequency signalling equipment must not
      recognize a combination consisting of two signalling frequencies used in
      the transmission direction considered, having a difference in level of
      20 dB or more.


      4.4.5.4 Influence of transient disturbances | see also Recommendation
              Q.458)

      The recognition of faulty signals due to short-lived transient
      conditions can largely be avoided if a multifrequency combination is
      recognized only after a specified minimum time, during which two, and
      only two, of the individual receivers are active and if the absence of
      multifrequency combinations is recognized only after a minimum time,
      during which all individual receivers are at rest.  These times are
      included in the operating and release times T0and TR.

      Typical transient disturbances, such as clicks, change of polarity,
      etc., generated by the switching equipment, must not change signals
      transferred from the receiving part of the signalling equipment to the
      register.

      It is recommended that Administrations formulate their own specification
      for a test method according to the type of disturbances encountered in
      their switching equipment.

                  [ For equipments already in service, the non-operation level
                  [ is -42 dBm.




 * 4.5 :  RANGE, SPEED AND RELIABILITY OF INTERREGISTER SIGNALLING

                             ____________________
                             Recommendation Q.457
                             »»»»»»»»»»»»»»»»»»»»
4.5.1 RANGE OF INTERREGISTER SIGNALLING
~~~~~
      4.5.1.1 Number of international links

      The number of international links switched in tandem for establishing an
      international connection using System R2 must not exceed 4 (see
      Recommendation Q.440).

      Assumptions for the transmission loss in 4-wire international links:

      i) nominal transmission loss at 800 Hz:  0.5 dB (Recommendation G.101, S
         5);

     ii) standard deviation of transmission loss variations with time must not
         exceed 1 dB [Recommendation G.151, S 3, a)];

    iii) the difference between the mean value and the nominal is assumed to
         be 0 [as in Recommendations G.122, S 1.2), and G.131, S 1].


      4.5.1.2 Number of national links

      a) Outgoing international R2 register and number of national 4-wire
         extension links in the country of origin

         The outgoing international R2 register is always provided with 4-wire
         multifrequency signalling equipment and the 4-wire loop will be open
         during interregister signalling.

         The outgoing international R2 register must be placed in an exchange
         from where the incoming international exchange in the country of
         destination is reached by not more than four 4-wire links switched in
         tandem (see Recommendation Q.440).

         It is understood that thenational 4-wire links in the country of
         origin shall have the same standard deviation of transmission loss
         variations with time (1 dB) as the international links and that, if
         these national links do not have the same nominal transmission loss
         as the international links (0.5 dB), appropriate compensation of the
         multifrequency combination levels will be made in both directions of
         transmission.

      b) Number (k) of national 4-wire extension links in the country of
         destination

         Not more than four national 4-wire extension links may be used.

         Forward transmission loss in the country of destination:

         i) The standard deviation of transmission loss variations with time
            in the national 4-wire extension links in the country of
            destination must not exceed 1 dB.

        ii) The nominal transmission loss at 800 Hz in the forward direction
            (Af) between the virtual switching point in the incoming
            international exchange and any incoming R2 register in the country
            of destination must not exceed:

            11.4 dB for a country using 3 national 4-wire extension links at
            the most

            or

            11.0 dB for a country using 4 national 4-wire extension links at
            the most,

            and must never be less than:


                                       A
                                     f min.
                              = -2.5 - 0.5 m + 2.3
                         \|
                           ________________________
                           (m + k) +(m + k + 1)0.04  dB.

            The values for the minimum forward transmission loss A f min.
            resulting from this formula are shown in Table 10/Q.457 (for the
            definitions of m and k see Recommendation Q.454).  In practice the
            forward transmission loss must not fall below these values.

            The formula has been determined as shown in Annex C to Section 4.

            When System R2 end-to-end signalling is applied in a national
            network, the 4-wire links involved may not conform to the
            characteristics specified by the CCITT for international circuits.
            Also, national transmission plans may be based on principles
            different from those of the international transmission plan.
            Consequently, the range over which end-to-end signalling is
            possible must be established using calculations, e.g.  as shown in
            Annex C to Section 4 (see also SS 4.5.1.3 and 4.5.1.4 below).

                                                        Table 10/Q.457, p.


      4.5.1.3 Total attenuation distortion

      It has been assumed that at all frequencies within the 530-1990 Hz band
      the overall attenuation distortion relative to 800 Hz between the
      outgoing international R2 register and any incoming R2 register will not
      exceed _ | dB.  Attention is drawn to the fact that on some national
      connections these assumptions might not be fulfilled.

      As type B test signals (see Recommendation Q.455) allow for a 5 dB
      difference in level between two adjacent signalling frequencies, and a 7
      dB difference between two non-adjacent signalling frequencies, a 4 dB
      attenuation distortion of the multi-link section can be allowed for two
      adjacent frequencies and a 6 dB distortion for two non-adjacent
      frequencies, provided that the level of the weakest signalling frequency
      is not lower than -35 dBm at the terminals of the receiving part of the
      multifrequency signalling equipment.

      The values 4 dB and 6 dB were obtained by allowing for a 1 dB difference
      in sending level.


      4.5.1.4 Intermodulation

      A multifrequency signalling system in conformity with the above
      specifications will allow satisfactory working over a multi-link section
      introducing intermodulation products from two signalling frequencies and
      falling within the 520-1160 Hz and 1360-2000 Hz bands, the level of each
      of such products being at least 24 dB below the highest signal frequency
      level.


4.5.2 Build-up and time specification of a complete forward compelled
~~~~~ signalling cycle

      Figure 18/Q.457 shows in detail the build-up and time sequence of a
      compelled signalling cycle.

                                                        Figure 18/Q.457 p.

      In this figure:

      TP\dFdenotes the transmission delay of the slower of the two frequencies
      of a forward multifrequency combination;

      TP\dBdenotes the transmission delay of the slower of the two frequencies
      of a backward multifrequency combination;

      T0and T `0 denote the operating times as defined in Recommendation
      Q.451;

      TRand T ` Rdenote the release times as defined in Recommendation Q.451;

      T int 1 , T int 2 and T int 3 denote the internal operation times as
      defined in Recommendation Q.451.

      If the values of T int 2 and T int 3 lie within certain limits, they do
      not contribute to the total duration of the compelled signalling cycle,
      as can be seen from Figure 18/Q.457.

      TS\d1and TS\d2denote respectively the time required for starting and
      stopping the sending of a multifrequency combination (switching-on or
      switching-off times, exclusive of logic operations).

      It thus appears that the total duration T of a complete compelled
      signalling cycle is given by the formula:

                                    T = 2 (T
                                       PF
                                       + T
                                       PB
                                       ) +

               |
               |
               |
                (T ` 0 + T `  RfR) DfR  +(T ` 0 + T `  RfR) AfR
                     T 0 + T RfR) DfR + (T 0 + T RfR) AfR
                                                               |
                                                               |
                                                               |

                                       + T
                                      int 1
                                       + (T
                                       S 1
                                        +
                                        T
                                       S 2
                                        )
                                        D
                                       + (T
                                       S 1
                                        +
                                        T
                                       S 2
                                        )
                                        A

      or

      The subscripts D and A apply respectively to the outgoing and the
      incoming registers.

      TP\dFand TP\dBdepend on the propagation characteristics of the forward
      and backward speech paths respectively and therefore cannot be
      specified.

      A value of 10 ms for TP\dFand TP\dBcan be considered as typical, e.g.
      for average terrestrial regional connections and 320 ms for circuits
      including a satellite link.

      The maximum value of T0 + TRhas been fixed at 70 ms.  A value of 35 ms
      could be taken in a certain number of cases as representing the minimum
      duration of T0 + TR.

      T int 1 , T int 2 and T int 3 are dependent on the type of exchange and
      therefore cannot be specified; but their contribution to the total
      duration of the compelled signalling cycle must be kept as small as
      possible.

      If the influence of T int 1 , T int 2 and T int 3 is ignored, and if the
      extreme values of T0 + TRand for TS\d1 + TS\d2are assumed to be
      identical for the outgoing and the incoming register, the extreme values
      of TS\d1 + TS\d2being taken as 5 ms TS\d1 + T2 10 ms and if the value of
      10 ms indicated above as typical is adopted for TP\dFand TP\dB, the
      probable extreme values of the compelled signalling cycle T would be:

      for terrestrial connections:  120 ms T 200 ms.

      for circuits including a satellite link:  1080 ms T 1440 ms.

      The signalling rates would be between approximately 8 and 5 signalling
      cycles per second for terrestrial circuits.  These values are not
      absolute limits; the signalling cycle could, for example, be longer on a
      complex connection or in the presence of noise or other conditions
      approaching those of type B test combinations (see Recommendation
      Q.455).


