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- 5 Changeover
- 5.1 General
- 5.1.1 The objective of the changeover procedure is to ensure that signalling
- traffic carried by the unavailable signalling link is diverted to the alternative
- signalling link(s) as quickly as possible while avoiding message loss,
- duplication or mis-sequencing. For this purpose, in the normal case the
- changeover procedure includes buffer updating and retrieval, which are performed
- before reopening the alternative signalling link(s) to the diverted traffic.
- Buffer updating consists of identifying all those messages in the retransmission
- buffer of the unavailable signalling link which have not been received by the far
- end. This is done by means of a hand-shake procedure, based on changeover
- messages, performed between the two ends of the unavailable signalling link.
- Retrieval consists of transferring the concerned messages to the transmission
- buffer(s) of the alternative link(s).
- 5.1.2 Changeover includes the procedures to be used in the case of
- unavailability (due to failure, blocking or inhibiting) of a signalling link, in
- order to divert the traffic pertaining to that signalling link to one or more
- alternative signalling links.
- These signalling links can be carrying their own signalling traffic and
- this is not interrupted by the changeover procedure.
- The different network configurations to which the changeover procedure may
- be applied are described in S 5.2.
- The criteria for initiation of changeover, as well as the basic actions to
- be performed, are described in S 5.3.
- Procedures necessary to cater for equipment failure or other abnormal
- conditions are also provided.
- 5.2 Network configurations for changeover
- 5.2.1 Signalling traffic diverted from an unavailable signalling link is routed
- by the concerned signalling point according to the rules specified in S 4. In
- summary, two alternative situations may arise (either for the whole diverted
- traffic or for traffic relating to each particular destination):
- i) traffic is diverted to one or more signalling links of the same link
- set, or
- ii) traffic is diverted to one or more different link sets.
- 5.2.2 As a result of these arrangements, and of the message routing function
- described in S 2, three different relationships between the new signalling link
- and the unavailable one can be identified, for each particular traffic flow.
- These three basic cases may be summarized as follows:
- a) the new signalling link is parallel to the unavailable one (see Figure
- 9/Q.704);
- b) the new signalling link belongs to a signalling route other than that
- to which the unavailable signalling link belongs, but this signalling
- route still passes through the signalling point at the far end of the
- unavailable signalling link (see Figure 10/Q.704);
- c) the new signalling link belongs to a signalling route other than that
- to which the unavailable signalling link belongs, and this signalling
- route does not pass through the signalling point acting as signalling
- transfer point, at the far end of the unavailable signalling link (see
- Figure 11/Q.704).
- Figure 9/Q.704 - CCITT 35830
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- Figure 10/Q.704 - CCITT 35840
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- Figure 11/Q.704 - CCITT 35850
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- Only in the case of c) does a possibility of message mis-sequencing exist:
- therefore its use should take into account the overall service dependability
- requirements described in Recommendation Q.706.
- 5.3 Changeover initiation and actions
- 5.3.1 Changeover is initiated at a signalling point when a signalling link is
- recognized as unavailable according to the criteria listed in S 3.2.2.
- The following actions are then performed:
- a) transmission and acceptance of message signal units on the concerned
- signalling link is terminated;
- b) transmission of link status signal units or fill in signal units, as
- described in Recommendation Q.703, S 5.3, takes place;
- c) the alternative signalling link(s) are determined according to the
- rules specified in S 4;
- d) a procedure to update the content of the retransmission buffer of the
- unavailable signalling link is performed as specified in S 5.4 below;
- e) signalling traffic is diverted to the alternative signalling link(s) as
- specified in S 5.5 below.
- In addition, if traffic toward a given destination is diverted to an
- alternative signalling link terminating in a signalling transfer point not
- currently used to carry traffic toward that destination, a transfer-prohibited
- procedure is performed as specified in S 13.2.
- 5.3.2 In the case when there is no traffic to transfer from the unavailable
- signalling link action, only item b) of S 5.3.1 is required.
