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7.2 Transmission requirements
7.2.1 Transmitting equipment
The check-tone frequency will be 2000 ▒ 20 Hz. For international
application the sending level of the check-tone will be -12 ▒ 1 dBm0.
7.2.2 Check-loop
The check-loop will have a loss of 0 dB, taking into account any
difference between the relative levels of the two paths at the point of
attachment.
7.2.3 Receiving equipment
The check-tone receiver will have the following characteristics:
a) Operating requirements
Check-tone frequency: 2000 ▒ 30 Hz
Check-tone level range for international application:
The absolute power level N of the check-tone shall be within the limits
(-18 + n) N (-6 + n) dBm where n is the relative power level at the
receiver input.
Recognition time: 30-60 ms
The frequency and level range tolerances allow for variations at the
sending end and for variations in line transmission that are considered
acceptable.
b) Non-operating requirements
Signal frequency: outside the frequency band 2000 ▒ 200 Hz
Signal level for international application: below or equal to -22 + n
dBm.
The limit is 10 dB below the nominal absolute level of the check-tone
at the input of the receiver. If the level falls below this point,
transmission is considered unacceptable.
Signal duration: shorter than 30 ms
The level range of (-18 + n) N (-6 + n) dBm will serve as a Go/No-go
check on the links in that part of the international connection served
by Signalling System No. 7.
c) Release requirements
If the receiver is used to test for the removal of check-tone (see S
7.3):
- after recognition of tone, interruptions of up to 15 ms shall be
ignored; this will prevent switching through the speech path
prematurely;
- the indication of tone removal should not be delayed more than 40
ms; and
- the release level of the receiver should be lower than -27 + n dBm
for international application.
7.3 Continuity-check procedure
Decision on whether continuity-check should be performed or not on a given
circuit should be made by an outgoing exchange according to the criteria
described in S 1.4. The outgoing exchange will indicate whether continuity-check
is required or not by the continuity-check indicator in the initial address
message (Recommendation Q.723, S 3.3.1) or by a continuity check request in a
continuity-check-test call (see Rec. Q.723 S 9 and Rec. Q.724, S 7.5). If it is
required, the outgoing exchange will connect a transceiver to the speech circuit
when it sends an initial address message. If continuity-check is not required
either on the incoming circuit or on the outgoing circuit, the outgoing exchange
can switch-through the speech path immediately after having sent the initial
address message.
A description of the procedure using the specification and description
language is given in the state transition diagrams in Figures 4/Q.724 and
5/Q.724. The Signalling System No. 7 exchange will send forward the continuity
signal after completion of all the following actions:
- the continuity-check performed on the outgoing circuit is completed;
- the speech path across the exchange has been checked and found correct
(see S 1.4); and
- if the continuity-check indicator in the received initial address
message indicates that continuity-check is being (has been) performed
on previous circuit(s), receipt of a continuity signal from the
preceding exchange.
The speech path may be switched through at an international transit or
PAGE20 Fascicle VI.8 - Rec. Q.724
incoming exchange and the transceiver disconnected after the continuity-check of
the circuit has been successfully completed. However, the switching through of
the speech path should be delayed until the residual check-tone has propagated
through the return path of the speech circuit.
This determination may be made by timing, or by using the check-tone
receiver to test for the removal of the check-tone, or other appropriate means.
As a national option the following single report procedure may be used to
assure that on terrestrial circuits a complete check has been made of both
directions of transmission in the face of high noise and in the double seizing
situations. With this procedure, the continuity check is not considered
successful until the check tone is recognized and its subsequent removal
recognized within the continuity check timing interval. On tone recognition it
must be ensured that at least 60 ms of continuity check tone has been sent. In
the double seizing case, this procedure will ensure that both ends will recognize
the check tone if both directions of transmission are within acceptable
transmission limits. The end originating the continuity check and, in the case of
double seizing, the control end send the continuity signal on successful
completion of the check. The exchange at the other end of circuit removes the
loop (or transceiver in the case of double seizing) on receipt of the continuity
signal. If this exchange is the last common channel signalling exchange, the
address-complete signal is not returned until either the loop (or transceiver or
in the double seizing case) is disconnected.
With the single report continuity check procedure, the first exchange that
has initiated the continuity check must delay through-connect until receipt of an
address complete signal to avoid the potential hazards associated with delayed
loop removal.
On receipt of the continuity signal in the following international
exchange, the continuity-check loop will be removed if inserted. Also, any digits
of the national number which were withheld may be released (see S 1.2).
If in an interworking situation a continuity check has to be performed on
one or more of the circuits involved in the connection preceding the interworking
point, appropriate measures must be taken to prevent alerting of the called party
until the continuity of such circuits has been verified. Interworking situations
which could be discriminated are:
a) Signalling System No. 7 -> any non No. 7 Signalling System.
b) International Signalling System No. 7 -> national Signalling System No.
7 not performing continuity check.
For a) the last digit(s) of the national number have to be withheld in any
(interworking) transit exchange or terminating exchange in case of DDI (direct
dialling in) or the alerting of the called party is postponed in the terminating
exchange in case of non-DDI.
For b) either the last digit(s) of the national number are withheld in the
incoming international transit exchange, a transit exchange in the national
network or the terminating exchange in case of DDI or the alerting of the called
party is postponed in the terminating exchange in case of non-DDI.
At the Signalling System No. 7 exchange, on failure of the outgoing
circuit to satisfy the continuity-check:
- the continuity-check transceiver will be removed and an automatic
repeat attempt will be made on another circuit,
- a continuity-failure signal will be sent to the following exchange.
A repeat of the continuity-check of the speech path will be made on the
failed outgoing circuit within 1-10 seconds of detection of the continuity-check
failure, in case of the initiation of the procedure has been made by an initial
address message.
The second continuity-check will be initiated by the Signalling System No.
7 exchange detecting the failure using the continuity-check-request signal.
If the repeated check passes on this call, the speech circuit will be
returned to idle with a clear-forward/release-guard sequence. If the second check
fails, the maintenance staff will be alerted that a failure has occurred and the
check will be repeated at intervals of 1-3 minutes. The repeated continuity-check
will only be finished when continuity is detected.
Fascicle VI.8 - Rec. Q.724 PAGE1
According to transmission maintenance requirements, Signalling System No.
7 may provide for:
a) a print-out each time a second continuity-check is started. In such
cases, the circuit involved should be identified;
b) a print-out each time a continuity-check results in a warning being
given to maintenance personnel.
Since a continuity-check failure can be caused by a faulty transceiver,
precautions should be taken to ensure a low probability of selecting a faulty one
for both the initial continuity-check and the second check, e.g. by ensuring the
selection of a different transceiver for each of the checks.
