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All drawings contained in this Recommendation have been done in Autocad.
Recommendation Q.724
SIGNALLING PROCEDURES
1 Normal call set-up
In this Recommendation the signalling procedures are described for the
normal call set-up of an international call. The messages and signals are defined
in Recommendation Q.722 and the format and content are given in Recommendation
Q.723.
1.1 Initial address message
An initial address message which is sent as the first message of a call
set-up generally includes all of the information required by the next
international exchange to route the call. The seizing function is implicit in the
reception of this initial address message.
The sending sequence of address information will be the country code (not
sent to an incoming international exchange) followed by the national
(significant) number. For calls to operator positions (code 11 and code 12),
refer to Recommendation Q.107 [1].
All digits required for routing the call through the international network
will be sent in the initial address message. On calls with a country code in the
address (except in the case of calls to special operators), the initial address
message will contain a minimum of 4 digits and should contain as many digits as
are available. All digits of the address may be included; however, the initial
address message can contain one digit in specific circumstances, e.g. national
applications.
Selection of the outgoing national circuit normally can start at the
incoming international exchange on receipt of the initial address message and
signalling can proceed on the first national link.
When no echo suppressor or nature-of-circuit indication is received from a
preceding circuit using a signalling system with fewer facilities, the indicators
will be considered as received no, unless exchange data indicates otherwise.
Note - When additional signalling information (e.g. related to
supplementary services) is to be sent, an initial address message with additional
information may be used.
1.2 Subsequent address message
The remaining digits, if any, of the address may be sent individually in
one-digit messages or in groups in multidigit messages. Efficiency can be gained
by grouping together as many digits as possible.
However, to prevent an increase in post-dialling delay in those cases
where overlap operation with subscribers' dialling is used, it may be desirable
to send the last few digits individually. With reference to the withholding of
digits, sufficient digits should be withheld to avoid the operation at subsequent
exchanges of the short 4-6 second timeout which may be used in certain cases to
determine the address complete condition. (See Recommendation Q.608, S 8.2.1).
Subsequent address messages can be sent on the national network as they
are received. If a continuity-check has to be performed on one or more of the
international circuits involved in the connection, appropriate measures [e.g. by
withholding the last digit(s) of the national number] must be taken at the last
common channel exchange to prevent ringing the called subscriber or alerting the
operator until the continuity of such speech circuits has been verified.
Note - If in the international network the code 0000 in the number of
address signals field is received the message is considered as faulty.
Fascicle VI.8 - Rec. Q.724 PAGE1
1.3 End-of-pulsing (ST) signal
The end-of-pulsing (ST) signal is always sent in the following situations:
a) semiautomatic calls,
b) test calls, and
c) when the end-of-pulsing signal is received from a preceding circuit.
In automatic working, the end-of-pulsing signal will be sent whenever the
outgoing international exchange is in a position to know, by digit analysis, that
the final digit has been sent. Digit analysis may consist of an examination of
the country code and counting the maximum (or fixed) number of digits of the
national number. In other cases, the end-of-pulsing signal is not sent and the
end-of-address information is determined by the receipt of one of the
address-complete signals from the incoming international exchange.
1.4 Continuity-check of the telephone circuits
Because the signalling in Signalling System No. 7 does not pass over the
speech path, facilities should be provided for making a continuity-check of the
speech path in the circumstances described below.
The application of the continuity-check depends on the type of the
transmission system used for the telephone circuit.
For transmission systems having some inherent fault indication features
giving an indication to the switching system in case of fault, a continuity check
is not required. This situation commonly occurs when fully digital circuits are
used. However, a per-call continuity check may be needed on fully digital
circuits when circuits or bundles of circuits in primary multiplex groups are
dropped and inserted en route between switches and alarm indications carried on
bits of the primary multiplex frame structure are lost in passing through an
intermediate transmission facility that does not relay them transparently.
Typically, per-call continuity checks may be needed when the transmission link
between switches contains a TDMA satellite system, a digital circuit
multiplication system or a digital access and crossconnection system, where fault
indications are lost.
When an initial address message is received with a request for a
continuity-check relating to a digital circuit having inherent fault indication,
one of the following actions is taken:
a) the continuity-check request is disregarded; or
b) a continuity-check loop is connected and the maintenance system is
alerted. In this case the call may fail since no continuity signal may
be received from the distant end.
Note - The reception of such a request could only be caused by an abnormal
condition such as administrative errors or the occurrence of signalling errors.
When the circuit type is unknown to a Signalling System No. 7 exchange, or
in an application where both analogue and digital circuits may be served, or when
no inherent fault indication is available, a continuity-check loop should always
be connected in the following cases:
i) when the exchange has the capability to process initial address
messages with continuity-check request and such messages are received;
ii) when continuity-check requests are received.
For analogue circuits with pilot supervision it is sufficient to perform
the continuity-check on a statistical basis or by test calls (see S 7.5)1). For
analogue circuits not using pilot supervision and for mixed circuits, i.e.
analogue and digital circuits, the continuity-check should be performed on a per
call basis. Within mixed connections, i.e. connections composed of circuits with
and without continuity-check on a per call basis, it shall be ensured that the
continuity signal be forwarded to the destination point although no
continuity-check may have been performed on one or more parts of the end-to-end
connection.
1) The application to the international circuits and the quantitative aspects (in
particular, the frequency of performing the continuity-check) are for further study.
PAGE1 Fascicle VI.8 - Rec. Q.724
The continuity-check is not intended to eliminate the need for routine
testing of the transmission path.