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- PAGE1 Fascicle VI.7 - Rec. Q.704
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- 5.3.3 If no alternative signalling link exists for signalling traffic towards
- one or more destinations, the concerned destination(s) are declared inaccessible
- and the following actions apply:
- i) the routing of the concerned signalling traffic is blocked and the
- concerned messages already stored in the transmission and
- retransmission buffers of the unavailable signalling link, as well as
- those received subsequently, are discarded10);
- ii) a command is sent to the User Part(s) (if any) in order to stop
- generating the concerned signalling traffic;
- iii) the transfer-prohibited procedure is performed, as specified in S
- 13.2;
- iv) the appropriate signalling link management procedures are performed, as
- specified in S 12.
- 5.3.4 In some cases of failures or in some network configurations, the normal
- buffer updating and retrieval procedures described in SS 5.4 and 5.5 cannot be
- accomplished. In such cases, the emergency changeover procedures described in S
- 5.6 apply.
- Other procedures to cover possible abnormal cases appear in S 5.7.
- 5.4 Buffer updating procedure
- 5.4.1 When a decision to changeover is made, a changeover order is sent to the
- remote signalling point. In the case that the changeover was initiated by the
- reception of a changeover order (see S 5.2) a changeover acknowledgement is sent
- instead.
- A changeover order is always acknowledged by a changeover acknowledgement,
- even when changeover has already been initiated in accordance with another
- criterion.
- No priority is given to the changeover order or changeover acknowledgement
- in relation to the normal traffic of the signalling link on which the message is
- sent.
- 5.4.2 The changeover order and changeover acknowledgement are signalling network
- management messages and contain the following information:
- - the label, indicating the destination and originating signalling points
- and the identity of the unavailable signalling link;
- - the changeover-order (or changeover-acknowledgement) signal; and
- - the forward sequence number of the last message signal unit accepted
- from the unavailable signalling link.
- Formats and codes of the changeover order and the changeover
- acknowledgement appear in S 15.
- 5.4.3 Upon reception of a changeover order or changeover acknowledgement, the
- retransmission buffer of the unavailable signalling link is updated (except as
- noted in S 5.6), according to the information contained in the message. The
- message signal units successive to that indicated by the message are those which
- have to be retransmitted on the alternative signalling link(s), according to the
- retrieval and diversion procedure.
- 5.5 Retrieval and diversion of traffic
- When the procedure to update the retransmission buffer content is
- completed, the following actions are performed:
- - the routing of the signalling traffic to be diverted is changed;
- - the signal traffic already stored in the transmission buffers and
- retransmission buffer of the unavailable signalling link is sent
- directly towards the new signalling link(s), according to the modified
- routing.
- The diverted signalling traffic will be sent towards the new signalling
- link(s) in such a way that the correct message sequence is maintained. The
- diverted traffic has no priority in relation to normal traffic already conveyed
- on the signalling link(s).
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- 10) The adequacy of this procedure to meet the acceptable dependability objective in
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- 5.6 Emergency changeover procedures
- 5.6.1 Due to the failure in a signalling terminal it may be impossible for the
- corresponding end of the faulty signalling link to determine the forward sequence
- number of the last message signal unit accepted over the unavailable link. In
- this case, the concerned end accomplishes, if possible, the buffer updating
- procedures described in S 5.4 but it makes use of an emergency changeover order
- or an emergency changeover acknowledgement instead of the corresponding normal
- message; these emergency messages, the format of which appears in S 15, do not
- contain the forward sequence number of the last accepted message signal unit.
- Furthermore, the signalling link is taken out of service, i.e. the concerned end
- initiates, if possible, the sending of out-of-service link status signal units on
- the unavailable link (see Recommendation Q.703, S 5.3).
- When the other end of the unavailable signalling link receives the
- emergency changeover order or acknowledgement, it accomplishes the changeover
- procedures described in SS 5.4 and 5.5, the only difference being that it does
- not perform either buffer updating or retrieval. Instead, it directly starts
- sending the signalling traffic not yet transmitted on the unavailable link on the
- alternative signalling link(s).