                             ____________________
                             Recommendation Q.458
                             »»»»»»»»»»»»»»»»»»»»
4.5.3  RELIABILITY OF INTERREGISTER SIGNALLING
~~~~~
      4.5.3.1 General

      In signal transmission, reliability and speed are to some extent
      conflict requirements:  the slower the signalling, the more reliable it
      is likely to be.  System R2 combines the two essential requirements of
      speed and reliability, since it is a compelled system, adapting its
      signalling speed to the working conditions with minimum loss of
      reliability.

      System R2 is protected against the acceptance of faulty information
      (multifrequency combinations consisting either of one frequency only or
      of more than two frequencies) by means of the 2-out-of-n method of
      checking the number of frequencies received.

      This protection is inoperative in the case of disturbances (noise,
      clicks, etc.) activating two, and only two, of the single frequency
      receivers; nor does it prevent the release of all the receivers (thus
      wrongly indicating the end of a multifrequency combination) in the event
      of an interruption caused by disturbances during transmission of a
      multifrequency combination.

      Disturbances causing the operation of two single frequency receivers or
      the release of all the receivers are due mainly to short-lived,
      transient conditions.  The recognition of faulty information due to such
      conditions can therefore largely be avoided by so designing the
      receiving part of the multifrequency signalling equipment that a
      multifrequency combination will be recognized only after a specified
      minimum time.  During this time two, and only two, of the individual
      receivers are active and the absence of multifrequency combinations will
      be recognized only after a specified minimum time, during which all
      individual receivers are at rest.  The degree of protection against
      faults due to the above-mentioned conditions is dependent on these
      times, which are included in the operating and release times defined for
      the receiving part of the multifrequency signalling equipment in
      Recommendation Q.451.


      4.5.3.2 Error rate for compelled working

      The test of the multifrequency signalling equipment as a whole consists
      in continuous, compelled transmission of multifrequency combinations.

      It must be ensured that all possible combinations of the forward and
      backward multifrequency combinations have equal probability of
      occurrence during the test period.

      The error rate is observed at the receiving part at both ends of the
      link and is defined, for each end, as the number of errors divided by
      the number of combinations sent by the corresponding sending parts at
      each end of the link.

      It is for each Administration to define the sources of permanent and
      impulsive noise to be applied to the interface between the sending and
      receiving parts in the light of its experience and local conditions.

      The compelled working may be tested, on the one hand, by using test
      combinations of Type A (see Recommendation Q.455) in the presence of
      noise at a power level of -40 dBm and a uniform power distribution in
      the 300-3400 Hz band (filtered white noise) and, on the other hand, by
      using Type B test combinations in the presence of noise at a power level
      of -45 dBm and a uniform power distribution in the 300-3400 Hz band.

      The error rates in these conditions will be:

      o for Type A test combinations and noise at -40 dBm:  10DlF2615;

      o for Type B test combinations and noise at -45 dBm:  10DlF2614.




                                     ANNEX A
                                 (to Section 4)

                           (see Recommendation Q.454)

   Development of formulae for the power level of signalling frequencies

   The formulae (1), (2), and (3) of Recommendation Q.454 describing the
nominal absolute power level N of a transmitted single signalling frequency
were derived as follows:

1.  The nominal transmission loss in the backward direction between the
    incoming R2 register and the outgoing international R2 register is given
    by (see Recommendation Q.457):

                                 Ab + 0.5 m  dB.

2.  On the assumption that the contribution of the national 2-wire links to
    transmission loss variations with time is negligible and with allowance
    for the standard deviation of transmission loss variations with time of
    the international and national 4-wire circuits and in the exchanges (see
    CCITT Recommendation Q.45:  ~ = 0.2 dB), the total transmission loss
    variation for a 1% probability of it being exceeded is given by:

                                     _ 2.3
                        \|
                          _________________________
                          (m + k)+(m + k + 1)(0.2)2  dB.



3.  Given a total attenuation distortion of _ | dB relative to 800 Hz (see
    Recommendation Q.457) and a tolerance of _ | dB on the nominal power level
    N , the lower and upper limits of the level received in the outgoing
    international R2 register will be:




                                      N - A
                                b - 0.5 m - 2.3
                     \|
                       _______________________
                       (m + k)+(m + k + 1)0.04  - 3 - 1 dBm



   and

                                      N - A
                                b - 0.5 m + 2.3
                     \|
                       _______________________
                       (m + k)+(m + k + 1)0.04  + 3 + 1 dBm.



   respectively.



4.  The receiving part of the multifrequency signalling equipment has a
    sensitivity range between -35 dBm and -5 dBm (see Recommendation Q.455).

    The minimum value of N  | s thus given by:


                                      N - A
                                b - 0.5 m + 2.3
                    \|
                      _______________________
                      (m + k)+(m + k + 1)0.04  - 4 = - 35 dBm



       hence
                                     N _" A
                                b + 0.5 m + 2.3
                      \|
                        ________________________
                        (m + k)+(m + k  + 1)0.04  - 31 dBm
       (1)



            The maximum value of N  | s given by:

                                      N - A
                                b - 0.5 m + 2.3
                    \|
                      _______________________
                      (m + k)+(m + k + 1)0.04  + 4 = - 5 dBm



       hence
                                      N  A
                                b + 0.5 m - 2.3
                       \|
                         ________________________
                         (m + k)+(m + k  + 1)0.04  - 9 dBm
       (3)












5.  The nominal level of the backward signals at the virtual switching point
    of the incoming international exchange must not be higher than -11.5 dBm;
    thus


                                      N - A
                                  b  - 11.5 dBm


    hence

                                      N  A
                                 b  - 11.5 dBm.
       (2)




                                     ANNEX B
                                 (to Section 4)

                           (see Recommendation Q.455)

   Possible method of improving protection from interruptions

   If the receivers are locked through an internal function of the registers
until the times Q and S (see Figure B-1) the intervals during which there is a
risk that interruptions may cause their release are reduced by PQ and RS
respectively.

   This method can be used on both terrestrial and satellite circuits.

   To ensure that the duration of the compelled signalling cycle is not
prolonged on terrestrial circuits (short transmission delay).  PQ and RS must
be calculated with:


                            TP\dF= TP\dB = 0 ms: PQ
                                  Ti\dn\dt\d1 +
                                TS\d1 + T0+ TS\d2


   and

                             RS  TS\d2 + TR+ TS\d2


   To ensure that the duration of the compelled signalling cycle is not
prolonged on satellite circuits (long transmission delay) PQ and RS must be
calculated with:


                             TP\dF+ TP\dB = 250 ms:
                       PQ  500 ms + Ti\dn\dt\d1 + TS\d1+









                                   T0+ TS\d2


   and


   RS  500 ms + TS\d2+ TR+ TS\d2.bp






                                                      Figure B-1/Q.455, p.

                                     ANNEX C
                                 (to Section 4)

                           (see Recommendation Q.457)

   Development of a formula for the admissible forward transmission loss in a
country of destination

   The formula used in Recommendation Q.457 for the calculation of the minimum
forward transmission loss A f min.  a country of destination was determined as
follows:

1.  According to Recommendation Q.454 the lowest and highest sending levels
    will be, respectively,

                              -11.5 - 1 = -12.5 dBm
    and

       -11.5 + 1 = -10.5 dBm.

2.  Let Afbe the nominal transmission loss at 800 Hz in the forward direction,
    between the virtual switching point in the incoming international exchange
    and the incoming R2 register concerned.

    The total nominal transmission loss between the outgoing international R2
    register and the incoming R2 register will then be:

                                       A
                                        f
                                   + 0.5 m dB.

3.  On the assumption that the contribution of the national 2-wire links to
    transmission loss variations with time is negligible, and that the
    standard deviation of transmission loss variation in the exchange is 0.2
    dB (see Recommendation Q.45), the total transmission loss variation,
    calculated for a 1% probability of it being exceeded will be:

                                     _ 2.3
                         \|
                           ________________________
                           (m + k)+(m + k + 1)0.2 2  dB.

4.  With allowance for a total attenuation distortion of _ | dB relative to
    800 Hz (see Recommendation Q.457) the lower and upper limits of the level
    of each frequency at the input of the incoming R2 register will be
    respectively:
                                   -12.5 - A
                                        f
                                  - 0.5 m - 2.3
                       \|
                         _______________________
                         (m + k)+(m + k + 1)0.04  - 3 dBm

    and
                                   -10.5 - A
                                        f
                                  - 0.5 m + 2.3
                       \|
                         _______________________
                         (m + k)+(m + k + 1)0.04  + 3 dBm.

5.  The receiving part of the multifrequency signalling equipment has a
    sensitivity range of -35 dBm to -5 dBm.

6.  The maximum value of Afis thus given by:

                                   -12.5 - A
                                     f max.
                                  - 0.5 m - 2.3
                    \|
                      _______________________
                      (m + k)+(m + k + 1)0.04  - 3 = - 35 dBm

    hence
                                       A
                                     f max.
                              = 19.5 - 0.5 m - 2.3

                         \|
                           _______________________
                           (m + k)+(m + k + 1)0.04  dB.


7.  The minimum value for Afis given by:

                                   -10.5 - A
                                     f min.
                                  - 0.5 m + 2.3
                     \|
                       _______________________
                       (m + k)+(m + k + 1)0.04  + 3 = - 5 dB

    hence
                                       A
                                     f min.
                              = -2.5 - 0.5 m + 2.3
                         \|
                           _______________________
                           (m + k)+(m + k + 1)0.04  dB.


   According to Recommendation Q.457, the maximum number of 4-wire links with
international characteristics is m = 4.

   The maximum number of national 4-wire extension links is k = 4.





                                  SECTION 5

                            SIGNALLING PROCEDURES

   A major characteristic of System R2 is the inherent flexibility in the
interregister signalling procedures which allow efficient signal information
transfer, adapted to the particular requirements relating to different call
types, traffic conditions and switching equipments.  This flexibility is
primarily achieved by the principle that the interregister signalling sequence
is controlled by the incoming R2 register by means of backward signals.

   In this context the function of an outgoing R2 register is basically to
respond with the forward signal requested by the incoming R2 register.  At the
incoming R2 register the determination of the backward signal to be sent, thus
controlling the signalling sequence, is closely related to the analysis
necessary to determine the routing and handling of the call.

   In this Section the signalling procedures are specified.  Although System
R2 in principle allows a high degree of freedom in the combination of these
procedures within the signalling sequence for call set-up, the procedures to
be used for normal international calls connected via terrestrial links are
described below.  These procedures may vary when a satellite link is employed
in a connection because the register at the incoming end of a satellite link
also acts as an outgoing R2 register (see Recommendation Q.440, S 4.1.1).  For
national application of System R2 the signalling sequences to be applied have
to be defined in accordance with the limitations of the present specifications
by the Administration concerned.



                             ____________________
                             Recommendation Q.460
                             »»»»»»»»»»»»»»»»»»»»
 * 5.1 :  NORMAL CALL SET-UP PROCEDURES FOR INTERNATIONAL WORKING

5.1.1 General
~~~~~
   During call set-up an outgoing international R2 register is connected to an
international link.  This register signals to at least one incoming R2
register situated in an international exchange and possibly, depending upon
the routing, to as many as 4 successive incoming R2 registers in international
exchanges and 4 successive incoming R2 registers situated in national
exchanges in the destination country where System R2 is employed.