The continuity-check of the speech circuit will be done, link-by-link, on
a per call basis or by a statistical method prior to the commencement of
conversation. Procedures and requirements are specified in S 7.
The actions to be taken when pilot supervision is used are described in S
9.
1.5 Cross-office check
For digital exchanges the requirements mentioned in Recommendation Q.504
[2] shall be met. For other exchanges Administrations shall ensure the
reliability of a connection through a switching machine (cross-office check)
either on a per call basis or by a statistical method. With either method, the
probability of the connection being established with an unacceptable speech path
transmission quality should not exceed 10-5 as the long-term average.
1.6 Address-complete signals
An address-complete signal will not be sent until the continuity signal
has been received and the cross-office check made, if they are applicable.
If the succeeding network does not provide electrical
called-party's-line-condition signals, the last Signalling System No.7 exchange
shall originate and send an address-complete signal when the end of address
signalling has been determined and a possible GRQ/GSM cycle has been completed:
a) by receipt of an end-of-pulsing signal;
b) by receipt of the maximum number of digits used in the national
numbering plan;
c) by analysis of the national (significant) number to indicate that a
sufficient number of digits has been received to route the call to the
called party;
d) by receipt of an end-of-selection signal from the succeeding network
(e.g. number received signal in Signalling System No. 4); or
e) exceptionally, if the succeeding network uses overlap signalling and
number analysis is not possible, by observing that 4 to 6 seconds have
elapsed since the last digit was received, and that no fresh
information has been received; in such circumstances, transmission to
the national network of the last digit received must be prevented until
the end of the waiting period which causes an address-complete signal
to be sent over the international circuit. In this way, it is ensured
that no national answer signal can arrive before an address-complete
signal has been sent.
Specifically, in cases d) and e) above, the address-complete charge signal
should be sent.
Note - If the succeeding network provides electrical
called-party's-line-condition signals, the last Signalling System No. 7 exchange
shall originate and send address-complet signal when that condition has been
received from the succeeding network and a possible GRQ/GSM cycle has been
completed.
If in normal operation, delay in the receipt of an address-complete or
equivalent signal from the succeeding network is expected, the last common
channel signalling exchange will originate and send an address-complete signal 15
to 20 seconds after receiving the latest address message. This time-out condition
is an upper limit considering the clauses of S 6.4.1 (20 to 30 seconds for
outgoing international exchanges in abnormal release conditions).
On receipt of an address-complete signal, the first Signalling System No.
7 exchange will through-connect the speech path of the interconnected circuit2).
2) It is envisaged that in the future evolution of the Telephone User Part (e.g. in the
context of an integrated services digital network) the through-connection immediately
after sending of the initial address message may become a mandatory requirement.
Fascicle VI.8 - Rec. Q.724 PAGE1
After an address-complete signal, only the following signals relating to
the call set-up may be sent in the backward direction:
a) in normal operation, one of the answer or release-guard signals;
b) call-failure signal; or
c) the national network congestion signal; or
d) the circuit group congestion signal.
Note - Cases b), c) and d) can only occur after an address complete signal
without subscriber free.
Any further information about the called-party's-line-condition will be
transmitted to the calling subscriber or operator as audible tones or
announcements.
The address-complete signal with the subscriber-free indication is sent
when it is known that the called subscriber's line is free (not busy). It must be
originated in the called subscriber's exchange, and therefore cannot be followed
by one of the unsuccessful backward set-up information signals.
If an incoming international exchange has sent a general request message,
then an address complete message must not be sent until a general forward set-up
information message has been received in response to that general forward set-up
information message.
1.7 Address-incomplete signal
The determination that the proper number of digits has not been received
can be made at once if the end-of-pulsing signal is received or by receipt of an
address-incomplete signal (or equivalent) from the national network. When overlap
working is used and the end-of-pulsing signal has not been received, the
address-incomplete signal will be sent by the last common channel Signalling
exchange 15 to 20 seconds after receipt of the latest digit.
Each Signalling System No. 7 exchange on receipt of the address-incomplete
signal will send the signal to the preceding Signalling System No. 7 exchange, if
any, and clear forward the connection. The first Signalling System No. 7 exchange
will send a suitable signal on the preceding circuit if the related signalling
system permits to do so; otherwise the appropriate tone or announcement for the
national network concerned will be sent to the calling party.
1.8 Congestion signals
As soon as the congestion condition is detected one of the congestion
signals (see Recommendation Q.722, S 3.4) is sent without waiting for the
completion of a possible continuity-check sequence.
Reception of a congestion signal at any Signalling System No. 7 exchange
will cause the clear-forward signal to be sent and cause an appropriate signal to
be sent to the preceding exchange if the signalling system allows this or an
appropriate tone or announcement to be sent to the originating subscriber or
operator.
1.9 Called-party's-line-condition signals
The called-party's-line-condition signals (see Recommendation Q.722, S
3.4) will be sent when the appropriate electrical signals are received at the
incoming international exchange from the national network.
The called-party's-line-condition signals will be sent without waiting for
the completion of a possible continuity check. On receipt of one of these
signals, the first Signalling System No. 7 exchange (or the outgoing
international exchange) will clear forward the connection and cause an
appropriate signal to be sent to the preceding exchange if the signalling system
allows this or an appropriate tone or announcement to be sent to the originating
subscriber or operator.
Each Signalling System No. 7 exchange on receipt of one of these signals
has to clear forward the connection.
1.10 Answer signals
The signals answer, charge and answer, no charge are sent as received from
the national network or from the succeeding international link.
The signals answer, charge and answer, no charge are used only as a result
of the first off-hook signal from the called party.