- The use of normal or emergency changeover messages depends on the local
- conditions of the sending signalling point only, in particular:
- - an emergency changeover order is acknowledged by a changeover
- acknowledgement if the local conditions are normal; and
- - a changeover order is acknowledged by an emergency changeover
- acknowledgement if there are local fault conditions.
- 5.6.2 Time-con changeover is initiated when
- the exchange of changeover messages is not possible or not
- desirable, i.e., if any (or several) of the following cases apply:
- i) No signalling path exists between the two ends of the unavailable link,
- so that the exchange of changeover messages is impossible.
- ii) Processor outage indication is received on a link. In this case, if the
- remote processor outage condition is only transitory, sending of a
- changeover order could result in failure of the link.
- iii) A signalling link currently carrying traffic has been marked
- (locally or remotely) inhibited. In this case, time controlled
- changeover is used to divert traffic for the inhibited link without
- causing the link to fail.
- When the concerned signalling point decides to initiate changeover in such
- circumstances, after the expiry of a time T1 (see S 16.8), it starts signalling
- traffic not yet transmitted on the unavailable signalling link on the alternative
- link(s); the purpose of withholding traffic for the time T1 (see S 16.8) is to
- reduce the probability of message mis-sequencing.
- An example of such a case appears in Recommendation Q.705, Annex A.
- In the abnormal case when the concerned signalling point is not aware of
- the situation, it will start the normal changeover procedure and send a
- changeover order; in this case it will receive no changeover message in response
- and the procedure will be completed as indicated in S 5.7.2. Possible reception
- of a transfer-prohibited message (sent by an involved signalling transfer point
- on reception of the changeover order, see S 13.2) will not affect changeover
- procedures.
- 5.6.3 Due to failures, it may be impossible for a signalling point to perform
- retrieval even if it has received the retrieval information from the far end of
- the unavailable signalling link. In this case, it starts sending new traffic on
- reception of the changeover message (or on time-out expiry, see SS 5.6.2 and
- 5.7.2); no further actions in addition to the other normal changeover procedures
- are performed.
- 5.7 Procedures in abnormal conditions
- 5.7.1 The procedures described in this section allow the completion of the
- changeover procedures in abnormal cases other than those described in S 5.6.
- 5.7.2 If no changeover message in response to a changeover order is received
- within a timer T2 (see S 16.8), new traffic is started on the alternative
- signalling link(s).
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- 5.7.3 If a changeover order or acknowledgement containing an unreasonable value
- of the forward sequence number is received, no buffer updating or retrieval is
- performed, and new traffic is started on the alternative signalling link(s).
- 5.7.4 If a changeover acknowledgement is received without having previously sent
- a changeover order, no action is taken.
- 5.7.5 If a changeover order is received relating to a particular signalling link
- after the completion of changeover from that signalling link, an emergency
- changeover acknowledgement is sent in response, without any further action.
- 6 Changeback
- 6.1 General
- 6.1.1 The objective of the changeback procedure is to ensure that signalling
- traffic is diverted from the alternative signalling link(s) to the signalling
- link made available as quickly as possible, while avoiding message loss,
- duplication or mis-sequencing. For this purpose (in the normal case), changeback
- includes a procedure to control the message sequence.
- 6.1.2 Changeback includes the basic procedures to be used to perform the
- opposite action to changeover, i.e. to divert traffic from the alternative
- signalling link(s) to a signalling link which has become available (i.e., it was
- uninhibited, restored or unblocked). The characteristics of the alternative
- signalling link(s) from which changeback can be made are described in S 5.2. In
- all the cases mentioned in S 5.2 the alternative signalling links can be carrying
- their own signalling traffic and this is not interrupted by the changeback
- procedures.
- Procedures necessary to cater for particular network configuration or
- other abnormal conditions are also provided.
- Note - The term "alternative signalling link(s)" refers to signalling
- link(s) terminating in the signalling point at which a changeback is initiated
- (see also S 4).