   The duration of the normal call set-up procedure can vary because different
phases of the signalling sequence between the outgoing international R2
register and successive incoming R2 registers may be repeated.



                             ____________________
                             Recommendation Q.462
                             »»»»»»»»»»»»»»»»»»»»
5.1.2 SIGNALLING BETWEEN THE OUTGOING INTERNATIONAL R2 REGISTER AND AN
~~~~~ INCOMING R2 REGISTER IN AN INTERNATIONAL EXCHANGE

   The address signals from an operator or a subscriber must be stored in an
outgoing international R2 register.  When a sufficient number of digits is
available an outgoing link is selected and a seizing (line) signal is sent.
When the seizing signal is recognized an incoming R2 register is associated
with the link.

   Immediately the outgoing link is seized the outgoing international R2
register sends the first interregister signal.

      5.1.2.1 Signalling to an international transit exchange

      When the outgoing link is to an international exchange from which a
      transit routing is required to the destination country, the first
      interregister signal sent is a country code indicator.  This is one of
      signals I-11, I-12, I-14 depending upon the requirement for echo
      suppressors (see Recommendation Q.479).

      On recognition of a country code indicator the incoming R2 register
      determines that the call is to be internationally transit switched.  The
      incoming R2 register sends signal A-1 when it requests the first digit
      of the country code.  The outgoing international R2 register sends this
      address digit (a signal I-1 to I-10).  The incoming R2 register may send
      signal A-1 to request the next digit.

      The incoming R2 register examines the address digit(s) and if a further
      digit (or digits) is required for routing, signal A-1 is sent to request
      the next digit.

      When sufficient digits are stored at the incoming exchange to permit the
      call to be routed to the next exchange, the backward signal is
      determined by the nature of the signalling system employed on the
      outgoing link.

      a) If the outgoing link employs System R2, one of two backward signals
         is sent immediately the outgoing link is seized:

         i) If the outgoing link is to another international transit exchange
            from which a transit routing is required to the destination
            country, signal A-11 is sent to request repetition of the country
            code indicator.

            On recognition of signal A-11 the outgoing international R2
            register sends a country code indicator as the first signal to be
            received by the incoming R2 register in the next international
            transit exchange.  This is one of signals I-12 or I-14.  If signal
            I-11 was sent initially, signal I-14 is sent subsequently.

            On recognition of a country code indicator the incoming R2
            register determines that the call is to be internationally transit
            switched.  The signalling procedure which follows is identical to
            that described above.


        ii) If the outgoing link is to an incoming international exchange in
            the destination country, signal A-12 is sent to request a language
            or discriminating digit.

            In both these cases after the backward signal is sent and the
            compelled signalling sequence is complete, the transit exchange
            releases the incoming R2 register and through-connects the
            speech-path between the outgoing exchange and the next exchange.

      b) If the outgoing link employs a signalling system other than System
         R2, then the acting incoming R2 register is the last incoming R2
         register.  The exchange seizes an outgoing international link to a
         further international transit exchange or an incoming international
         exchange in the destination country.  Signalling continues between
         the outgoing international R2 register and the last incoming R2
         register and interworking takes place with the other signalling
         system.

         If congestion is encountered signal A-15 is sent, if necessary in
         pulse form.  After the compelled signalling sequence is complete or
         after the end of the pulse signal the transit exchange dismisses the
         register.

         On recognition of signal A-15 the outgoing exchange may initiate a
         repeat attempt, a re-routing or cause the return of congestion
         information to the calling subscriber.  In all cases the outgoing
         (multi-)link section is released.


      5.1.2.2 Signalling to an incoming international exchange in the
              destination country

      When the outgoing international link is a direct link to an incoming
      international exchange in the destination country the first
      interregister signal sent is a language or discriminating digit.

      Alternately, if the outgoing multi-link section is routed via one, two
      or three international transit exchanges then on recognition of signal
      A-12 the outgoing international R2 register sends a language or
      discriminating digit as the first signal to be received by the incoming
      R2 register in the terminal international exchange in the destination
      country.

      The first signal A-12 received from an international transit exchange by
      the outgoing international R2 register informs it that an international
      link terminating at an incoming international exchange has been added to
      the (multi-)link section.

      In both cases, on recognition of a language or discriminating digit (a
      signal I-1 to I-10), the incoming R2 register determines that the call
      is to be routed to the national network, and selects the next backward
      signal:

      i) The incoming R2 register may send signal A-14 to request information
         about the requirement for echo suppression.

         o If an incoming half-echo suppressor is required the outgoing
           international R2 register sends signal I-14.

           In response to signal I-14 the incoming R2 register sends signal
           A-1 to request the first digit of the national (significant)
           number.  In response to signal A-1 the outgoing international R2
           register sends the first digit of the national (significant)
           number.

        o If no echo suppressor is required the outgoing international R2
          register sends the first digit of the national (significant) number.

     ii) Alternatively, if it is known that no echo suppressor is required to
         be inserted the incoming R2 register may send signal A-1 to request
         the first digit of the national (signi- ficant) number.  In response
         to signal A-1 the outgoing international R2 register sends the first
         digit of the national (significant) number.

         The incoming R2 register examines the first digit of the national
         (significant) number and if a further digit (or digits) is required
         for routing, signal A-1 is sent to request the next digit.

         When sufficient digits are stored at the incoming exchange to permit
         the call to be routed to the next exchange the backward signal (if
         any) is determined by the nature of the signalling system employed on
         the outgoing link and the national routing principles.

         a) If the outgoing national link employs System R2 and
            international/national end-to-end signalling is used, a backward
            signal may be sent after the outgoing link is seized to request
            the address digit required as the first signal to be received by
            the incoming R2 register in the following national exchange.

            The appropriate signal is one of signals A-1, A-2, A-7, A-8 or
            A-12.  These signals may be sent after any digit, and can be
            repeated provided they do not conflict with the logic procedure.

            However, if the address digit on-line  | s the digit  required
            as the first signal to be received by the incoming R2 register in
            the next exchange, it is possible for the transit exchange to
            release the incoming R2 register and through-connect the
            speech-path after the outgoing link is seized without sending a
            backward signal.

            Otherwise, after the appropriate backward signal is sent and the
            compelled signalling sequence is complete the transit exchange
            releases the register and through-connects the speech path.

         b) If the outgoing national link employs System R2 but end-to-end
            international/national signalling cannot be used, the register in
            the incoming international exchange relays the multifrequency
            signals:  it acts as an outgoing R2 register.  It requests the
            remainder of the address digits by repetitive use of signal A-1.
            The digits received by the outgoing R2 register are retransmitted
            over the outgoing national link at the request of the incoming R2
            register in the following national exchange(s) (see Recommendation
            Q.478).

        c) If the outgoing link employs a signalling system other than System
           R2 then the acting incoming R2 register is the last incoming R2
           register.  The exchange seizes a national link.  Signalling
           continues between the outgoing international R2 register and the
           last incoming R2 register and interworking takes place with the
           other signalling system.

           If congestion is encountered, congestion signal A-4 or A-15 is
           transmitted (if necessary in pulse form) and the incoming R2
           register is dismissed.

           On recognition of the congestion signal A-4 the outgoing exchange
           releases the outgoing link or connection and causes the return of
           congestion information to the calling subscriber.

           On recognition of the congestion signal A-15 the outgoing exchange
           may initiate a repeat attempt, re-routing or cause the return of
           congestion information to the calling subscriber.  In all cases the
           outgoing link or connection is released.

           It is desirable to use the congestion signal A-15 solely in the
           case of incoming international terminal traffic when repeat attempt
           or re-routing may be expected to be successful.

                             ____________________
                             Recommendation Q.463
                             »»»»»»»»»»»»»»»»»»»»
5.1.3 SIGNALLING BETWEEN THE OUTGOING INTERNATIONAL R2 REGISTER AND AN
~~~~~ INCOMING R2 REGISTER IN A | NATIONAL EXCHANGE IN THE DESTINATION COUNTRY


      5.1.3.1 Signalling to a national transit exchange

      The outgoing international R2 register sends the requested address digit
      as the first signal to be received by the incoming R register in the
      national transit exchange in the destination country.

      The incoming R2 register examines the digit and if a further digit (or
      digits) is required for routing, signal A-1 is sent to request the next
      digit.