1.11 Clear-back signal
A clear-back signal must not disconnect the speech path at a Signalling
System No. 7 exchange. The requirements for the release of a connection in the
event that a clear-forward signal is not received are given in Recommendation
Q.118 [3].
1.12 Reanswer and clear-back signal sequences
PAGE1 Fascicle VI.8 - Rec. Q.724
Subsequent off-hook, on-hook signals from the called party, such as will
result from switch-hook flashing, will cause the following sequence of signals to
be sent:
- clear-back,
- reanswer,
- clear-back,
- reanswer,
- etc.
It is necessary that a flashing sequence be retransmitted to the operator
(or the preceding link) and that the final condition of the circuit represents
the final position of the called party's switch hook.
1.13 Forward-transfer signal
The forward-transfer signal may be sent in semiautomatic working in either
of the following two cases:
a) following a call switched automatically to a subscriber, or following a
call established via a special operator, the controlling operator
wishes to call in an assistance operator. On receipt of the
forward-transfer signal at the incoming international exchange, an
assistance operator is called in;
b) following a call via code 11 and 12, the controlling operator wishes to
recall the incoming operator at the incoming international exchange.
Receipt of the forward-transfer signal at the incoming international
exchange recalls the incoming operator on calls completed via the
operator positions at the exchange.
1.14 Clear-forward and release-guard sequences
The clear-forward signal is overriding and all exchanges must be in a
position to respond by releasing the circuit and sending a release-guard signal
at any time during the progress of a call and even if the circuit is in the idle
condition. If sent while a circuit is blocked it will not result in unblocking
the circuit concerned (see S 5). The fact that the circuit is blocked will not
delay the transmission of the release-guard signal.
1.15 Reset of circuits and circuit groups
In systems which maintain circuit status in memory there may be occasions
when the memory becomes mutilated. In such a case the circuits must be reset to
the idle condition in both exchanges to make them available for new traffic.
Since the exchange with the mutilated memory does not know whether the circuits
are idle, busy outgoing, busy incoming, blocked, etc., reset-circuit signals or a
circuit group reset message should be sent as appropriate for the affected
circuits. The reset-circuit signal may also be sent, in certain cases, when a
signalling fault occurs (see SS 6.2 and 6.5).
1.15.1 Reset-circuit signal
If only a few circuits are concerned a reset-circuit signal should be sent
for each affected circuit.
On receipt of a reset-circuit signal the unaffected exchange will:
a) accept the signal as a clear-forward signal and respond by sending a
release-guard signal, after the circuit has been made idle, if it is
the incoming exchange on a connection in any state of call set-up or
during a call;
b) accept the signal as a clear-back or call-failure signal, whichever is
appropriate, and respond by sending a clear-forward signal immediately
if it is the outgoing exchange on a connection;
c) accept the signal as a clear-forward signal and respond by sending a
release-guard signal if the circuit is in the idle condition;
d) if it has previously sent a blocking signal, or if it is unable to
release the circuit as described above, respond by the blocking signal.
If an incoming or outgoing call is in progress, this call should be
disconnected and the circuit returned to the idle (blocked) state. A
clear-forward or release-guard signal may be sent. The blocking signal
should be acknowledged by the affected exchange. If the acknowledgement
is not received, the repetition procedure specified in S 6.4.4 should
be followed;
Fascicle VI.8 - Rec. Q.724 PAGE1
e) if it had previously received the blocking signal, respond by
disconnecting any connected call, remove the blocked condition and
restore the circuit to the idle state. If an outgoing call had been in
progress, respond with a clear-forward or, in all other cases, a
release-guard signal;
f) if a reset-circuit signal is received after the sending of an initial
address message but before receipt of a backward signal relating to
that call, clear the circuit and make an automatic repeat attempt on
another circuit if appropriate.
g) if a reset-circuit signal is received after having sent a reset-circuit
signal, respond by a release-guard signal. The circuit should be
restored to traffic;
h) send an appropriate clearing signal on an interconnected circuit (e.g.,
clear-forward, or a suitable backward signal).
The affected exchange will then reconstruct its memory according to the
received acknowledgement to the reset-circuit signal, and respond to the signals
received in the normal way, i.e. release-guard in response to a clear-forward,
blocking-acknowledgement in response to a blocking signal.
In addition, an interconnected circuit may be cleared by the use of an
appropriate signal. If no acknowledgement to the reset-circuit signal is received
before 4-15 seconds, the reset-circuit signal should be repeated. If an
acknowledgement for the signal is not received within 1 minute after the sending
of the initial reset-circuit signal, maintenance personnel should be notified to
permit manual restoration procedures. However, the sending of the reset-circuit
signal should continue at 1-minute intervals until maintenance intervention
occurs.
1.15.2 Circuit group reset message
If a considerable number of circuits or all circuits are affected by the
memory mutilation, circuit group reset messages should be used to make these
circuits available for new traffic.
Since the effect of erroneous circuit group reset messages generated by
undetected errors may seriously affect the quality of service, each circuit group
reset message has to be sent twice.
On receipt of two circuit group reset messages with 5 seconds for the same
group or parts thereof the unaffected exchange will:
i) If the range field is not coded all zero:
a) restore the circuits involved to the idle state;
b) send the appropriate group blocking message(s) if it had previously
sent a hardware failure oriented and/or software generated group
blocking message;
c) respond by a circuit group reset-acknowledgement message in which
the status indicator bits of the circuits available for service or
blocked for reasons of hardware failure or a software generated
alarm are coded 0 and the status indicator bit of all circuits
blocked for maintenance reasons are set to 1.
ii) If the range field is coded all zero (national option)
a) send the appropriate group blocking message(s) if it had previously
sent a hardware oriented and/or a software generated group blocking
message;
b) start the restoration of the circuits on a per circuit basis in the
same way as after receipt of a reset circuit for each circuit
within the group (see S 1.15.1);
c) respond by a circuit group reset-acknowledgement message indicating
that the restoration of the circuits concerned was started.