7.4 Continuity-check timing
7.4.1 Time-out period
The continuity-check is considered to have failed if the receiver has not
responded within a period determined by the Administration concerned. This period
should not exceed two seconds.
The time-out period of the continuity-check should always exceed the
continuity recognition time, TCR, given by:
TCR = 2TP + TIAM + TTC + TL + TR - TT
where
TP One-way propagation time of the speech circuit and the signalling
link (where these times are the same),
TTC Speech interpolation clip time for two speech interpolation systems
in series (for connections not using speech interpolation TTC = 0),
TR Receiver response time,
TL Loop connecting time (maximum),
TT Transceiver connecting time (minimum),
TIAM Emission time of the longest initial address message.
If retransmission of an initial address message is to be included in TCR,
the following formula may be used:
TCR = 4TP + 2TIAM + TFISU + 2TX + TL + TR - TT
where
TFISU Emission time of a fill-in signal unit (length of a fill-in signal
unit),
TX Time between receiving an initial address message and emitting a
signal unit containing an acknowledgement for that initial address
message, or
time between receiving a signal unit asking for retransmission and
emitting the initial address message to be retransmitted.
7.4.2 Switching of continuity-check equipment
The connection and disconnection of the equipment used for the
continuity-check and also the disabling and subsequent enabling of echo
suppressors should be related to the following stages of progress in the
establishment of the connection:
a) Preparation at Signalling System No. 7 exchange applying the
transceiver - Action should be initiated when the initial address
message is available for transmission in the Message Transfer Part.
b) Preparation at Signalling System No. 7 exchange connecting the
check-loop - Action should be initiated at the moment of recognition of
the initial address message received.
c) Disconnection at Signalling System No. 7 exchange connecting the
check-loop - Action follows the receipt of the continuity signal, the
continuity-failure signal or the clear-forward signal, or the emission
of signals indicating that the call cannot be established, e.g.
circuit-group-congestion signal.
d) Disconnection at Signalling System No. 7 exchange applying the
transceiver - Action should be initiated on the successful completion
or the failure of the continuity-check.
Exceptionally, if disconnection has not previously occurred, action should
be initiated at the moment of recognition of the address-complete signals, the
answer signals, signals indicating that the call cannot be established, or on the
emission of a clear-forward signal.
It is recommended that the mean time, both for the connection and for the
disconnection, is less than 100 ms. A mean time of 200 ms should not be exceeded.
7.5 Continuity-check test calls
7.5.1 The following procedure may be used in the cases when continuity-check is
PAGE20 Fascicle VI.8 - Rec. Q.724
performed by test calls. This procedure is used to test a single interexchange
circuit, which must be idle when the procedure is initiated.
7.5.2 When the outgoing Signalling System No. 7 exchange intends to initiate the
procedure, it sends to the following exchange a continuity-check-request message
and it connects the transceiver to the outgoing speech circuit. On receipt of the
continuity-check-request message, the following exchange connects the loop to the
involved circuit. On detection of the backward tone within the time-out specified
in S 7.4.1, the outgoing exchange will disconnect the transceiver and the circuit
will be returned to idle with a clear-forward/release-guard sequence.
7.5.3 In the case that no backward tone is detected within the specified
time-out, the same actions apply as in the case of continuity-check failure
during normal call set-up, see S 7.3 (the clause referring to the repeat attempt
is not relevant in this case).
7.5.4 If an exchange receives an initial address message relating to a circuit
for which it has sent a continuity-check-request message (i.e. in case of
collision on a both-way operated circuit), it will abort the continuity-check
test call, disconnect the transceiver and complete the incoming call.
An exchange receiving a continuity-check-request message after having sent
an initial address message, will ignore it and continue the call set-up
procedure.
8 Continuity-check for 2-wire speech circuits
In general the same procedure as described in S 7 is used for the
continuity-check of 2-wire speech circuits except the check-loop which has to be
replaced by a transponder and the fact that in the backward direction the
frequency 1780 ▒ 20 Hz is used.
9 Interruption control for multiplex systems
9.1 Digital circuits
When fully digital circuits are applied between two exchanges, which have
some inherent fault indication features giving an indication to the switching
system in case of fault (cf. S 1.4), the switching system should inhibit new
local seizures of the concerned circuits when the fault indication arises and for
as long as it persists.
9.2 FDM circuits
9.2.1 General
Interruption of the pilot in frequency-division multiplex systems
corresponds to loss of continuity of speech circuits or a considerable reduction
of level. Therefore a switching equipment monitoring this indication (see S 1.4)
should inhibit local seizure of the concerned speech circuits in case of
interruption. Moreover, seizure by the remote exchange should be prevented, as
long as the interruption persists, by sending blocking and unblocking signals as
specified in S 9.2.2.
When interruption control is implemented, possible use of the
specifications contained in Recommendation Q. 416 [5] could be applied.
Fascicle VI.8 - Rec. Q.724 PAGE1
9.2.2 Blocking and unblocking of speech circuits
Blocking signals are sent to the other end, with regard to the relevant
speech circuits, whenever an interruption is detected which lasts more than 4-15
seconds.
When an interruption indicated terminates, unblocking signals are sent to
the other end after 4-15 seconds, provided that blocking signals were previously
sent on occurrence of the interruption.
10 Supplementary services
10.1 General
The supplementary services general descriptions in an ISDN environment are
covered by other Recommendations, e.g.: Recommendations Q.80 to Q.83 and Q.85 to
Q.87.
In principle, many of these descriptions might be applied also in
telephone dedicated digital/analogue networks.
This Recommendation includes variants of supplementary services procedures
and/or descriptions. It contains its own supplementary services descriptions for
the services presented in this chapter.
In this part the signalling procedures related to a number of
supplementary services are also described. The messages and signals are defined
in Recommendation Q.722 and the format and the content are given in
Recommendation Q.723.
10.2 Closed User Group
10.2.1 General
The closed user group (CUG) facilities enable users to form groups with
different combinations of restrictions for access from or to the users having one
or more of these facilities. The following CUG facilities are standardized:
a) closed user group - this is the basic facility that enables a user to
belong to one or more CUGs;
b) closed user group with outgoing access - this is an extension to a)
which also enables the user to make outgoing calls to the open part of
the network, and to users having the incoming access capability see c)
below;
c) closed user group with incoming access - this is a variant of a) which
also enables the user to receive incoming calls from the open part of
the networks, and from users having the outgoing access capability see
b) above;
d) incoming calls barred within the closed user group - this is a
supplementary facility to a), b) or c) which, when used, applies per
user per CUG;
e) outgoing calls barred within the closed user group - this is a
supplementary facility to a), b) or c) which, when used, applies per
user per CUG.