      When sufficient digits are stored at the incoming exchange to permit the
      call to be routed to the next exchange the backward signal (if any) is
      determined by the nature of the signalling system employed on the
      outgoing link and the national routing principles.

      a) If the outgoing national link employs System R2 a backward signal may
         be sent after the outgoing link is seized to request the address
         digit required as the first signal to be received by the incoming R2
         register in the next exchange.  The signalling procedure which occurs
         is similar to that described in S 5.1.2.2 a) above.

      b) If the outgoing link employs System R2 but end-to-end
         international/national signalling cannot be used, the register in the
         national exchange relays the interregister signals:  it acts as an
         outgoing R2 register.  The digits received by this outgoing R2
         register are retransmitted over the outgoing link at the request of
         the incoming R2 register in the following exchange(s) (see
         Recommendation Q.478).

      c) If the outgoing link employs a signalling system other than System R2
         then the acting incoming R2 register is the last incoming R2
         register.  The exchange seizes an outgoing national link to the next
         national exchange.  Signalling continues between the outgoing
         international R2 register and the last incoming R2 register and
         interworking takes place with the other signalling system.

      If congestion is encountered signal A-4 is sent (if necessary in pulse
      form) and the incoming R2 register is released.

      On recognition of congestion signal A-4 the outgoing exchange releases
      the outgoing multi-link section and causes the return of congestion
      information to the calling subscriber.


      5.1.3.2 Signalling to a national exchange to which the called subscriber
              is connected

      When the outgoing multi-link section is routed to the national exchange
      to which the called subscriber is connected the acting incoming R2
      register is the last incoming R2 register:  the outgoing international
      R2 register sends the requested address digit as the first signal to be
      received by the last incoming R2 register and signalling continues as
      described below.


                             ____________________
                             Recommendation Q.464
                             »»»»»»»»»»»»»»»»»»»»
5.1.4 SIGNALLING BETWEEN THE OUTGOING INTERNATIONAL R2 REGISTER AND THE LAST
~~~~~ INCOMING R2 REGISTER


      5.1.4.1 General

      The usual System R2 signalling procedure is to request in succession the
      remainder of the address digits stored in the outgoing international R2
      register by the repetitive use of signal A-1 until it is determined at
      the incoming end that the complete address information has been received
      by the incoming exchange or that the call cannot be routed.

      System R2 enables transmission of information concerning a large number
      of different conditions of called subscriber's line or reasons why call
      set-up has failed.  But this can only be sent if the switching systems
      and other signalling systems employed on the remaining links of the
      connection offer the possibility of differentiating several line
      conditions.  Group B-signals are provided for this purpose.

      Transition from Group A to Group B meanings is indicated by means of
      address-complete signal A-3.  However, if the incoming exchange is
      unable to send any signals concerning the condition of the called
      subscriber's line it is unnecessary to send signal A-3 followed by a
      Group B signal.  In such cases the address-complete signal A-6, provided
      for the purpose is used.

      If congestion is encountered after transmission of the address-complete
      signal A-3 the congestion signal B-4 is sent in place of signal A-4 or
      A-15.


      5.1.4.2 When the last incoming R2 register is able to transmit the
              condition of the called subscriber's line

      When the condition of the called subscriber's line can be determined the
      incoming R2 register can send signals conveying this information after
      receipt of the address digits.

      As soon as the last address digit is received the last incoming R2
      register sends the address-complete signal A-3 to announce changeover to
      the transmission of Group B signals.  In response the outgoing
      international R2 register sends the appropri- ate calling party's
      category signal (II-7 to II-10).  The last incoming R2 register
      acknowledges this with the relevant Group B signal indicating the
      condition of the called subscriber's line.

      Immediately the compelled signalling sequence is complete the incoming
      R2 register is released and depending upon the Group B signal sent the
      speech-path may be through-connected.

      When the condition of the called subscriber's line is determined by an
      electrical signal to the last incoming R2 register and if the called
      subscriber's line is free the address-complete signal A-6 can be sent
      instead of signal A-3, possibly in pulse form.  This will be the last
      interregister signal.  After signal A-6 is sent the incoming R2 register
      is released, the speech-path is through-connected and ringing tone
      returned to the calling subscriber.  Outgoing international R2 registers
      must be able to interpret all Group B signals.

      On recognition of the last backward signal the outgoing exchange
      releases the outgoing international R2 register and either
      through-connects the speech-path or releases the outgoing connection and
      causes the return of an appropriate tone or special announcement or both
      alternately to the calling subscriber.


      5.1.4.3 When the last incoming R2 register is not able to transmit the
              condition of the called subscriber's line

      In this case the last incoming R2 register sends the address-complete
      signal A-6, possibly in pulse form, as the last interregister signal.
      After this is sent, the last incoming R2 register is released and the
      speech-path is through-connected.

      When the last incoming R2 register is in the exchange to which the
      called subscriber is connected, an appropriate tone is returned to the
      calling subscriber from this exchange.

      On recognition of address-complete signal A-6, the outgoing exchange
      releases the outgoing international R2 register and through-connects the
      speech-path.  The calling subscriber will then hear ringing tone, busy
      tone, special information tone or a recorded announcement alternately
      with special information tone sent by the incoming equipment.


                             ____________________
                             Recommendation Q.465
                             »»»»»»»»»»»»»»»»»»»»
5.1.5 PARTICULAR CASES
~~~~~
      5.1.5.1 Unallocated number

      When, after the reception of any digit, the incoming R2 register
      determines that the address information corresponds to an unallocated
      number, the address-complete signal A-3 is sent immediately without
      requesting all the address digits.  In response the outgoing
      international R2 register sends the appropriate Group II signal.  This
      is then acknowledged by signal B-5 unallocated number.


      5.1.5.2 Congestion in the national network

      If congestion is encountered in the national network the incoming R2
      register sends congestion signal A-4, possibly in pulse form.  However,
      if address-complete signal A-3 has already been sent, congestion signal
      B-4 is sent in acknowledgement of the Group II signal which commences
      the last compelled signalling sequence.


      5.1.5.3 Operator calls

      The procedures described in Recommendations Q.462-Q.464 are also valid
      for a semi-automatic call.  However, in this case the address
      information is always terminated by the end-of-pulsing signal I-15.

      For code 11 or code 12 calls only a limited number of backward signals
      can be employed as the last interregister signal (e.g.  signal A-4, A-6
      or B-6).


      5.1.5.4 Request of calling party's category

      At any time calling party's category information may be requested by the
      incoming R2 register interrupting the normal transmission of address
      information.  The incoming R2 register sends signal A-5 in
      acknowledgement of a Group I signal and the outgoing international R2
      register sends the appropriate Group II signal (a signal II-7 to II-10)
      in reply.  If this Group II signal is acknowledged by a signal different
      from address-complete signal A-3 or signal A-5 the next forward signal
      sent will be one of the Group I.



                             ____________________
                             Recommendation Q.466
                             »»»»»»»»»»»»»»»»»»»»
5.1.6 SUPERVISION AND RELEASE OF THE CALL
~~~~~
   An answer signal is sent when the called subscriber's off-hook condition is
detected.  Each transit exchange in the connection, relays this line signal.
At the outgoing international exchange, receipt of the answer signal normally
causes call charging to take place and metering for international accounting
purposes to start.

   When the called subscriber's on-hook condition is detected a clear-back
signal is sent and relayed by each transit exchange.

   When the calling subscriber's on-hook condition is detected a clear-forward
signal is relayed by the outgoing international exchange over the outgoing
international link.  On receipt of the clear-forward signal on an incoming
link, release operations are initiated and the clear-forward signal is
repeated forward on the outgoing link.

   Immediately release operations at an exchange are complete (although the
outgoing circuit may still be held) a release-guard sequence is initiated on
the incoming link.  After recognition of the end of the release-guard sequence
at the outgoing exchange the link reverts to the idle state.

   Supervision of a call must be in accordance with Recommendation Q.118.


                             ____________________
                             Recommendation Q.468
                             »»»»»»»»»»»»»»»»»»»»
 * 5.2 :  ROUTING AND NUMBERING FOR INTERNATIONAL WORKING (See Recommendations
          Q.107 and Q.107 | fIbis , Fascicle VI.1.)


 * 5.3 :  TERMINATION OF INTERREGISTER SIGNALLING


                             ____________________
                             Recommendation Q.470
                             »»»»»»»»»»»»»»»»»»»»
5.3.1 AT AN INCOMING R2 REGISTER SITUATED IN A TRANSIT EXCHANGE
~~~~~
      5.3.1.1 Successful routing

      Normally, interregister signalling is terminated in one of the following
      ways:

      a) The last forward interregister signal received by the incoming R2
         register at the transit exchange is not acknowledged from that
         register.  After an outgoing link to the next exchange is seized, the
         register is dismissed and the speech-path is through-connected.  The
         forward signal remains on-line and is the first signal received by
         the next incoming R2 register.  Provision must be made for the
         switching operations to be completed so as to ensure that the signal
         will remain on-line and be received by the succeeding register.

      b) The last forward interregister signal received by the incoming R2
         register at the transit exchange is acknowledged by a backward signal
         (A-1, A-2, A-7, A-8, A-11 or A-12) requesting transmission of a
         clearly specified signal as the first to be received by the next
         incoming R2 register.  When the compelled signalling sequence is
         complete the register is dismissed and the speech-path is
         through-connected.