PAGE1 Fascicle VI.8 - Rec. Q.724
iii) Independent from the coding of the range field the following
actions should take place in the unaffected exchange after receipt of
two circuit group reset signals within 5 seconds:
a) if it had previously received (a) blocking signal(s) or (a)
blocking message(s) for one or more of the circuit(s) involved the
blocked condition will be removed and the circuits will be made
available for service;
b) if a circuit group reset message is received after having sent a
circuit group reset message or (a) reset circuit signal(s) the
circuits involved in both the sent and the received
message/signal(s) are made available for service;
c) appropriate signals should be sent on interconnected circuits to
release them.
The affected exchange will then reconstruct its memory according to the
possibly received blocking messages and the received circuit group
reset-acknowledgement message. It will respond to the possibly received group
blocking messages in the normal way.
If no acknowledgement to a circuit group reset message is received before
4-15 seconds the circuit group reset message should be repeated (twice). If
acknowledgement for the message is not received within 1 minute after sending the
initial circuit group reset message maintenance personnel should be notified to
permit manual restoration procedures. However, the sending of the circuit group
reset message should continue at 1 minute intervals until maintenance
intervention occurs.
1.16 Analysis of digit information for routing
(See Recommendation Q.107 bis.)
1.17 Diagrams showing signal sequence
Some examples of call set-up sequences are shown diagrammatically (Tables
1/Q.724 and 2/Q.724).
1.18 Use of the General Request Message and the General Forward Set-up
Information Message (GRQ/GSM)
The following procedures shall be applicable to exchanges generating or
receiving GRQ or GSM messages:
a) The GRQ/GSM protocol can only be initiated during call set-up.
b) A unique GSM must be sent in response to a GRQ and must only contain
answers to all requests contained in the GRQ.
c) At a transit exchange, once a GRQ has been sent, there is no
requirement to wait for the resultant GSM before setting up a
connection to a succeeding exchange, unless the information requested
is necessary for routing/analysis functions for that call.
d) An exchange having sent a GRQ should wait until the GSM is received
before sending an Address Complete Message (ACM). However, in a whole
Signalling System No. 7 international network there is no requirement
in the international transit exchange to delay sending the ACM, even if
the GRQ/GSM cycle is not completed (i.e. ignore GSM).
e) A subsequent GRQ must not be sent from the same exchange before a reply
(GSM) has been received in response to the previous GRQ. Consequently
any GRQ's received by an exchange subsequent to the first GRQ and prior
to replying with a GSM shall be ignored.
f) The GRQ-GSM interchange shall always take place on a link-by-link
basis. This means that an exchange receiving a GRQ for which it does
not hold the information, must initiate a separate GRQ/GSM cycle on the
preceding link.
g) Information received in the GSM, other than that specifically requested
in the associated GRQ, will be ignored.
h) An exchange shall store any information gained on a call by using the
GRQ/GSM interchange or receipt of an IAM/IAI, until the call is
completed successfully or failed.
i) If a call attempt fails (e.g., receipt of CGC, NCC, CFL, etc.) during
the period when an exchange is waiting for a GSM, then the appropriate
backward call failure shall be sent without waiting for the GSM.
Fascicle VI.8 - Rec. Q.724 PAGE1
j) Failure to receive a GSM in response to a GRQ will result in the
preceding exchange failing the call due to non-receipt of the ACM (T2
timer expires in 20-30 seconds).
TABLE 1/Q.724
Semiautomatic (SA) and automatic (A) terminal traffic
(error-free operation assumed)
Outgoing international exchange Incoming international exchange
Normal call to a free subscriber
(using continuity-check)
Address signals from the national
network are analysed
The outgoing circuit is seized.
The initial address message is Initial The address message is analysed to
sent: address determine:
-all address signals including ST message a) - the circuit to be sized
in "en bloc" operation, or - country code not included
-all available address signals in - nature-of-circuit (satellite or
overlap operation (a minute of 4 terrestrial)
digits) - echo suppressor control
- calling-party's-category
- continuity-check control.
The echo suppressor, if present,
is disabled so that the speech The echo suppressor, if present,
path continuity-check may be is disabled so that the speech
performed. path continuity-check may be
performed. The loop for the speech
The transceiver for the speech path continuity-check is attached
path continuity-check is attached, to the incoming circuit
and the check-tone is transmitted
on the outgoing circuit
Check-tone
a)
When the speech path continuity- Set-up the call in the national
check and cross-office check have network begins when enough address
been completed, and if the signals are received for routing
incoming circuit is used with (overlap operation).
common, channel signalling, when
the continuity signal has been
received from the national Check-tone
network, the continuity
PAGE1 Fascicle VI.8 - Rec. Q.724
signal is sent, and the
transceiver is removed.
(When the continuity-check fails, Continuity The check-loop is removed and the
the continuity-failure signal is echo suppressor, if present, is
sent forward. An automatic repeat Subsequent enabled. Address signals are
attempt is made.) The echo address passed into the national network.
suppressor, if present, is enabled messages The speech path is switched
as appropriate. The remaining through.
address signals are sent forward
in overlap operation.