A user may belong to one or more CUGs. In the case where a user belongs to
more than one CUG, one of these is nominated as the preferential CUG of that
user. Each user belonging to at least one CUG has either the closed user group
facility or one or both of the closed user group with outgoing access and the
closed user group with incoming access facilities. For each CUG to which a user
belongs, either or none of the incoming calls barred within the closed user group
or outgoing calls barred within the closed user group facilities may apply for
that user. Different combinations of CUG facilities may apply for different users
belonging to the same CUG.
The realization of the CUG facilities is done by the provision of
interlock codes and is based on various validation checks at call set-up,
determining whether or not a requested call to or from a user having a CUG
facility is allowed. In particular, a validation check is performed by
verification that both the calling and called parties belong to the same CUG as
indicated by interlock codes.
The data for each CUG that a user belongs to can either be stored,
associated to the user at the local exchange to which the user is connected
(decentralized administration of CUG data) or in dedicated point(s) in the
network. (Centralized administration of CUG data.)
The validation checks at call set-up when using decentralized
administration of the CUG data are performed in the originating and destination
exchange. When using centralized administration of CUG data most of the
validation checks are made in the dedicated point(s), and a minimum of the CUG
PAGE20 Fascicle VI.8 - Rec. Q.724
data is stored in the local exchanges.
In S 10.2.2 the call set-up procedures based on decentralized
administration of CUG data is specified.
The centralized administration of CUG data is not specified in this
Recommendation as it requires non-circuit related protocols.
10.2.2 Call set-up procedure with decentralized administration of CUG data
10.2.2.1 Originating exchange
The actions at the originating exchange at call set-up from a user
belonging to a CUG depends on whether the user belongs to one or more CUGs and on
the combination of CUG facilities that applies.
a) CUG selection
For each CUG that a user belongs to, the interlock code assigned to the
CUG is stored, associated to the user at the local exchange. In the
case where a user belongs to more than one CUG, a selection of the CUG
concerned, and thus of the corresponding interlock code, is required at
call set-up. This selection is based on the following criteria:
In the case where the calling party makes a facility request including
an index identifying a particular CUG, this CUG is selected by the
originating exchange.
In the case where the calling party makes no facility request
identifying a particular CUG, the originating exchange selects the
preferential (or only) CUG.
Thus in the case where the calling party belongs to a CUG, no facility
request concerning CUG facilities is made if:
i) the user belongs to one CUG only;
ii) a user who belongs to more than one CUG (with or without outgoing
access) makes a call within the preferential CUG;
iii) a user having the closed user group with outgoing access
facility makes an outgoing access call.
A facility request is always required for a call within any CUG other
than the preferential CUG.
b) Call set-up from a user having the closed user group or the closed user
group with incoming access facility
In this case the CUG selection is performed in accordance with a)
above.
The case where a user has both the closed user group with incoming
access and closed user group with outgoing access facilities is handled
in accordance with c) below.
In the case where the outgoing calls barred within the closed user
group facility does not apply for the selected CUG, the call is set-up
at the originating exchange. The initial address message forwarded to
the next exchange then includes the interlock code of the selected CUG
together with an indication that the call is a CUG call.
In the case where the outgoing calls barred within the closed user
group facility applies for the selected CUG, the call is rejected and
the access barred signal is returned to the calling party.
c) Call set-up from a user having the closed user group with outgoing
access facility
In this case the call is regarded as either an outgoing access call or
a call within the preferential (or only) CUG, unless the calling party
makes a facility request identifying a particular CUG for the call.
In the case where the outgoing calls barred within the closed user
group facility does not apply for the selected CUG, the call is set up
at the originating exchange. The initial address message forwarded to
the next exchange then includes the interlock code of the selected CUG
together with an indication that the call is a CUG for which outgoing
access is allowed.
In the case where the outgoing calls barred within the closed user
group facility applies for the preferential (or only) CUG, the call is
regarded as an outgoing access call. In this case the call is set up at
the originating exchange and no interlock code or CUG call indication
Fascicle VI.8 - Rec. Q.724 PAGE1
is included in the initial address message forwarded to the next
exchange.
In the case where the calling party makes a facility request
identifying a particular CUG and the outgoing calls barred within the
closed user group applies for this CUG, the call is rejected and an
access barred signal is sent to the calling party.
10.2.2.2 Transit exchange
With the possible exception of some gateway exchanges, each transit
exchange sets up a CUG call as an ordinary call. The information related to the
CUG facilities received from the preceding exchange, i.e. an interlock code, a
CUG call indication and possibly an indication that outgoing access is allowed,
is forwarded to the succeeding exchange.
In the case of an international CUG call, no special functions are
required at the gateway exchange provided that the international interlock code
assigned to the international CUG concerned is used in the national network.
However, in the case where a national interlock code other than the applicable
international interlock code is used within a national network, interlock code
conversion is required at the gateway (or corresponding) exchange.
10.2.2.3 Destination exchange
At the destination exchange a validation check of the acceptability of a
call is made where either the calling party (as indicated by a CUG call
indication in the initial address message received) or the called party belongs
to CUG. The call is connected only in cases where the information received checks
with the information stored at the destination exchange, as specified in the
following. In cases where a call is rejected because of incompatible CUG
information an unsuccessful backward set-up information message including the
access barred signal is sent towards the originating exchange.
a) Calls to a user having the closed user group or the closed user group
with outgoing access facility
In this case an incoming call is accepted only when:
i) it is a CUG call, including the case where outgoing access is
allowed, and
ii) correspondence is found between the interlock code received and an
interlock code associated with the called party, and
iii) the incoming calls barred within the closed user group facility
does not apply for the CUG identified by the interlock code
received.
If all the above conditions are not met, the call is rejected.
b) Calls to a user having the closed user group with incoming access
facility
In this case an incoming call is accepted when it is:
i) an ordinary call;
ii) a CUG call for which outgoing access is not allowed, if both
conditions specified in ii) and iii) of a) above are met;
iii) a CUG call for which outgoing access is allowed.
c) CUG calls to a user not belonging to any CUG
In the case where the incoming call is:
i) a CUG call for which outgoing access is allowed, it is accepted;
ii) a CUG call for which outgoing access is not allowed, it is
rejected.
10.2.3 International interlock code
Each international CUG is assigned a unique International CUG number (ICN)
according to the administrative rules defined in Recommendation X.180.
10.3 Users access to the calling line identification
10.3.1 General
Users access to the calling line identification is a user facility that
enables a user to be informed at incoming calls of the identity of the calling
line. When provided, the facility applies to all incoming calls except when the
calling party has the calling line identity presentation restricted facility or
when the complete identity of the calling line is not available at the
destination exchange.