      Because it is not possible to send signals A-2, A-7, A-8, A-11 or A-12
      in pulse form, precautions are necessary to avoid acknowledging the last
      address digit (on automatic calls) until the signalling system employed
      on the outgoing link is known.  If signal A-1 is sent in acknowledgement
      of the last address digit and if the outgoing link employs System R2 it
      may not then be possible to send one of these signals and therefore
      end-to-end signalling to the next exchange is no longer feasible (see
      also Recommendation Q.474).

      In international working signals A-2, A-7 and A-8 may be used to
      acknowledge receipt of any forward signal.

      Signal A-11 must be used to request a country code indicator.

      Signal A-12 must generally be used to request the language or
      discriminating digit.  However, in the case where an incoming R2
      register is equipped with only 5 backward signalling frequencies only
      one of the signals A-2, A-7 or A-8 can be used for this purpose.
      Attention is drawn to the fact that in this case the language or
      discriminating digit may not be sent by all outgoing international R2
      registers.

      Although it is not normally necessary to request a repeat of a digit n
      because it remains on the line until acknowledged (but see

      Recommendation Q.476) such repetition may prove necessary after an
      interruption of digit transmission (e.g.  to request infor- mation
      concerning the calling party's category) or if the comple- tion of
      switching operations referred to in method a) above, can- not be
      guaranteed with a time compatible with the time-out of the outgoing
      (international) R2 register (see Recommendation Q.476).  The procedure
      is then as follows:

      Signal A-2 is sent, eliciting the digit n -1; this is immediately
      acknowledged by signal A-1 to elicit the required digit n .  When the
      digit n in question is the first digit in the store of the outgoing R2
      register this procedure is not applicable.

      At an international transit exchange method b) above, must be used and
      only one of the signals A-11 or A-12 applies.

      When the outgoing link connects to a further international transit
      exchange signal A-11 must be used to request a country code indicator.
      On recognition of signal A-11 the outgoing international R2 register
      must send a country code indicator (signal I-12 or I-14, see
      Recommendation Q.479) as the first for ward signal to be received by the
      next incoming R2 register.

      Signal I-12 or I-14 may be requested as many times as necessary by
      sending signal A-11.

      When the outgoing link connects to an incoming international terminal
      exchange signal A-12 must be used to request the language or
      discriminating digit.  On recognition of signal A-12 the outgoing
      international R2 register must send the language or discriminating digit
      (a signal I-1 to I-10) as the first forward signal to be received by the
      next incoming R2 register.

      On recognition of signal A-12 (sent from an international transit
      exchange) an outgoing international R2 register is informed that an
      international link connected to an incoming international terminal
      exchange has been added to the (multi-)link section and that call set-up
      is now in progress in the national destination network.  This is of
      importance when signal A-9 or A-10 are, in the originating country,
      amongst those used to set-up international calls.


      5.3.1.2 Congestion

      If it is impossible to set-up the desired connection in the transit
      exchange the incoming R2 register terminates interregister signalling by
      transmission of the congestion signal A-4 or A-15.  The backward signal
      may serve as acknowledgement of the last forward signal received by the
      incoming R2 register at the transit exchange or is sent in pulse form.

      The two congestion signals A-4 and A-15 are provided to enable an
      outgoing international R2 register to determine whether congestion is
      occurring in the international network or in that of the destination
      country so that provision for repeat attempt or re-routing can be made
      in the former case:

      o Signal A-15 is sent from an international exchange.

      o Signal A-4 is sent from a national exchange or possibly from a
        terminal international exchange.  [See also S 5.1.2.2 c).]

      Because receipt of congestion signal A-15 by an outgoing international
      R2 register may initiate repeat attempt or re-routing it is possible to
      transmit congestion signal A-4 from international exchanges where repeat
      attempt or re-routing may be expected useless.


                             ____________________
                             Recommendation Q.471
                            »»»»»»»»»»»»»»»»»»»»
5.3.2 AT THE LAST INCOMING R2 REGISTER SITUATED IN THE EXCHANGE TO WHICH THE
~~~~~ CALLED SUBSCRIBER IS CONNECTED

      5.3.2.1 Successful routing

      When a call has been completely set up by means of System R2
      interregister signalling, the incoming R2 register terminates
      interregister signalling immediately on receipt of the entire number.

      The following criteria are used to determine whether the number received
      by the incoming R2 register is complete:

      a) analysis - to determine the last digit;

      b) electrical conditions given by the switching equipment succeeding the
         incoming R2 register;

      c) receipt of the end-of-pulsing signal (I-15);

      d) the assumption, after a specified time has elapsed, that no further
         digits will be sent (see Recommendation Q.476).

      When criterion a) (analysis) is applied:

      o if the incoming R2 register is equipped to send Group B signals to
        provide information about the condition of the called subscriber's
        line the address-complete signal A-3 is transmitted on receipt of the
        last digit.  As soon as it is known whether the connection with the
        subscriber's line can be established, only the appropriate Group B
        signal needs to be sent.  The use of the Group B signals is detailed
        in Recommendation Q.474;

      o if the incoming R2 register is not equipped to receive information
        about the condition of the called subscriber's line, the
        address-complete signal A-6 is sent immediately after reception of the
        last digit and no Group B signal will be transmitted.

      In both cases the time interval between the end of a signal A-6 or a
      Group B signal and the start of the subsequent answer signal must not be
      less than 75 ms.

      When criterion b) (electrical conditions) is applied:

      It is recommended that, to avoid delay in sending the answer signal, no
      Group B signal should be sent when the called subscriber's line is free,
      and that the setting-up of speech conditions be ensured by sending
      address-complete signal A-6 immediately the electrical conditions are
      recognized.  The time interval between the end of signal A-6 and the
      start of transmission of the subse- quent answer signal must be not less
      than 75 ms (see also Recommendations Q.412 and Q.475).

      Criterion c) (end-of-pulsing) can be applied only if the incoming R2
      register is equipped to receive the 6 forward signalling frequencies
      (see also Recommendation Q.473).  When signal I-15 is received and
      recognized the last incoming R2 register can perform in the way
      described under criterion a).

      When criterion d) (time-out) is applied:

      Address-complete signal A-6 must be sent in pulse form as soon as the
      specified time has elapsed.  The time between the end of signal A-6 and
      the start of transmission of the subsequent answer signal must be, as
      indicated above for criterion b) , not less than 75 ms (see also
      Recommendations Q.412 and Q.472).

      It may happen, however, that the called subscriber answers before the
      specified time has elapsed.  In such exceptional circumstances the pulse
      signal A-6 must be sent immediately the answer signal is recognized.  In
      this case the time between the end of signal A-6 and the start of the
      transmission of the subsequent answer signal must be 75 ms or more but
      less than 150 ms.  The calling subscriber will not hear the ringing
      tone.

      This disadvantage can be avoided by not using a received digit to set up
      the call until after the following digit has been received or until a
      certain time has elapsed.  This procedure, however, may give rise to
      difficulties if the time-out devices provided in the switching equipment
      succeeding the incoming R2 register are set for too short a delay [see
      also Recommendation Q.120, S 1.5.5.2 b) iv)].


      5.3.2.2 Congestion

      An incoming R2 register should terminate interregister signalling
      immediately any conditions preventing complete setting-up of a call have
      been recognized.

      If congestion is encountered congestion signal A-4 is sent, possibly in
      pulse form.  However if address-complete signal A-3 has already been
      sent then congestion signal B-4 is sent in acknowledgement of the Group
      II signal which commences the last compelled signalling sequence.


                             ____________________
                             Recommendation Q.472
                             »»»»»»»»»»»»»»»»»»»»
5.3.3 AT THE LAST INCOMING R2 REGISTER SITUATED IN A TRANSIT EXCHANGE
~~~~~
      5.3.3.1 Successful routing

      Interregister signalling to such a register can be terminated after all
      the address digits are received.  In order to determine that the number
      is complete the same criteria as described in Recommendation Q.471 are
      used.

      When criterion a) (analysis) is applied:

      1) if the signalling system employed on the outgoing link enables the
         condition of the called subscriber's line to be sent backwards within
         an acceptable period compared to the time-out of the outgoing
         international R2 register, the last incom- ing R2 register can react
         in accordance with one of the following methods:

         i) address-complete signal A-3 is sent in acknowledgement of the last
            address digit followed by the relevant Group B signal depending
            upon the condition of the called subscriber's line;

        ii) signal A-1 is sent in acknowledgement of the last address digit,
            and signal I-15 if received, to deliberately suspend compelled
            signalling and later,

            o when the condition of the called subscriber's line is known,
              address-complete signal A-3 is sent in pulse form followed by
              the relevant Group B signal.  This makes it possible to avoid
              maintaining on-line the Group II signal sent after reception of
              signal A-3,

            o when the called subscriber's line is free, it is preferable to
              send the address-complete signal A-6 in pulse form,

      2) if the signalling system employed on the outgoing link does not
         enable the condition of the called subscriber's line to be
         transmitted backwards or if such information is only available at the
         last incoming R2 register after a delay incompatible with the
         time-out of the outgoing international R2 register, then
         address-complete signal A-6 is sent in acknowledgement of the last
         address digit.

         When criterion b) (electrical conditions) is applied:

         Only signal A-1 can be used to acknowledge each address digit.

         Upon receipt of an electrical signal on the outgoing link, the last
         incoming R2 register acts in one of the following ways:

         o if the condition of the called subscriber's line is not known or is
           known to be free , address-complete signal A-6 is sent in pulse
           form;

         o if the condition of the called subscriber's line is known to be
           other than free , address-complete signal A-3 is sent in pulse
           form, followed by the relevant Group B signal.