On receipt of the address-complete Address
signal, registers (if any) are complete The address messages are analysed
released and the speech path to determined that all the
through-connected, the address required address signals have been
signals are erased. Subsequent receive (where applicable)b). Set-
supervisory signals are handled by up of the speech path is
the processor as appropriate completed. Subsequent supervisory
signals are handled by the
processor as appropriate.
The operator (SA), or the calling Audible
subscriber (A) hears ringing tone. ringing ton Ringing tone of the country of
e
destination is sent back.
On receipt of the answer signal, Answer
charging a) mea-surement of call Signals from the national network
duration and conversation begin. are passed to the outgoing
international exchange as follows
The called subscriber answers
(charge or not charge)
"Clear-back" is recognized Clear-back The called subscriber hangs up.
SA:A clearing supervisory signal SA and A: After 2-3 min., if
to given to the controlling
operator
Fascicle VI.8 - Rec. Q.724 PAGE1
there is no clear-forward
signal, the national part
of the connection is
released.
A:After 1-2 min., if there is no
clear-forward signal, the
international connection is
released and charging and
measurement of the call duration
are ceased.
The outgoing operator (SA) or the Clear- "Clear-forward" is recognized. The
calling subscriber (A) clears. forward connection is released, and "clear-
When the outgoing equip-ment is forward" is sent to the national
released, the clear-forward signal network of destination
is sent.
"Release-guard" is recognized, and Release- When the incoming equipment has
the outgoing circuit is made guard released, a release-guard signal
available for new traffic is sent back. The circuit is made
available for new traffic
a) Solid arrows denote common channel signals; dotted arrows are tones sent via the speech
path (check-tone and audible tones).
b) Address-complete signal may come from the national network.
c) Unless a no-charge answer or address-complete signal has been received.
TABLE 2/Q.724 (sheet 1 of 4)
Semiautomatic (SA) and automatic (A) transit traffic (error-free operation
assumed)
a) Solid arrows denote common channel signals; dotted arrows are tones sent
via the speech path (check-tone and audible tones).
PAGE1 Fascicle VI.8 - Rec. Q.724
TABLE 2/Q.724 (sheet 2 of 4)
Tables are in file named "924T2-E.DOC"
b) The address-complete signal may come from the national network.
c) Unless a no-charge answer or address-complete signal has been received.
Fascicle VI.8 - Rec. Q.724 PAGE1
TABLE 2/Q.724 (sheet 3 of 4)
PAGE1 Fascicle VI.8 - Rec. Q.724
TABLE 2/Q.724 (sheet 4 of 4)
Fascicle VI.8 - Rec. Q.724 PAGE1
2 Dual seizure with both-way operation
2.1 Dual seizure
Since Signalling System No. 7 circuits have the capability of both-way
operation, it is possible that the two exchanges will attempt to seize the same
circuit at approximately the same time.
2.2 Unguarded interval
Considering that with Signalling System No. 7:
a) signalling data link propagation time may be relatively long,
b) there may be significant delay due to retransmissions,
c) quasi-associated operation may add extra message transfer time(s) at
signalling transfer points,
the unguarded interval during which dual seizure can occur may be relatively long
in some instances. The exchange must therefore detect dual seizure and take
action as defined in S 2.5.
2.3 Detection of dual seizure
A dual seizure is detected by an exchange from the fact that it receives
an initial address message for a circuit for which it has sent an initial address
message (see also S 7.5.1).
2.4 Preventive action
Different methods for circuit selection can be envisaged to minimize the
occurrence of dual seizure. In the following, two methods are described. Further
study is required to determine the field of application of each method and to
ensure that the two methods do interwork satisfactorily.
Other methods for circuit selection may also be used provided that they
give the same degree of protection against dual seizure also when one of the
methods specified is used at the other end.
Method 1
An opposite order of selection is used at each terminal exchange of a
both-way circuit group.
Method 2
Each terminal exchange of a both-way circuit group has priority access to
the group of circuits which it is controlling (see S 2.5). Of this group the
circuit which has been released the longest is selected (first-in - first-out).
In addition each terminal exchange of a both-way circuit group has nonpriority
access to the group of circuits which it is noncontrolling. Of this group the
latest released circuit is selected (last-in - first-out).
For call control purposes a both-way circuit group can be subdivided into
subgroups in an exchange.
It is necessary to take preventive action in cases where Signalling System
No. 7 uses a signalling data link with long propagation time.
2.5 Action to be taken on detection of dual seizure
Each exchange will control one half of the circuits in a both-way circuit
group. On detection of a dual seizure, the call being processed by the control
exchange for that circuit will be completed and the received initial address
message will be disregarded.
Under these conditions, the call being processed by the control exchange
will be allowed to complete although, when continuity-check has to be performed,
the continuity of the circuit may have been checked in the direction from
noncontrol to control only. The call being processed by the noncontrol exchange
will be backed off, switches released, the continuity-check transceiver removed,
and the check-loop connected unless or until a continuity signal has been
received from the control exchange. A clear-forward signal will not be sent. The
noncontrol exchange will make an automatic repeat attempt on the same or on an
alternative route.
For the purpose of resolution of dual seizure on both-way circuits, the
exchange with the higher signalling point code will control all even-numbered
circuits (circuit identification code) and the other exchange the odd-numbered
circuits. The designation of control may also be used for maintenance control
purposes.
3 Automatic repeat attempt
Automatic repeat attempt, as defined in Recommendation Q.12 [4], is
provided in Signalling System No. 7.