The calling line identity is the telephone number of the calling party.
The calling line identity presentation restricted facility enables a user
to prohibit the forwarding of the calling line identity to the called party.
In the case where a national network does not always provide the calling
PAGE20 Fascicle VI.8 - Rec. Q.724
line identity facility, the calling line identity is the known part of the
telephone number at the interworking point (e.g. Trunk Code).
In the case where the calling is a PABX the network will send the
telephone number of the PABX or, in alternative the full DDI number. The latter
case is possible if the PABX provides the calling line identification facility to
the network.
The information indicating that a user has the calling identity or the
calling line identity presentation restricted facility is available in the
exchange to which the user is connected.
10.3.2 Call set-up procedure
The call control procedure and the information included in call control
messages vary depending on whether the calling party has indicated to use the
calling line identity presentation restricted facility for this call and whether
the calling line identity is included in the initial address message.
Two different call control procedures can be used to provide the calling
line identity facility. Both procedures are specified for international use:
10.3.2.1 The calling line identity is included in the initial address message
In the case where the calling party has indicated the calling line
identity restricted facility, the initial address message includes the calling
line identity restricted request indicator.
In the case where the complete identity of the calling party is not
available or not allowed to be forwarded outside the network:
a) in international network no information regarding the calling line
identity is included;
b) in national networks, the known part of the calling line identity could
be included. In this case an incomplete calling line identity indicator
is included in the message.
The calling party address is sent to the called party.
In the case where the destination exchange receives the calling party
address restricted request indicator or a calling party incomplete address
indicator, the calling line identity is not forwarded to the called party.
10.3.2.2 The calling line identity is not included in the initial address
message
In the case where the called party has the user access to the calling line
identification facility, a request is sent towards the originating exchange. The
request is included in a general request message.
When receiving the request for calling line identity the
originating/interworking exchange sends a response including the calling line
identity. In the case where the calling party has the calling line identity
presentation restricted facility the response sent from the originating exchange
includes the calling line identity presentation restricted request indicator. The
response is included in a general forward set-up information message. The
information included in the response in addition to the calling line identity
presentation restricted indicator (where applicable) is as follows:
a) in the case where the complete identity of calling line is known, the
originating exchange includes the complete telephone number of the
calling party;
b) in the case where the complete identity of the calling party address is
not available or is not allowed to be forwarded outside the network,
the response includes:
i) in international networks the calling line identity unavailable
signal;
ii) in national networks, in addition to the calling line identity
unavailable signal, the response can include the known part of the
calling line identity. In this case the response includes the
incomplete calling line identity indicator.
The calling party address is sent to the called party.
In the case where the destination exchange receives the calling party
address restricted request indicator or a calling party incomplete address
indicator, the calling line identity is not forwarded to the called party.
The destination exchange must not connect through until the complete
Fascicle VI.8 - Rec. Q.724 PAGE1
calling line identity has been sent to the called party or the called party has
been notified that the calling line address identity will not be forwarded.
10.4 Redirection of calls
10.4.1 General
The redirection of calls facility enables a user to have calls to a
telephone number, for which the facility is subscribed, redirected to another
predetermined number during periods when the facility is activated.
The redirection of calls rejected facilities enables a user to have
redirected calls to his telephone number automatically rejected during periods
when the facility is activated.
The redirection of calls information prohibited facility enables the user,
who has activated the redirection of calls facility, to prevent the calling party
from being informed that the call is redirected.
Depending on the possibilities offered by the Administration facility,
activation and deactivation may be made:
a) by the user by means of user controlled activation and deactivation
procedures;
b) by the network at predetermined times;
c) by the Administration on request of the user.
User controlled procedures for inquiry of the status of the facility (i.e.
whether the facility is activated or deactivated) may also be provided.
A call may only be redirected once. Redirected calls are subject to the
same restrictions as other calls where a closed user group is involved.
10.4.2 Call set-up procedure not involving other facilities affecting the
procedure
Information that a user has the redirection of calls rejected facility is
stored at the exchange to which the user is connected. When a redirected call
arrives at such a user, the call is rejected in the same manner as if this user
had activated the redirection of calls facility.
Information that a user has the redirection of calls information
prohibited facility is stored at the exchange, where the user is connected,
together with the redirection address.
Information that a subscriber has the redirection of calls facility
activated is stored together with the redirection address, at the exchange to
which the user is connected. When such a user is called, the call is set up to
the redirection address in accordance with the following:
10.4.2.1 The redirection address is at the same exchange
In this case the destination exchange connects the call to the redirection
address and returns an address complete message including the call forwarding
indicator. In the case where the called party has the redirection of calls
information prohibited facility activated the address complete message includes
the redirection of calls information prohibited indicator. When receiving the
call forwarding indicator the originating exchange sends a signal to inform the
calling party that the call has been redirected, except for the case when the
address complete message includes the redirection of calls information prohibited
indicator. In this case no information related to the redirection of calls
facility is sent to the calling party.
In the case where the user at the redirection address has the redirection
of calls or the redirection of calls rejected facility activated, the destination
exchange rejects the call and returns an indication in an unsuccessful backward
set-up message.
10.4.2.2 The redirection address is at another exchange
In this case the call is set-up to the redirection address in accordance
with the following procedure.
The call forwarding procedure is based on the principle that the
connection is extended forward from the destination exchange to the new
destination exchange.
i) The first destination exchange sets up the forward connection to the
redirection address. The initial address message forwarded includes a
redirected call indicator and the redirection address and redirection
of calls information prohibited indicator (if applicable). In national
networks the first called party address and the called line identity
(if applicable) and the calling line identity presentation prohibited
indicator (if applicable) could also be included in the initial address
message.
PAGE20 Fascicle VI.8 - Rec. Q.724
ii) Upon receipt of the redirected call the new destination exchange
connects or rejects the call in accordance with S 10.4.2.1. The
redirected call indicator received is used to prevent a further
redirection. The first called party address could be used for special
acceptance tests, or be sent to the calling party.
iii) In the case where the call is connected to the redirection address
the destination exchange will send an address complete message
including the call forwarding indicator and the redirection of calls
information prohibited indicator (if applicable). The call forwarding
indicator is used to inform the originating/controlling exchange, that
the first destination exchange performs the charging for the redirected
call. It could also be used to indicate to the calling party that the
call is redirected. Except for the case, when the address complete
message includes the redirection of calls information prohibited
indicator. In this case no information relating to the redirection of
calls facility is sent to the new called party.
iv) When the first destination exchange receives a message, e.g. request
for calling line identity from the new destination exchange, it sends
it further backwards to the originating exchange.