         Criterion c) (end-of-pulsing) is only applicable if the last incoming
         R2 register can receive 6 forward signalling frequencies (see also
         Recommendation Q.473).  In this case, when end-of-pulsing signal I-15
         is received and recognized, the last incoming R2 register can perform
         in the way described under criterion a) .

          When criterion d) (time-out) is applied:

          Address-complete signal A-6 is sent in pulse form after the
          specified time has elapsed (see Recommendation Q.476).


      5.3.3.2 Congestion

      If congestion is encountered, the procedure described in Recommendation
      Q.470 is followed.  However, if address-complete signal A-3 has already
      been sent then congestion signal B-4 is sent in acknowledgement of the
      Group II signal which commences the last compelled signalling cycle.


                             ____________________
                             Recommendation Q.473
                             »»»»»»»»»»»»»»»»»»»»
5.3.4 USE OF END-OF-PULSING SIGNAL I-15 IN INTERNATIONAL WORKING
~~~~~
   In international working, the end-of-pulsing signal I-15 is used in
accordance with Recommendation Q.468.  Signal I-15 (end-of-pulsing) is sent
immediately after the last digit.

   In national working, signal I-15 may be used too.

   In semi-automatic operation, calls to operators' positions are always
terminated by transmission of signal I-15.  This signal can only be
interpreted if the incoming R2 register is equipped for reception of 6 forward
frequencies.  However, when an incoming R2 register is equipped for reception
of only 5 forward signalling frequencies no provision can be made for
recognition of signal I-15.  Such an incoming R2 register will then act as
though signal I-15 had not been sent.  Consequently the next interregister
signal is only one of the signals A-3, A-4 or A-6 sent in pulse form.

      5.3.4.1 Procedures to be followed after receipt of end-of-pulsing signal
              I-15 by the last incoming R2 register (situated in a transit
      exchange or in the exchange to which the called subscriber is connected)

      Incoming R2 registers equipped for the reception of all 6 forward
      signalling frequencies can acknowledge receipt of the end-of-pulsing
      signal I-15 by sending an appropriate backward signal to complete the
      compelled signalling cycle.  Interregister signalling is terminated
      according to the procedures specified in Recommendations Q.471 or Q.472
      when criterion a) (analysis) is applied.

      In particular, if the last incoming R2 register acknowledges signal I-15
      with signal A-1 the outgoing international R2 register will not send any
      signal and the incoming R2 register can only send one of signals A-3,
      A-4, A-6 or A-15 in pulse form.

      Since there is no compulsion to equip incoming R2 registers in national
      exchanges with receivers for all 6 forward signalling frequencies, the
      signal I-15 sent by an outgoing international R2 register may not be
      recognized by the incoming R2 register.  In such cases other criteria
      can be used to determine whether the number received by the incoming R2
      register is complete.

      When the incoming R2 register determines that the number received is
      complete using criteria other than c) (end-of-pulsing) it may be that
      the last digit of the subscriber number is ack- nowledged by signal A-3,
      A-4, A-6 or A-15.  In this case interregister signalling is terminated
      without request for the signal I-15, in the normal way according to the
      procedures specified in Recommendations Q.471 or Q.472 (a compelled
      signalling cycle including signal I-15 is saved).


      5.3.4.2 Procedure to be followed after receipt of signal I-15 by an
              incoming R2 register situated in a transit exchange

      Signal A-1, A-2, A-7, A-8, A-11 or A-12 may be sent in acknowledgement
      of signal I-15 (end-of-pulsing).  However, at a transit exchange
      precautions are necessary to avoid acknowledgement of signal I-15 before
      the signalling system employed on the outgoing link has been identified.
      If signal A-1 is sent in acknowledgement of signal I-15 and if the
      outgoing link employs System R2, it is not then possible to send
      backward signal A-2, A-7, A-8, A-11 or A-12 after the end of the
      compelled signalling cycle because these signals cannot be sent in pulse
      form.  Therefore end-to-end signalling to the next exchange is no longer
      possible.



                             ____________________
                             Recommendation Q.474
                             »»»»»»»»»»»»»»»»»»»»
5.3.5 USE OF GROUP B SIGNALS
~~~~~
   Group B signals are used to transmit information about the condition of
switching equipment in the incoming exchange or the called subscriber's line,
to the outgoing international R2 register, which can then take the necessary
action.

   The address-complete signal A-3 is sent by an incoming R2 register to
announce changeover to sending Group B signals.  In addition, signal A-3
indicates that the incoming R2 register has received all the Group I forward
signals it requires from the outgoing international R2 register.  On
recognition of signal A-3 an outgoing R2 register sends a Group II signal.
The incoming R2 register may interpret the Group II signal giving information
about calling party's category, in order to control the switching operations
accordingly (e.g.  suppression of automatic ringing for operator initiated
calls or to prevent subscriber access to data transmission terminals).
Finally, a Group II forward signal is acknowledged by any Group B backward
signal.

   In cases where outgoing R2 registers are able to interpret all Group B
signals there is generally no need to provide equipment at the incoming end
able to send, in addition to Group B signals, the tones and/or announcements
corresponding to those signals, except in the case of ringing tone.

   In general, outgoing R2 registers must be equipped with devices permitting,
after reception of signal A-3:

   o the exchange of an additional cycle of interregister signals before the
     register is released;

   o changeover from Group A to Group B meanings of backward signals.

   o Outgoing international R2 registers, however, must be able to interpret
     all Group B signals.


      5.3.5.1 Procedures to be followed by an outgoing international R2
              register on receipt of Group B signals

      Signal B-1 is interpreted by an outgoing international R2 register as
      signal B-6:  the register is released and the speech-path is
      through-connected.  A subsequent answer signal initiates call charging.

      Signal B-2 send special information tone is sent by the incoming R2
      register:

      o when the number of the called party has been changed,

      o when the three following conditions are fulfilled together:

       i) the called party's line condition does not fit with one of the
          meanings of the present Group B signals,

      ii) it does not lead to setting up the speech path,

     iii) it is not incompatible with return of special information tone to
          the calling party.

      After recognizing signal B-2, the outgoing international R2 register
      clears forward and causes the transmission of only special information
      tone.

      The subscriber's line busy signal B-3 is sent by the incoming register
      when the called subscriber's line is engaged.  On recognition of this
      signal, the outgoing register releases the connection and causes
      transmission of the busy tone.

      When the congestion condition is encountered following the changeover
      from Group A signals to Group B signals, the congestion signal B-4 shall
      be transmitted on the conditions specified for congestion signal A-4.
      In all cases recognition of the signal causes release of the call and
      transmission of congestion information.

      After recognizing unallocated number signal B-5, the outgoing
      international R2 register clears the call and causes transmission of
      special information tone or a recorded announcement and special
      information tone, alternately, to the calling party.

      After recognizing signal B-6, the outgoing international R2 register
      sets up speech conditions so that a caller may hear the ringing tone.
      In this case, a subsequent answer signal activates the call-charging
      mechanism.

      After recognizing signal B-7 the outgoing international R2 register sets
      up speech conditions, so that a caller may hear the ringing tone.  In
      this case, a subsequent answer signal does not activate the
      call-charging mechanism.  However, in an outgoing international R2
      register signal B-7 may be interpreted as B-6 if there is no
      international agreement on non-chargeable calls.

      After recognizing signal B-8 subscriber's line out of order, the
      outgoing international R2 register clears forward and causes the
      transmission of a special information tone or recorded announcement and
      special information tone alternately to the calling party.

      Receipt of signal B-9 or B-10 by an outgoing international R2 register
      causes release of the outgoing connection and return of special
      information tone to the calling subscriber, i.e.  these signals should
      be interpreted as signal B-2.

      If an outgoing international R2 register receives one of signals B-11 to
      B-15 the call should be released and indication of this returned to the
      calling subscriber or operator, i.e.  this signal should be interpreted
      as signal B-4.



      5.3.5.2 Special procedures for national working

      Outgoing R2 registers in national exchanges may not be able to recognize
      and interpret Group B signals.  In such networks it is essential that
      the equipment at the incoming end must transmit not only the Group B
      signals but also the corresponding tones and/or announcements, when the
      incoming R2 register does not know whether the outgoing R2 register is
      capable of interpreting Group B signals.

      In the case where the incoming R2 register is able to make only two or
      three distinctions of conditions of the called subscriber's line the
      following procedure may be adopted:  When the only distinction that can
      be made is between subscriber's line free and subscriber's line engaged,

      i) signal B-3 is sent if the line is engaged;

     ii) otherwise signal B-6, or signal A-6 alone is sent so the caller can
         hear the ringing tone sent by the incoming equipment.

      In the case where outgoing R2 registers in national exchanges are only
      able to interpret a limited number of Group B signals or are indeed not
      able to interpret any, it is essential that such equipment be able to
      respond to signal A-3 in the specific manner and at least to recognize
      the next backward signal (which is a Group B signal) as indicating the
      end of interregister signalling.


      5.3.5.3 Possible application of signal B-1 in national working

      Signal B-1, for example, may be used to indicate that the call must
      remain under the control of the incoming equipment, insofar as this is
      possible (e.g.  for maintenance reasons, to trace malicious calls,
      etc.).  The incoming exchange sets up speech conditions, so that the
      calling subscriber can hear the ringing tone.  At present such a use of
      signal B-1 in international working is not envisaged.