An automatic repeat attempt will be made:
- upon failure of the continuity-check (see S 7.3);
- on detection of dual seizure (at the noncontrol exchange) (see S 2.5);
PAGE1 Fascicle VI.8 - Rec. Q.724
- on receipt of the blocking signal after sending an initial address
message and before any backward signal has been received (see S 6);
- on receipt of a reset-circuit signal after sending an initial address
message and before a backward signal has been received;
- on receipt of unreasonable signalling information after sending an
initial address message and before one of the backward signals required
for call set-up has been received.
4 Speed of switching and signal transfer in international
exchanges
4.1 Outgoing international exchange
At the outgoing international exchange:
- if overlap operation is used, the sending of the initial address
message shall take place as soon as sufficient digits are received and
analyzed to permit the selection of an outgoing circuit;
- if "en bloc" operation is used, the initial address message should be
sent as soon as all the digits of the address including the
end-of-pulsing signal are available and the outgoing circuit has been
chosen.
4.2 International transit exchange
At the international transit exchange, the selection of an outgoing
circuit should begin as soon as the digits necessary to determine the routing
have been received and analyzed.
4.3 Incoming international exchange
At the incoming international exchange:
- if overlap operation is used in the national network, the setting-up of
the national part of the connection should start as soon as a
sufficient number of digits has been received for routing;
- if "en bloc" operation is used in the national network, the setting-up
of the national part of the connection should start as soon as all the
digits of the address including the end-of-pulsing signal have been
received.
5 Blocking and unblocking of circuits and circuit groups
The circuit blocking (unblocking) signal and the group blocking
(unblocking) message are provided to permit the switching equipment or
maintenance personnel to remove from (and return to) traffic, the distant
terminal(s) at a circuit or circuit group because of fault or to permit testing.
Specific conditions for automatic sending of blocking and unblocking signals and
messages by the switching equipment in case of use of the interruption control on
interexchange circuits appear in S 9.
Since circuits served by Signalling System No. 7 have both-way capability,
the blocking signal or a group blocking message can be originated by either
exchange. The receipt of the blocking signal or a group blocking message will
have the effect of prohibiting calls on the relevant circuit(s) outgoing from
that exchange until an unblocking signal or the appropriate group unblocking
message is received, but will not in itself prohibit calls incoming to that
exchange. Acknowledgement sequences are always required for the blocking and
unblocking signals as well as for the group blocking and group unblocking
messages, using the blocking-acknowledgement signal, the
unblocking-acknowledgement signal, the appropriate group blocking-acknowledgement
message and the appropriate group unblocking-acknowledgement message,
respectively. The acknowledgement is not sent until the appropriate action,
either blocking or unblocking, has been taken. The clear forward signal should
not override a blocking condition and return circuits to service which might be
faulty. (A) blocked circuit(s) will be returned to service on transmission of the
unblocking-acknowledgement signal or the appropriate group
unblocking-acknowledgement message at one exchange and on receipt of the
unblocking-acknowledgement signal or the appropriate group
unblocking-acknowledgement message at the other exchange.
A circuit that has been maintenance blocked by a blocking signal can be
unblocked by either an unblocking signal or a maintenance oriented group
unblocking message. A circuit that has been maintenance blocked by a maintenance
Fascicle VI.8 - Rec. Q.724 PAGE1
oriented group blocking message can be unblocked by either an unblocking signal
or a maintenance oriented group unblocking message.
5.1 Other actions on receipt of a blocking signal
In the event of the receipt of a blocking signal:
- after an initial address message has been sent, and
- before a backward signal relating to that call has been received,
an automatic repeat attempt will be made on another circuit. The exchange
receiving the blocking signal should clear forward the original attempt in the
normal manner after sending the blocking-acknowledgement signal.
If the blocking signal for a circuit is received:
- in the outgoing exchange after at least one backward signal relating to
a call has been received, or
- in the incoming exchange after at least one backward signal relating to
a call has been sent,
the exchange will not seize that circuit for subsequent calls.
The fact that the circuit is engaged on a call will not delay transmission
of the blocking (unblocking)-acknowledgement signal.
If a blocking signal is sent and subsequently an initial address message
is received in the opposite direction, the following action is taken:
- for test calls, the call should be accepted, if possible. In the case
where the test call cannot be accepted, the blocking signal must be
returned;
- for calls other than test calls, the blocking signal must be returned.
Blocking of a circuit that has not been withdrawn from service by use of
the blocking signal should not exceed five minutes, after which an alarm should
be given at each terminal of the circuit. Should a call be in progress on the
circuit involved, the five minutes time will commence when that call is cleared.
If the work on the circuit must exceed five minutes, the circuit should be
withdrawn from service.
5.2 Group blocking and unblocking messages
The following group blocking (unblocking) messages and the appropriate
acknowledgement messages are provided:
- maintenance oriented group blocking (unblocking) message;
- hardware failure oriented group blocking (unblocking) message;
- software generated group blocking (unblocking) message (national
option).
The range of circuits to be blocked (unblocked) is dependent on the coding
of the range field:
- if the range field is not coded all zero, the circuits indicated in the
status field have to be blocked (unblocked);
- if the range field is coded all zero all circuits of the predetermined
circuit group have to be blocked (unblocked).
The same rule applies to the acknowledgements.
Since the effect of erroneous group blocking (unblocking) messages
generated by undetected errors may seriously affect the quality of service, each
group blocking (unblocking) message has to be sent twice. Therefore, at the
receiving exchange actions only take place after a blocking (unblocking) message
was received twice within 5 seconds.
For the circuits blocked for maintenance reasons the same conditions apply
and the same actions have to be taken as described in S 5.1.