10.4.3 Calls involving other facilities affecting the procedure
10.4.3.1 Calls involving a closed user group facility
Redirected calls are subject to the restrictions applying for the closed
user group (CUG) facilities.
- In the case where the call is a CUG call, or the originally called
party has a CUG facility, the call is rejected before redirection
unless the validation check requirements applying for the CUG
facility(ies) concerned are satisfied.
- In the case where the call is a CUG call, or the user at the
redirection address has a CUG facility, the call is rejected unless the
validation check requirements applying for the CUG facility(ies)
concerned are satisfied.
- In the case where:
i) the call is a CUG call and,
ii) the redirection address is at an exchange other than the first
destination exchange, and
iii) the procedure for setting up the call to the redirection address
is in accordance with S 10.4.2.2 (i.e. call forwarding procedure),
the first destination has to send the CUG information received (e.g.
the CUG call indication and the interlock code) forward to the new
destination exchange in the initial address message.
10.4.3.2 The redirection address has the user's access to the calling party
identification
In the case where a redirected call arrives at a user, who has the users
access to the calling party address identification facility, the succeeding
actions at the redirection exchange depend on if the calling party address is
available at the original called exchange.
In the case where the calling party address is not available, a request
for the calling party address is sent to the preceding exchange(s) in accordance
with S 10.3.2.2. When the new destination exchange has the calling party address
available, it sends it to the new called party unless the calling party address
presentation restricted indicator is received at the new destination exchange.
10.4.3.3 The redirection address has the malicious call identification
capability
In the case where a call arrives at a user marked as an MCI user, the call
set-up procedure depends on whether the calling party address and/or the original
called party address is included in the initial address message and if the hold
option should apply for the call.
a) The hold option does not apply for the call. In this case the call
control procedure depends on whether the calling party address and/or
the original called party address is included in the initial address
message.
Fascicle VI.8 - Rec. Q.724 PAGE1
In the case where one or both of the addresses are not available, a
request is sent to the preceding exchange(s). The request will indicate
which address(es) are requested.
As a response the preceding (e.g. the originating or the original
called) exchange will include the concerned address(es), which has been
requested.
b) The hold options applies for the call. In this case the call set-up
procedure depends on whether the calling party address and/or the
original called party address is included in the initial address
message. In this case a request is sent to the preceding exchange(s)
indicating that the holding of the circuit is required.
In the case where one or both of the address(es) are not available, a
request is sent to the preceding exchange(s).
In their response the preceding (e.g. original called or originating)
include the addresses concerned, which have been requested and apply
the holding of circuit.
In the case of interworking, the interworking exchange will send in
addition to the information specified in S 10.5.3, the original called
party address.
When the original called exchange receives the request when both
addresses are not available in this exchange, it repeats the request to
the originating exchange. When the original called exchange receives
the response it repeats the response towards the destination exchange.
When the original called exchange receives the delayed release message,
it sends it forward to the destination exchange.
10.5 Network access to the calling line identification
10.5.1 General
The network access to the calling line identification is a network
capability which enables a network to obtain the calling party address inside or
outside their own network. The capability is used for example for malicious call
identification, charging, etc.
10.5.2 Malicious call identification (MCI)
The malicious call identification gives the possibility to obtain by an
appropriate request the identification of the calling line and the original
called party (in the case of a redirected call). The identification request
provokes in the destination exchange, the print-out of the following items:
- called line identity;
- calling line identity and possibly the original called line identity;
- time and date of the call.
The same print-out may be, optionally, obtained in the originating
exchange.
The identification request can either be activated before, during or after
the conversation phase.
Two different options of the utility are defined namely:
a) MCI with hold (national use);
b) MCI without hold.
One or both options should be provided in a national network.
In case a), the holding of the connection is requested in addition to the
identification of the calling party. In case b), only the identification of the
calling line is requested.
In case a), the clearing of the connection is subject to called party
clearing.
10.5.3 Call set-up procedure
In case of an incoming call to a user having the MCI facility the call
set-up procedure depends on whether the calling line identity is included in the
initial address message and which options, without hold or with hold, the called
party has been assigned:
a) if the calling line identity is included in the initial address
message:
- in the case where the called party has the MCI without hold
indication, the calling party address and possibly the original
called address is stored in the destination exchange;
- in the case where the called party has the MCI with hold
indication, the calling party address and possibly the original
called party address is stored at the destination exchange, and a
PAGE20 Fascicle VI.8 - Rec. Q.724
request for holding of the circuit is sent to the originating
exchange.
Fascicle VI.8 - Rec. Q.724 PAGE1
b) if the calling line identity is not included in the initial address
message:
- in the case where the called party has the MCI without hold
indication, a request is sent to the originating exchange
containing the calling line identity request;
- in the case where the called party has the hold indication, the
request will include requests for the holding of the circuit and
for calling line identity.
In addition to the information mentioned above the request will also
include the MCI facility encountered indicator. The request will be sent in a
general request message.
When receiving the MCI request the transit exchange normally repeats the
request. However, in two cases the transit exchange acts in another way:
- In the case of interworking with networks that do not provide the
calling line identification facility, the relevant transit exchange
will send a response including the identity of the transit exchange.
The identity of the transit exchange could either be the known part of
the calling party address in that exchange or, in national networks,
the signalling point code of the transit exchange. In addition to the
identity of the transit exchange the response can also include the
identity of the incoming trunk. The interworking exchange may also
arrange the holding of the incoming trunk even if not explicitly
requested (i.e. also in the option "MCI without hold"). In the case
where the MCI request also includes the hold request the transit
exchange will make the clearing of circuit subject to the called party
clearing.
- In the case where the MCI cannot operate (due to administrative or
technical reasons), the relevant exchange includes in the MCI response
message the MCI not provided indicator.
At the receipt of the MCI request, the originating exchange sends a
general forward set-up information message containing the calling line identity
and the hold indicator. If holding of the connection is provided the clearing of
the circuit will be subject to the called party clearing (i.e. subject to the
receipt of the clear-back signal). When the identification request is made the
destination exchange produces the print-out of the related MCI information and
sends backwards, optionally, the MCI print-out request (for further study)
message to obtain the print-out of the same information in the originating
exchange.
10.5.4 Clearing procedures
In the case where no holding of the circuit is requested, the normal
release procedure will apply.
In the case where the holding of the circuit is requested, the following
procedures apply at the originating exchange and the destination exchange:
a) In the case where the calling party hangs up first, the originating
exchange will apply the hold of the connection and stop the charging
(if applicable). Moreover, the originating exchange may send forward
the optional "calling party clear signal".
