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 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.

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:

T CR = 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:

T CR = 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 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.

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 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 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 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 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.

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.

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 request for holding of the circuit is sent to the originating exchange.

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).

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.

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.

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

ý provisional values

Tue2 = 5-10 s

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.

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

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

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

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 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)