For the circuits blocked for reasons of hardware failure or software
generated alarm, the following actions will be taken:
- the maintenance personnel will be alerted;
- all interconnected circuits will be released by the appropriate
signals;
- the affected circuits will be set to the condition idle/hardware or
software blocked without any exchange of clearing signals.
6 Release of international connections and associated equipment
6.1 Normal release conditions
Connections are normally released in the forward direction as a result of
the receipt of a clear-forward signal from the preceding exchange.
In addition, the normal release of connections (or circuits) occurs as
follows:
- on continuity-check failure (see S 7.3);
- on receipt of an address-incomplete signal (see S 1.7);
PAGE1 Fascicle VI.8 - Rec. Q.724
- on receipt of one of the congestion signals (see S 1.8);
- on receipt of one of the called-party's-line-condition signals (see S
1.9);
- on receipt of the blocking signal or the maintenance oriented group
blocking message after sending an initial address message and before a
backward signal relating to that call has been received (see S 5);
- on receipt of unreasonable signalling information after sending an
initial address message and before one of the backward signals required
for call set-up has been received (see S 6.5).
If the conditions for the normal release of connections as described above
are not fulfilled, release is provided as follows:
- in the release under abnormal conditions (see S 6.4);
- on receipt of a call-failure signal (see S 6.3);
- on failure to receive a clear-forward signal after sending a clear-back
signal (see S 6.4);
- on failure to receive an answer signal (see S 6.4);
- on receipt of a reset-circuit signal or circuit group reset message
(see S 1.15).
Address and routing information are released from memory in each of the
exchanges of a connection as described in the following subsections.
6.1.1 Outgoing international exchange
Address and routing information stored at the outgoing international
exchanges can be erased on receipt of one of the following backward signals:
a) one of the address-complete signals,
b) the address-incomplete signal,
c) one of the congestion signals,
d) one of the called-party's-line-condition signals,
e) the call-failure signal,
or when the connection is cleared earlier and no automatic repeat attempt has to
be made.
6.1.2 Incoming international exchange
Address and routing information stored at the incoming international
exchange can be erased on receipt of one of the backward signals indicated in S
6.1.1 (or equivalent) from a national signalling system, or when one of the
following signals has been originated and sent to the outgoing international
exchange:
a) one of the address-complete signals,
b) the address-incomplete signal,
c) one of the congestion signals,
d) the call-failure signal,
e) the reset-circuit signal, or circuit group reset message,
or on receipt of a clear-forward signal.
6.1.3 International transit exchange
Address and routing information stored at an international transit
exchange can be erased on receipt of one of the backward signals indicated in S
6.1.1, on receipt of a clear-forward signal, or when one of the congestion
signals is originated in that exchange.
6.2 Abnormal release conditions - Clear-forward, release-guard sequences
6.2.1 Inability to release in response to a clear-forward signal
If an exchange is unable to return the circuit to the idle condition in
response to a clear-forward signal, it should remove the circuit from service and
send the blocking signal. Upon receipt of the blocking-acknowledgement signal,
the release-guard signal is sent in acknowledgement of the original clear-forward
signal.
6.2.2 Inability to release in response to a backward signal
If an exchange is unable to release a circuit in response to an
address-incomplete, congestion, called-party's-line-condition or call-failure
signal, it should remove the circuit from service by sending the blocking signal.
Upon receipt of the blocking-acknowledgement signal, the clear-forward signal
should be sent in reply to the original backward signal.
6.2.3 Failure to receive a release-guard signal in response to a clear-forward
Fascicle VI.8 - Rec. Q.724 PAGE1
signal
If a release-guard signal is not received in response to a clear-forward
signal before 4-15 seconds, the clear-forward signal will be repeated.
If, after sending a clear-forward signal, a release-guard signal is not
received within a period of one minute after the first clear-forward signal, the
maintenance personnel shall be alerted. The repetition of the clear-forward
signal is ceased, and circuit reset is initiated.
6.3 Call-failure signal
The call-failure signal is sent as the result of time-out situations,
described in S 6.4 and whenever a call attempt fails and other specific signals
do not apply, viz:
- the address-incomplete signal,
- the congestion signals, or
- the called-party's-line-condition signals.
Reception of the call-failure signal at any Signalling System No. 7
exchange will cause the clear-forward signal to be sent and, if the signalling
system permits to do so, the appropriate signal to be sent to the preceding
exchange or the appropriate tone or announcement to be sent to the national
network.
Failure to receive a clear-forward signal within 4-15 seconds of sending a
call-failure signal causes the latter to be repeated. If no clear-forward signal
is received within 1 minute of sending the call-failure signal, repetition of the
call-failure signal is ceased, maintenance personnel is alerted and circuit reset
initiated.
6.4 Abnormal release condition - other sequences
If the conditions for normal release as covered in S 6.1 are not
fulfilled, release will take place under the following conditions:
6.4.1 Outgoing international exchange
An outgoing international exchange shall:
a) release all equipment and clear forward the connection on failure to
meet the conditions for normal release of address and routing
information as covered in S 6.1.1 before 20-30 seconds after sending
the latest address message;
b) release all equipment and clear forward the connection on failure to
receive an answer signal within the interval specified in
Recommendation Q.118 [3];
c) release all equipment and clear forward the connection on failure to
receive a clear-forward signal from the national network after having
received a clear-back signal within the interval specified in
Recommendation Q.118 [3].