When receiving the calling party clear signal an intermediate charging
point stops the charging (if applicable) and forwards the calling party
clear signal to the succeeding exchange.
When receiving the calling party clear signal the destination exchange
starts a timer T1, if the identification request is not received.
The value of T is a national option.
b) In the case where the identification request is made before the called
party disconnects, no clear-back signal will be sent until appropriate
action has been taken (e.g. maintenance action). If applicable T1 is
stopped when the identification request is received.
c) When the called party disconnects the destination exchange may start a
timer T2 to allow for making the identification request after the
conversation is terminated.
The succeeding actions at the destination exchange will depend on whether
an identification request has been made or not.
In the case where the request was not made identification request, the
expiration of the timer T2 will result in sending of the clear-back message. The
timer T1 is stopped (if applicable).
PAGE20 Fascicle VI.8 - Rec. Q.724
In the case where the called party makes the request for identification is
made before the timer T2 expires, no clear-back signal will be sent until
appropriate actions have been taken. The timers T2 and T1 (if applicable) are
stopped when receiving the identification request is made.
11 Digital connectivity
11.1 General
The digital connectivity is a user facility that enables a user to
establish a fully digital path at 64 kbit/s user-to-user. It is an optional
facility assigned to the user and provided on a call request basis or specific
category.
11.2 Call set-up procedure
In the case of a call for which the digital connectivity is required, the
IAM/IAI message includes the all digital path required indicator.
On recognition of this request each exchange (originating/transit) makes a
check on the possibility to route the call on a digital path:
- if the check is positive the call is routed and the request of this
facility is forwarded to the succeeding exchange;
- if negative, the call is rejected and one of the following unsuccessful
signals is sent backwards:
- congestion or call-failure signal in case where a digital path
exists but it is not possible to complete the call due to
congestion or failure (see Recommendation Q.722, S 3.4).
- digital path not provided in case where a routing that allows a
complete digital path doesn't exist.
In the destination exchange, at the reception of an incoming call with the
digital connectivity request, the appropriate validation check is made and, if
positive, the call is completed using the standard procedures. In the negative
case the call is rejected and the access barred signal is sent backwards.
12 Echo suppressor control
12.1 General
The echo suppressor control signalling procedure is used on per call basis
to convey information between exchanges about the demand and ability to insert
echo suppressors.
The procedure is mainly intended to be used in the case where the echo
suppressors are provided in pools.
The procedure is initiated by the exchange which upon analysis of an
initial address message of a call realizes that the call is to be routed on a
connection for which echo suppressor is necessary, and no indication is received
that an outgoing half-echo suppressor is already included (see Note).
The exchange shall always be able to insert outgoing half-echo
suppressors.
One of the exchanges succeeding the above-mentioned exchange shall always
be able to insert incoming half-echo suppressors.
The procedure is for application in national networks and could be applied
in the international network upon bilateral agreement.
Note - In the case where this exchange knows that there is no echo
suppressor situated in the preceding network the procedure is not initiated.
12.2 Actions at the exchange initiating the echo suppressor control procedure
Upon receipt of an initial address message the following actions are taken
if no indication is received that an outgoing half-echo suppressor is already
included:
- a request for outgoing half-echo suppressor is sent in the backward
direction;
- a timer T is started (see Note);
- an outgoing half-echo suppressor is reserved;
- the initial address message is sent on with the indication outgoing
half-echo suppressor included.
Fascicle VI.8 - Rec. Q.724 PAGE1
Upon receipt of a response on the outgoing half-echo suppressor request
the following actions are taken:
a) if the response is negative:
- the reserved outgoing half-echo suppressor is included;
- the timer T is stopped;
b) if the response is positive:
- the reserved outgoing half-echo suppressor is released;
- the timer T is stopped.
Note - If response on the request for outgoing half-echo suppressor has
not been received before timer T has expired, then the reserved half-echo
suppressor is included.
12.3 Actions at the originating exchange
Upon receipt of a request for outgoing half-echo suppressor the following
actions are taken:
a) if the originating exchange is not able to insert outgoing half-echo
suppressor:
- a negative response is sent in the forward direction;
b) if the originating exchange is able to insert outgoing half-echo
suppressor:
- a half-echo suppressor is included;
- a positive response is sent in the forward direction.
12.4 Actions at an intermediate exchange
12.4.1 The exchange being able to insert a half-echo suppressor
Upon receipt of a request for outgoing half-echo suppressor the following
actions are taken (see Note 1):
- an outgoing half-echo suppressor is reserved;
- the request message is sent on;
- a timer T is started (see Note 2).
Note 1 - If the intermediate exchange knows that there is no echo
suppressor in the preceding network the intermediate exchange performs actions in
accordance with S 12.3.
Note 2 - If response on the request for outgoing half-echo suppressor has
not been received before timer T has expired, then the reserved half-echo
suppressor is included and a positive response is sent in the forward direction.
Upon receipt of a response on the outgoing half-echo suppressor request
the following actions are taken:
a) the response is negative:
- the reserved outgoing half-echo suppressor is included;
- the timer T is stopped;
- a positive response is sent in forward direction;
b) the response is positive:
- the reserved outgoing half-echo suppressor is released;
- the timer T is stopped;
- the response is sent on.
Upon receipt of an initial address message with the indication "outgoing
half-echo suppressor included" the following actions are taken:
- an incoming half-echo suppressor is reserved;
- the initial address message is sent on.
PAGE20 Fascicle VI.8 - Rec. Q.724
Upon receipt of an address complete message with an indication on incoming
half-echo suppressor the following actions are taken:
a) the indication is negative:
- the reserved incoming half-echo suppressor is included;
- the address complete message is sent on with a positive indication;
b) the indication is positive:
- the reserved incoming half-echo suppressor is released;
- the address complete message is sent on.
12.4.2 The exchange not being able to insert half-echo suppressor
No special actions are required.
12.5 Actions at the destination exchange
Upon receipt of an initial address message with the indication "outgoing
half-echo suppressor included" the following actions are taken:
a) if the destination exchange is not able to insert an incoming half-echo
suppressor:
- a negative indication on the inclusion of incoming half-echo
suppressor is given in the address complete message;
b) if the destination exchange is able to insert incoming half-echo
suppressor:
- a half-echo suppressor is included;
- a positive indication on the inclusion of incoming half-echo
suppressor is given in the address complete message.