                             ____________________
                             Recommendation Q.475
                             »»»»»»»»»»»»»»»»»»»»
 * 5.4 :  NORMAL RELEASE OF OUTGOING AND INCOMING R2 REGISTERS

   An outgoing international R2 register must be released once it has received
an appropriate backward interregister signal terminating the interregister
signalling, or on receipt of a clear-forward (line) signal from the preceding
link.

   An incoming R2 register must be released once it has accomplished the
necessary switching control and terminated the interregister signalling
required, or on receipt of a clear-forward (line) signal from the preceding
link.

   The last recognized interregister signal preceding setting up of speech
conditions will normally be a backward signal:  e.g.  address-complete signal
A-6, subscriber's line free signals B-6 or B-7.

   The multifrequency combination receivers at the two ends must be
disconnected before the switching equipment passes to the speech condition;
this procedure eliminates any possibility of their operating or being held
under the influence of speech or line signals.  The following conditions
regarding the duration of the various phases should be observed (see Figure
20/Q.475):

   a) the multifrequency signalling equipment of the incoming R2 register must
      be disconnected within 30 ms after recognition of the end of
      transmission of the last backward signal;

   b) the multifrequency signalling equipment of the outgoing R2 register must
      be disconnected within 30 ms after recognition of the end of the last
      backward signal;

   c) at the outgoing exchange speech conditions must be set up between 30 and
      60 ms after recognition of the end of the last backward signal.
      However, at the exchange where the outgoing international R2 register is
      situated, the setting-up of speech conditions depends upon the
      signalling system employed on the preceding link;

   d) at the incoming exchange at least 75 ms must elapse between the end of
      transmission of the last backward signal and the setting up of speech
      conditions.

                                                      Figure 20/Q.475 p.13


                             ____________________
                             Recommendation Q.476
                             »»»»»»»»»»»»»»»»»»»»
 * 5.5 ; ABNORMAL RELEASE OF OUTGOING AND INCOMING R2 REGISTERS

   To limit the holding time of R2 registers, when interregister signalling is
interrupted either by a fault or by any other cause, all R2 registers must be
equipped with devices for continuous supervision of the time taken by the
various phases of interregister signalling.  The time-out delay of these
devices must be as short as possible, but long enough not to interrupt normal
operation.


5.5.1 Time-out of outgoing international R2 register
~~~~~
   In an outgoing international R2 register, the intervals during which a
forward multifrequency combination is transmitted and the intervals during
which no such combination is transmitted are supervised separately.

      5.5.1.1 Supervision during sending of forward multifrequency
              combinations

      The lower limit of the time-out delay is a function of the time required
      for the switching procedures in a transit exchange.

      On this basis, the time-out delay is specified as 15 _ 3 seconds.

      The supervision device will start functioning at the beginning of the
      transmission of a forward multifrequency combination and be reset with
      the deactivation of the senders involved.  It will start again at the
      beginning of the transmission of the next forward multifrequency
      combination.


      5.5.1.2 Supervision during intervals when no forward multifrequency
              combination is being sent

      The lower limit of the time-out delay is a function of:

      a) the maximum permissible time interval between dialling of two
         successive digits by the subscriber;

      b) the time-out delay specified for incoming R2 registers (see S 5.5.2
         below).

      On this basis, the time-out delay is specified to be longer than 24
      seconds (a longer delay and an upper limit may be specified by each
      Administration).

      If this specification is observed an incoming R2 register, which has
      acknowledged the last received digit with the signal A-1, is bound to be
      released before the supervision device of the outgoing international R2
      register initiates the alarm condition.


      5.5.1.3 Procedure to be followed if time-out occurs

      If time-out occurs, the time supervision devices mentioned in SS 5.5.1.1
      and 5.5.1.2 above will bring about operations producing:

      o return of an appropriate signal and/or audible tone to inform the
        calling party,

      o release of the outgoing international R2 register and of the
        connection as far as the latter is not necessary for the
        above-mentioned operation.

      Fault recording equipment may start functioning and/or a delayed alarm
      may alert the technical staff.


      5.5.1.4 Time-out of outgoing R2 register

      It is recommended that the same principles outlined in SS 5.5.1.1 to
      5.5.1.3 above be applied by analogy to outgoing R2 registers.


5.5.2 Time-out of incoming R2 register
~~~~~
   The time-out device shall supervise the interval elapsing between seizure
of the register and recognition of the first forward multifrequency
combination as well as the interval elapsing between the recognition of two
successive multifrequency combinations in the forward direction.

      5.5.2.1 Time-out delay

      The lower limit of the time-out delay is a function of:

      a) the maximum permissible time interval between the recognition of 2
         successive forward multifrequency combinations; this time interval
         may in certain cases be influenced by the maximum permissible time
         interval between dialling of 2 successive digits by the subscriber;

      b) the maximum time required for setting-up the call under conditions
         which slow down the interregister signalling.

      In view of the desirability expressed in S 5.5.1.2 above that the
      incoming R2 register be released before expiry of the time-out delay
      specified for the outgoing international R2 register, an upper limit
      should be fixed as well.

      On this basis the time-out delay should be specified within the range of
      8-24 seconds.  A minimum delay of 15 seconds corresponding to the
      time-out delays in other CCITT standardized signalling systems is to be
      preferred.

      For incoming R2 registers using the criterion d) (time-out) indicated in
      Recommendation Q.471 to determine completion of the number, the time
      referred to there as the specified time may exceptionally be shorter
      than 8 seconds, but never less than 4 seconds.


      5.5.2.2 Procedure to be followed if time-out occurs

      If time-out occurs, the time supervision device will bring about
      operations producing:

      o sending of a congestion signal (A-4 or A-15) in pulse form;

      o release of the incoming R2 register and other equipment in the
        incoming exchange;

      o on time-out of the initial digit:

        i) line signalling, analogue version:  establishment of the blocked
           state on the incoming circuit until recognition of the
           clear-forward signal (see Recommendation Q.412, abnormal
           conditions);

       ii) line signalling, digital version:  no further action required.

      Fault recording equipment may start functioning and/or a delayed alarm
      may alert the technical staff.



                             ____________________
                             Recommendation Q.478
                             »»»»»»»»»»»»»»»»»»»»
 * 5.6 :  RELAY AND REGENERATION OF R2 INTERREGISTER SIGNALS BY AN OUTGOING R2
          REGISTER IN A TRANSIT EXCHANGE

   When the overall multi-link connection is divided into end-to-end
signalling sections, regeneration of R2 interregister signals is required by
either an outgoing international R2 register or an outgoing R2 register (see
Recommendation Q.440).

   There are three possible procedures for relaying interregister signals by
an outgoing R2 register:

   a) the outgoing R2 register acknowledges each signal received over the
      incoming link by transmitting the appropriate backward signal; this
      operation is independent of the operations entailed in retransmitting on
      the outgoing link;

   b) the forward address signal with rank n + 1 is acknowledged on the
      incoming link as soon as the forward address signal with rank n is
      acknowledged on the outgoing link;

   c) as soon as a forward signal is received on the incoming link, a signal
      is sent on the outgoing link; the acknowledgement signal is sent on the
      incoming link only when such a signal has been received on the outgoing
      link.

   Methods a) and b) provide the most rapid transfer of information and are
therefore the preferable methods of relaying the information necessary for
setting up the call.  It is essential, however, that the outgoing R2 register
be provided with adequate storage capacity in the case of method a) .

   Method b) can only be applied after method a) .

   Method c) should be used for relaying information relating to the
procedures for the termination of interregister signalling.

   The changeover from method a) or b) to method c) may require transmission
of an address-complete signal A-3 in pulse form as indicated in Recommendation
Q.442 (see Figure 21/Q.478).

   The procedure for disconnecting the multifrequency combination receivers
and setting-up speech conditions for each link, incoming and outgoing, is
given in Recommendation Q.475.

   When method a) or b) is used, the time-outs of the R2 registers associated
with the first signalling section may expire if the signalling on the second
signalling section is too slow.  Relatively long time-outs are recommended
(see Recommendation Q.476).

Note - The use of signal A-3 on circuits with very long transmission delay,
       e.g.  satellite circuits, for relaying such information (method c) may
       result in premature release in certain incoming local exchanges with
       very short time guard.  The problem may be avoided if the outgoing R2
       register immediately following the satellite link uses signal A-5 to
       obtain the calling party's category information previously to the
       receipt of signal A-3 from those incoming local exchanges.

                                                      Figure 21/Q.478 p.14

                             ____________________
                             Recommendation Q.479
                             »»»»»»»»»»»»»»»»»»»»
 * 5.7 :  ECHO-SUPPRESSOR CONTROL - SIGNALLING REQUIREMENTS

5.7.1 Introduction
~~~~~
   Recommendations Q.42 and Q.115 give fundamental requirements with which the
employment of echo suppressors must comply.

   There are two methods of connecting echo suppressors to a circuit.  One
method is by use of permanently connected echo suppressors and the other is by
the insertion of echo suppressors from a pool, when required.

   In System R2 (see Recommendation Q.441) two forward signals (I-12 and I-14)
and one backward signal (A-14) are provided to indicate whether or not an
incoming half-echo suppressor (IHES) is required.

   A third forward signal (I-11) is also available for use by bilateral
agreement to indicate that an outgoing half-echo suppres- sor (OHES) must be
employed.

   The following principles must be taken into account.


5.7.2 Principles of echo-suppressor control
~~~~~
      5.7.2.1 The outgoing international exchange may be able to determine the
              need for echo suppressors in the connection by analysing the
              received country code.