6.4.2 Incoming international exchange
An incoming international exchange shall:
a) release all equipment, clear forward the connection into the national
network and send back a call-failure signal in the following cases:
- on failure to receive a continuity or continuity-failure signal if
applicable (see Recommendation Q.723, S 3.3.1) before 10-15 seconds
after receipt of the initial address message; or
- on failure to receive one of the backward signals indicated in S
6.1.1 (or equivalent) from a national network (where expected)
before 20-30 seconds after receipt of the latest address message,
unless the timing for sending the address-incomplete signal (see S
1.7) is provided; or
- on receipt of an address-incomplete signal after an
address-complete signal has been generated;
b) send the call-failure signal on failure to receive a clear-forward
signal for the incoming circuit before 4-15 seconds after sending an
address-incomplete, congestion, call-failure or a
called-party's-line-condition signal indicating inability to complete
the call.
If a clear-forward signal is not received within a period of one minute
after sending the call-failure signal, the repetition of the
call-failure signal should be ceased, maintenance personnel should be
alerted, and a reset-circuit signal should be sent for the concerned
circuit.
c) release all equipment and clear forward the connection into the
PAGE1 Fascicle VI.8 - Rec. Q.724
national network on failure to receive a clear-forward signal after
sending a clear-back signal within the interval specified in
Recommendation Q.118 [3].
6.4.3 International transit exchange
An international transit exchange shall:
a) release all equipment, clear forward the connection and send back the
call-failure signal in the following cases:
- on failure to receive a continuity or continuity-failure signal if
applicable (see Recommendation Q.723, S 3.3.1) before 10-15 seconds
after receipt of the initial address message; or
- on failure to meet the conditions for normal release as covered in
S 6.1.3 before 20-30 seconds after sending the latest address
message; or
b) send the call-failure signal on failure to receive a clear-forward
signal for the incoming circuit before 4-15 seconds after sending an
address-incomplete, congestion, call-failure or a
called-party's-line-condition signal indicating inability to complete
the call.
If a clear-forward signal is not received within a period of one minute
after sending the call-failure signal, the repetition of the
call-failure signal should be ceased, maintenance personnel should be
alerted, and a reset-circuit signal should be sent for the concerned
circuit.
6.4.4 Failure in the blocking/unblocking sequence
An exchange will repeat the blocking (unblocking) signal or the group
blocking (unblocking) messages on failure to receive the appropriate
acknowledgement in response to one of these signals/messages before 4-15 seconds
(see S 5).
If an acknowledgement is not received within a period of one minute after
sending the initial blocking (unblocking) signal or group blocking (unblocking)
messages, maintenance personnel should be alerted, the repetition of the blocking
(unblocking) signal or group blocking (unblocking) messages should be continued
at one minute intervals.
6.5 Receipt of unreasonable signalling information
The Message Transfer Part of the signalling system will avoid
mis-sequencing, or double delivery, of messages with a high reliability
(Recommendation Q. 706, S 2). However, undetected errors at the signalling link
level and exchange malfunctions may produce signalling information in messages
that is either ambiguous or inappropriate.
In order to resolve some possible ambiguities in the state of a circuit
when unreasonable signals are received the following will apply:
a) if a clear-forward signal is received relating to an idle circuit it
will be acknowledged with a release-guard signal;
b) if a release-guard signal is received relating to a circuit for which a
clear-forward signal has not been sent, the following actions will be
undertaken:
- if the circuit is idle, the release-guard signal is discarded;
- if the circuit is seized by a call, the release-guard signal is
considered as an ordinary unreasonable information (see item g));
c) if a blocking signal is received for a blocked circuit, a
blocking-acknowledgement signal will be sent;
d) if an unblocking signal is received for an unblocked circuit, an
unblocking-acknowledgement signal will be sent;
e) if a blocking-acknowledgement signal for which no blocking signal has
been sent is received:
- relating to a circuit blocked by sending a blocking signal, the
blocking-acknowledgement signal will be discarded,
- relating to a circuit which is not blocked by sending a blocking
signal, an unblocking signal will be sent;
f) if an unblocking-acknowledgement signal for which no unblocking signal
has been sent, is received:
Fascicle VI.8 - Rec. Q.724 PAGE1
- relating to a circuit blocked by sending a blocking signal, the
blocking signal will be sent,
- relating to a circuit which is not blocked by sending a blocking
signal, the unblocking-acknowledgement signal will be discarded;
g) if other unreasonable signalling information is received, the following
actions will be undertaken:
- if the circuit is idle, the reset-circuit signal is sent;
- if the circuit is seized by a call, after receipt of a backward
signal required for the call set-up, the unreasonable signalling
information is discarded;
- if the circuit is seized by a call, before receipt of a backward
signal required for the call set-up, the reset-circuit signal is
sent. If the circuit is seized by an incoming call, the call will
be released. If the circuit is seized by an outgoing call, an
automatic repeat attempt is provided on another circuit.
7 Continuity-check for 4-wire speech circuits
7.1 General
This specification relates only to that part of a 4-wire connection served
by Signalling System No. 7. The part of the speech path to be checked may include
a circuit with speech interpolation. As the presence of active echo suppressors
in the circuit would interfere with the continuity-check, it is necessary to
disable the suppressors during the check and to re-enable them, if required,
after the check has been completed.
The transceiver (check-tone transmitter and receiver) is connected to the
go and return paths of the outgoing circuit at the first and each succeeding
exchange, excluding the last exchange, in that part of the connection served by
Signalling System No. 7. The check-loop should be connected to the go and return
paths of the incoming circuit at each exchange except the first in that part of
the connection served by Signalling System No. 7. A continuity-check is
considered successful when a tone is sent on the go path and is received on the
return path within acceptable transmission and timing limits.
PAGE1 Fascicle VI.8 - Rec. Q.724
Fascicle VI.8 - Rec. Q.724 PAGE1