13 Congestion control
13.1 Exchange congestion control
13.1.1 Automatic congestion control
Automatic Congestion Control (ACC) is used when an exchange is in an
overload condition (see also Recommendation Q.542, S 5.4.5). Two levels of
congestion are distinguished, a less severe congestion threshold (congestion
level 1) and a more severe congestion threshold (congestion level 2). If either
of the two congestion levels is reached, an automatic congestion control
information message may be sent to the adjacent exchanges indicating the level of
congestion (congestion level 1 or 2). The adjacent exchanges, when receiving an
automatic congestion control information message, should reduce their traffic to
the overload affected exchange.
The automatic congestion control information message is sent by the
overloaded exchange after receiving the clear-forward signal and before sending
the release-guard signal for a circuit. If the overloaded exchange returns to
normal traffic load, no more automatic congestion control information messages
are sent. The adjacent exchanges then, after a predetermined time, automatically
return to their normal status.
13.2 Telephone User Part signalling congestion control
13.2.1 General
On receipt of congestion indication primitives, CIP (see also
Recommendation Q.704, S 10.2.3), the TUP should reduce traffic load (call
attempts) into the affected direction in several steps.
13.2.2 Procedure
When the first CIP is received by the TUP, the traffic load into the
affected direction is reduced by one step. At the same time, two timers Tue1 and
Tue2 are started. During Tue1, all the following received CIPs for the same
direction are ignored in order not to reduce traffic too rapidly. Reception of a
CIP after the expiry of Tue1, but still during Tue2, will decrease the traffic
load by one more step and restart Tue1 and Tue2.
If Tue2 expires (i.e. no CIPs have been received during the corresponding
period), traffic will be increased by one step and Tue2 will be restarted unless
full traffic load has been resumed.
Tue1 = 300-600 ms ü
ì provisional values
Tue2 = 5-10 s Φ
The number of steps of traffic reduction and the type and/or amount of
increase/decrease of traffic load at the various steps are considered to be an
implementation dependent function.
Fascicle VI.8 - Rec. Q.724 PAGE1
14 Telephone User Part outage
When a Telephone user part outage occurs, actions should be taken as
follows:
- The user parts at the nodes connected to the failing node should
receive an indication from the user's flow control functions and react
by stopping the seizure of circuits to that failing node and by routing
the traffic on alternative routes.
- In the user part which has previously failed, after the initialization
procedures, the resumption of the signalling relation is obtained by
sending circuit group messages in all the circuits affected by the
outage, as specified in S 1.15 (Reset of circuits and circuit groups).
15 State transition diagrams
15.1 General
This section contains the description of the signalling procedures
described in this Recommendation in the form of state transition diagrams
according to the CCITT Specification and Description Language (SDL).
In order to facilitate functional description, the Telephone User Part
signalling procedure function is divided into functional blocks, as shown in
Figure 1/Q.724; state transition diagrams are provided for each functional block,
as shown below:
- Signalling procedure control (SPRC): Figure 2/Q.724
- Call processing control (CPC): Figure 3/Q.724
- Continuity-check outgoing (CCO): Figure 4/Q.724
- Continuity-check incoming (CCI): Figure 5/Q.724
- Continuity-recheck outgoing (CRO): Figure 6/Q.724
- Continuity-recheck incoming (CRI): Figure 7/Q.724
- Blocking and unblocking signal sending (BLS): Figure 8/Q.724
- Blocking and unblocking signal reception (BLR): Figure 9/Q.724
- Circuit reset (CRS): Figure 10/Q.724
- Circuit group control (CGC): Figure 11/Q.724
- Circuit group reset sending (CGRS): Figure 12/Q.724
- Circuit group reset receipt (CGRR): Figure 13/Q.724
- Maintenance oriented circuit group blocking and unblocking sending
(MBUS): Figure 14/Q.724
- Maintenance oriented circuit group blocking and unblocking receipt
(MBUR): Figure 15/Q.724
- Hardware failure oriented circuit group blocking and unblocking sending
(HBUS): Figure 16/Q.724
- Hardware failure oriented circuit group blocking and unblocking receipt
(HBUR): Figure 17/Q.724
- Software generated circuit group blocking and unblocking sending
(SBUS): Figure 18/Q.724
- Software generated circuit group blocking and unblocking receipt
(SBUR): Figure 19/Q.724
The detailed functional breakdown shown in the diagrams is intended to
illustrate a reference model and to assist interpretation of the text in the
earlier sections. The state transition diagrams are intended to show precisely
the behaviour of the signalling system as viewed from a remote location. It must
be emphasized that the functional partitioning shown in the diagrams is used only
to facilitate understanding of the system behaviour and is not intended to
specify the functional partitioning to be adopted in a practical implementation
of the signalling system.
15.2 Drafting conventions
a) Abbreviations used in Figures 1/Q.724 to 19/Q.724 are listed in S 15.3.
b) External inputs and outputs are used for interactions with different
functional blocks. Internal inputs and outputs are used for
interactions within each functional block, e.g. to indicate control of
time-outs.
c) External inputs and outputs contain as part of their name, the
abbreviations of their source and destination functional block names,
with an arrow in between, e.g. Start CPC->CCO.
d) For interexchange signals or signal messages, external input and output
symbols are used as shown below to indicate the direction of each
signal on message.
Figure - CCITT 41230
PAGE20 Fascicle VI.8 - Rec. Q.724
Note - The functions covered by Figures 1/Q.724 to 19/Q.724 are limited in
the following points:
- they refer only to call processing functions in international transit
exchanges;
- they do not necessarily cover all the abnormal situations.
However, they include some operations on receipt of unreasonable
signalling information as specified in S 6.5.