      5.7.2.2 In direct traffic between 2 countries the use of echo
              suppressors generally takes place according to fixed rules.

      Echo-suppressor control signalling is not, therefore, necessary in these
      cases unless it is applied for the sake of uniformity.


      5.7.2.3 In international transit traffic the outgoing half-echo
              suppressor (OHES) will normally be employed at the outgoing
              international exchange, and the incoming half-echo suppressor
              (IHES) at the incoming international exchange, unless other
              arrangements are agreed upon with the Administration of the
              international transit exchange(s) (see S 5.7.2.5 below).

      However, in cases where an outgoing satellite link is selected by a
      transit exchange without the knowledge of the outgoing inter-national
      exchange, the signal I-12 may be sent over the satellite link.  In such
      cases half-echo suppressors are required and will normally be
      permanently fitted at each end of the satellite link.

       5.7.2.4 An incoming international exchange, equipped for the connection
               of IHES, must ask the outgoing international exchange whether
               or not an IHES must be employed unless this is known from other
               criteria (e.g.  line classification).  It will do this by
               sending the backward signal A-14 as an acknowledgement of the
               discriminating or language digit (Z-digit).

      When a satellite link is used in the connection the exchange situated at
      the incoming end of the satellite link responds to the A-14 signal
      instead of the outgoing international exchange (see Recommendation Q.7).

      5.7.2.5 It may be agreed upon bilaterally that in international transit
              traffic the OHES (IHES) will not be employed in the outgoing
              (incoming) international exchange, but in the international
              transit exchange, e.g.  in the case where most of the traffic on
              the circuit group between the outgoing exchange and the transit
              exchange (between the transit exchange and the incoming
              exchange) needs no echo-suppressor.

              a) When echo suppressors are needed and the OHES must be
                 employed at the international transit exchange the outgoing
                 international exchange sends the signal I-11 as a country
                 code indicator.

                 If an international connection is routed via two or more
                 transit exchanges, signal I-11 must not be sent beyond the
                 first transit exchange.  Therefore the outgoing exchange,
                 after having sent the signal I-11 once, must send the signal
                 I-14 if the country code indicator is requested again (signal
                 A-11).

              b) When echo suppressors are needed and the IHES must be
                 employed at the international transit exchange this is known
                 by the international transit exchange.  In this case the
                 incoming international exchange will not send signal A-14.

      5.7.2.6 When echo suppressors are employed they will remain disabled
              until the answer signal is received.  This condition is
              necessary in order to permit compelled interregister signalling
              to (e.g.  national) exchanges further on in the connection.

      Alternatively, when the ability exists to detect that interregister
      signalling has been completed, the echo-suppressor may be enabled at
      that time without awaiting the answer signal.


5.7.3 Examples of echo-suppressor control signalling
~~~~~
   The following situations may arise:

      5.7.3.1 Direct interregister signalling between two countries A and B.

              a) No echo-suppressor control signalling is applied.

                 This may be for one of two reasons.  Either the connection
                 does not normally require echo suppressors or echo
                 suppressors are required and are permanently connected to the
                 circuit.

                 The signalling procedure is indicated in Table 11/Q.479,
                 column a.  If echo suppressors are necessary, the OHES is
                 employed at A and the IHES is employed at B.

              b) Echo-suppressor control signalling is applied.

                 There are two cases:

                 i) No echo suppressors are required (see column b of Table
                    11/Q.479).

                ii) Echo suppressors are required (see column c of Table
                    11/Q.479).  The OHES is employed at A and the IHES is
                    employed at B.


      5.7.3.2 Traffic between two countries A and D via two international
              transit exchanges B and C

              There are two cases:


      5.7.3.2.1 The connection is via terrestrial circuits only.

                a) No echo suppressors are required (see column a of Table
                   12/Q.479).

                b) Echo suppressors are required (see columns b, c, d, e of
                   Table 12/Q.479).

                   The following examples may be mentioned:

                   o OHES in A:  IHES in D (column b);

                   o OHES in B:  IHES in D (column c).

               The country code indicator I-11 is used by bilateral agreement
               and indicates that B must employ the OHES.  When the country
               code indicator is to be sent to C, B asks A to do this by
               sending signal A-11.  A, now sends signal I-14 instead of
               signal I-11, because B has employed the OHES.  The signal A-14,
               which is sent from D as an acknowledgement of the Z-digit will,
               of course, also be answered by signal I-14.

                   o OHES in A:  IHES in C (column d).

               In accordance with S 5.7.2.5 b) above, C knows that D cannot
               employ an IHES and C itself will therefore make the connection.
               D will naturally not send signal A-14.

                   o OHES in B:  IHES in C (column e).


      5.7.3.2.2 The connection includes a satellite link.


                 The following examples are given as typical situations that
                 may arise:

                a) The first link in the connection is via satellite.

                   In Table 13/Q.479, P and Q both know that echo suppressors
                   are required.

                   If Q has a permanently connected IHES then:

                   o OHES in P; IHES in Q (column a).

                When R or S is able to employ an IHES then:

                   o OHES in P; IHES in R (column c);

                   o OHES in P; IHES in S (column b).


               b) A later link in the connection is via satellite.  In Table
                  14/Q.479:

               When P knows that Q-R is via satellite:

                   o OHES in P; IHES in R or S (column b or d).

               When P does not know that Q-R is via satellite:


                   o OHES in Q; IHES in R or S (column a or c).

                                                     Tableau 11/Q.479 p.15

                                                     Tableau 12/Q.479 p.16

          Tableau 13/Q.479 (disposition a l'italienne c | te-a-c | te avec
                                            Tableau 14: page pleine), p.17

                    Tableau 14/Q.479 (a l'italienne avec tableau 13), p.18

                             ____________________
                             Recommendation Q.480
                             »»»»»»»»»»»»»»»»»»»»
 * 5.8 :  MISCELLANEOUS PROCEDURES

5.8.1 Nature of circuit procedure for international working
~~~~~
   An incoming R2 register in an international transit exchange or in the
country of destination can elicit the nature of the circuit as soon as at
least one forward signal has been received from an outgoing register.

   The incoming R2 register requests nature of circuit by sending the backward
signal A-13.  The outgoing R2 register, if capable, sends the nature of the
circuit indicator signal I-13 or I-14 in reply.

   Signal A-13 can be sent after any forward signal (Groups I and II) and
particularly after any address digit, but it can only be sent prior to any
transmission of signal A-3.

   If the outgoing R2 register does not have the capability of providing the
nature of circuit, signal I-12 (request not accepted) is sent in reply to
signal A-13.  The incoming R2 register will then ask for the following address
digit by sending signal A-1, for example.  If the outgoing R2 register
receives signal A-13 again it will send signal I-12 again.

   For existing equipment if the outgoing international R2 register does not
have the capability of providing the nature of the circuit, but is capable of
sending identification information as defined in the Yellow Book , the
procedure starts with the first digit of the country code.


5.8.2 Identification procedure for national working
~~~~~
   System R2 makes provision for the signalling necessary to identify the
calling subscriber's line:  for example, by repetition of signal A-5 or by
using one of signals A-9 or A-10.  For the time being this procedure is
restricted to national working only; outgoing international R2 registers
prevent its use over international links (see S 5.8.4).  The outgoing R2
register at the incoming end of an international satellite link must similarly
prevent its use via that link.


5.8.3 Processing of Group II signals reserved for national use
~~~~~
   The Group II signals reserved for national use must be converted in the
outgoing international R2 register to Group II signals used for international
working.

   The conversion must be carried out as follows:

           II-1 must be converted to II-7

           II-2 must be converted to II-7 or II-9

           II-3 must be converted to II-7

           II-4 must be converted to II-7

           II-5 must be converted to II-7 or II-10

           II-6 must be converted to II-8

           II-11 to II-15 must be converted to II-7.

   Since there are no Recommendations yet regarding handling of priority calls
in automatic international working, conversion of signal II-2 to signal II-9
must be the subject of bilateral agreement.

   If an incoming R2 register is situated in a national exchange and because
signals II-7 to II-10 are not used for national working, then analysis of
Group II signals enables a distinction to be made between nationally and
internationally originated calls.

   If an incoming R2 register knows a call is of international origin and if a
Group II signal reserved for national use is received, signal A-4 or B-4
(congestion on the national network) should be sent in acknowledgement.  This
procedure cannot be applied if an incoming R2 register in the destination
country is employed for national and international working and is not able to
detect the origin of the call.


5.8.4 Procedures to be followed by outgoing international R2 registers on
~~~~~ receipt of particular backward signals

   An outgoing international R2 register must send the appropriate calling
party's category signal (II-7 to II-10) in reply to signals A-3 and A-5.

   On recognition of one of the signals A-9 or A-10 an outgoing international
R2 register sends signal I-12 (request not accepted) in response.  Therefore,
national incoming R2 registers using these signals should be equipped to
receive signal I-12.

   The incoming national R2 register receiving I-12 has to determine the
appropriate international standardized signal to be sent in response to signal
I-12 and which substitutes for signals A-9 or A-10.

   In reply to signal A-14 an outgoing international R2 register sends:

   o signal I-14 when an incoming half-echo supressor is required;

   o the next address digit (a signal I-1 to I-10) when no incoming half-echo
     suppressor is required.

   If an outgoing international R2 register receives a backward signal
requesting an impossibility in the logical procedure (e.g.  receipt of signal
A-8 after sending the country code indicator), the call shall be released.