15.3 Abbreviations and timers used in Figures 1/Q.724 to 19/Q.724
General
BBR Circuit blocked by reception of the blocking signal
BBS Circuit blocked by sending the blocking signal
CC Continuity-check
CCT Telephone circuit
ICC Incoming trunk circuit
NOK Not OK
OGC Outgoing trunk circuit
Functional block names (See Figure 1/Q.724)
BLR Blocking and unblocking signal reception
BLS Blocking and unblocking signal sending
CCI Continuity-check incoming
CCO Continuity-check outgoing
CGC Circuit group control
CGRR Circuit group reset receipt
CGRS Circuit group reset sending
CPC Call processing control
CRI Continuity-recheck incoming
CRO Continuity-recheck outgoing
CRS Circuit-reset
HBUR Hardware failure oriented circuit group blocking and unblocking
receipt
HBUS Hardware failure oriented circuit group blocking and unblocking
sending
L3 Level 3 (Signalling network functions)
L4 Level 4 (Telephone user part)
MBUR Maintenance oriented circuit group blocking and unblocking receipt
MBUS Maintenance oriented circuit group blocking and unblocking sending
SBUR Software generated circuit group blocking and unblocking receipt
SBUS Software generated circuit group blocking and unblocking sending
Fascicle VI.8 - Rec. Q.724 PAGE1
SPRC Signalling procedure control
Messages and signals
ACM Address complete message
ADC Address complete signal, charge
ADI Address incomplete signal
ADN Address complete signal, no charge
ADX Address complete signal, coin box
AFC Address complete signal, charge, subscriber free
AFN Address complete signal, no charge, subscriber free
AFX Address complete signal, coin box, subscriber free
ANC Answer signal, charge
ANN Answer signal, no charge
BLA Blocking-acknowledgement signal
BLO Blocking signal
CBK Clear-back signal
CCF Continuity-failure signal
CCH Continuity-check indicator:
- 0: CC not required
- 1: CC required on this circuit
- 2: CC is being (has been) performed on a previous circuit
CCR Continuity-check-request signal
CFL Call-failure signal
CGC Circuit-group-congestion signal
CLF Clear-forward signal
COT Continuity signal
FOT Forward-transfer signal
GRA Circuit group reset-acknowledgement message
GRS Circuit group reset message
HBA Hardware failure oriented group blocking-acknowledgement message
HGB Hardware failure oriented group blocking message
HGU Hardware failure oriented group unblocking message
HUA Hardware failure oriented group unblocking-acknowledgement message
IAM Initial address message
PAGE20 Fascicle VI.8 - Rec. Q.724
LOS Line-out-of-service signal
MBA Maintenance oriented group blocking-acknowledgement message
MGB Maintenance oriented group blocking message
MGU Maintenance oriented group unblocking message
MUA Maintenance oriented group unblocking-acknowledgement message
NNC National-network-congestion signal
RAN Reanswer signal
RLG Release-guard signal
RSC Reset-circuit signal
SAO Subsequent address message with one signal
SAM Subsequent address message
SBA Software generated group blocking-acknowledgement message
SEC Switching-equipment-congestion signal
SGB Software generated group blocking message
SSB Subscriber-busy signal (electrical)
SST Send-special-information-tone signal
SUA Software generated group unblocking-acknowledgement message
UBA Unblocking-acknowledgement signal
UBL Unblocking signal
UNN Unallocated-number signal
Timers
T1 Timer "waiting for continuity or continuity-failure signal" [10-15
seconds, see S 6.4.3 a)]
T2 Timer "waiting for address-complete signal" [20-30 seconds, see S 6.4.3
a)]
T3 Timer "waiting for clear-forward signal after sending unsuccessful
message" [4-15 seconds, see S 6.4.3 b)]
T4 Timer "waiting for clear-forward signal after sending call-failure
signal" [4-15 seconds, see S 6.4.3 b)]
T5 Timer "stop sending call-failure messages on time out" [1 minute, see S
6.4.3 b)]
T6 Timer "waiting for release-guard signal" (4-15 seconds, see S 6.2.3)
T7 Timer "stop sending clear-forward signal on time out" (1 minute, see S
6.2.3)
T8 Timer "waiting for backward check-tone" (should not exceed 2 seconds,
see S 7.4.1)
T9 Timer "delay to start first-time continuity-recheck" (1-10 seconds, see
S 7.3)
T10 Timer "delay for multiple retests of continuity" (1-3 minutes, see S
7.3)
T11 Timer "waiting to alert maintenance personnel following initiation of
blocking" (5 minutes, see S 5)
T12 Timer "waiting for blocking-acknowledgement signal" (4-15 seconds, see
Fascicle VI.8 - Rec. Q.724 PAGE1
S 6.4.4)
T13 Timer "waiting to alert maintenance personnel on failure to receive
BLA" (1 minute, see S 6.4.4)
T14 Timer "delay to repeat sending of blocking signals" (1 minute, see S
5.1)
T15 Timer "waiting for unblocking acknowledgement" (4-15 seconds, see S
6.4.4)
T16 Timer "waiting to alert maintenance personnel on failure to receive
unblocking acknowledgement" (1 minute, see S 6.4.4)
T17 Timer "delay to repeat sending of unblocking acknowledgement" (1
minute, see S 5.1)
T18 Timer "waiting for a response to the reset-circuit signal" (4-15
seconds, see S 1.15)
T19 Timer "delay to send the reset-circuit signal" (1 minute, see S 1.15)
T20 Timer "waiting for second group reset message" (5 seconds, see S
1.15.2)
T21 Timer "waiting for circuit group reset acknowledgement message" (4-15
seconds, see S 1.15)
T22 Timer "delay to send the circuit group reset message" (1 minute, see S
1.15)
T23 Timer "waiting for second maintenance oriented group blocking message"
(5 seconds, see S 5.2)
T24 Timer "waiting for second maintenance oriented group unblocking
message" (5 seconds, see S 5.2)
T25 Timer "waiting to alert maintenance personnel following initiation of
maintenance oriented group blocking" (5 minutes, see S 5)
T26 Timer "waiting for maintenance oriented group blocking acknowledgement
message" (4-15 seconds, see S 6.4.4)
T27 Timer "delay to send the maintenance oriented group blocking message"
(1 minute, S 6.4.4)
T28 Timer "waiting for maintenance oriented group unblocking
acknowledgement message" (4-15 seconds, see S 6.4.4)
T29 Timer "delay to send the maintenance oriented group unblocking message"
(1 minute, see S 6.4.4)
T30 Timer "waiting for second hardware failure oriented group blocking
message" (5 seconds, see S 5.2)
T31 Timer "waiting for second hardware failure oriented group unblocking
message" (5 seconds, see S 5.2)
T32 Timer "waiting for hardware failure oriented group blocking
acknowledgement message" (4-15 seconds, see S 6.4.4)
T33 Timer "delay to send hardware failure oriented group blocking message"
(1 minute, see S 6.4.4)
T34 Timer "waiting for hardware failure oriented group unblocking
acknowledgement message" (4-15 seconds, see S 6.4.4)
T35 Timer "delay to send hardware failure oriented group unblocking
message" (1 minute, see S 6.4.4)
T36 Timer "waiting for second software generated group blocking message" (5
seconds, see S 5.2)
T37 Timer "waiting for second software generated group unblocking message"
(5 seconds, see S 5.2)
T38 Timer "waiting for software generated group blocking acknowledgement
message" (4-15 seconds, see S 6.4.4)
T39 Timer "delay to send software generated group blocking message" (1
minute, see S 6.4.4)
T40 Timer "waiting for software generated group unblocking acknowledgement
message" (4-15 seconds, see S 6.4.4)
T41 Timer "delay to send software generated group unblocking message" (1
minute, see S 6.4.4)
PAGE20 Fascicle VI.8 - Rec. Q.724
Fascicle VI.8 - Rec. Q.724 PAGE1