- 6.2 Changeback initiation and actions
- 6.2.1 Changeback is initiated at a signalling point when a signalling link is
- restored, unblocked or uninhibited, and therefore it becomes once again
- available, according to the criteria listed in SS 3.2.3 and 3.2.7. The following
- actions are then performed:
- a) the alternative signalling link(s) are determined, to which traffic
- normally carried by the signalling link made available was previously
- diverted (e.g., on occurrence of a changeover);
- b) signalling traffic is diverted (if appropriate, according to the
- criteria specified in S 4) to the concerned signalling link by means of
- the sequence control procedure specified in S 6.3; traffic diversion
- can be performed at the discretion of the signalling point initiating
- changeback, as follows:
- i) individually for each traffic flow (i.e., on destination basis);
- ii) individually for each alternative signalling link (i.e., for all
- the destinations previously diverted on that alternative
- signalling link);
- iii) at the same time for a number of, or for all the alternative
- signalling links.
- On occurrence of changeback, it may happen that traffic towards a given
- destination is no longer routed via a given adjacent signalling transfer point,
- towards which a transfer-prohibited procedure was previously performed on
- occurrence of changeover (see S 5.3.1); in this case a transfer-allowed procedure
- is performed, as specified in S 13.3.
- In addition, if traffic towards a given destination is diverted to an
- alternative signalling link terminating in a signalling transfer point not
- currently used to carry traffic toward that destination, a transfer-prohibited
- procedure is performed as specified in S 13.2.
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- 6.2.2 In the case when there is no traffic to transfer to the signalling link
- made available, none of the previous actions are performed.
- 6.2.3 In the case that the signalling link made available can be used to carry
- signalling traffic toward a destination which was previously declared
- inaccessible, the following actions apply:
- i) the routing of the concerned signalling traffic is unblocked and
- transmission of the concerned messages (if any) is immediately started
- on the link made available;
- ii) a command is sent to the User Part(s) (if any) in order to restart
- generating the concerned signalling traffic;
- iii) the transfer-allowed procedure is performed, as specified in S
- 13.3. However, in national networks, when the recovered link is not on
- the normal route for that destination, the transfer-restricted11)
- procedure may be performed as specified in S 13.5.
- 6.2.4 In the case that the signalling link made available is used to carry
- signalling traffic towards a destination which was previously declared
- restricted, the following actions apply:
- i) the concerned signalling traffic is rediverted and transmission of the
- concerned messages (if any) is immediately started on the link made
- available;
- ii) when the recovered link is on the normal route for that destination,
- the status of the route is changed to available; otherwise, the status
- of the route remains unchanged.
- 6.2.5 If the signalling point at the far end of the link made available
- currently is inaccessible, from the signalling point initiating changeback (see S
- 9 on Signalling Point Restart), the sequence control procedure specified in S 6.3
- (which requires communication between the two concerned signalling points) does
- not apply; instead, the time-controlled diversion specified in S 6.4 is
- performed. This is made also when the concerned signalling points are accessible,
- but there is no signalling route to it using the same outgoing signalling link(s)
- (or one of the same signalling links) from which traffic will be diverted.
- 6.3 Sequence control procedure
- 6.3.1 When a decision is made at a given signalling point to divert a given
- traffic flow (towards one or more destinations) from an alternative signalling
- link to the signalling link made available, the following actions are performed
- if possible (see S 6.4):
- i) transmission of the concerned traffic on the alternative signalling
- link is stopped; such traffic is stored in a changeback buffer;
- ii) a changeback declaration is sent to the remote signalling point of the
- signalling link made available via the concerned alternative signalling
- link; this message indicates that no more message signal units relating
- to the traffic being diverted to the link made available will be sent
- on the alternative signalling link.
- 6.3.2 The concerned signalling point will restart diverted traffic over the
- signalling link made available when it receives a changeback acknowledgement from
- the far signalling point of the link made available; this message indicates that
- all signal messages relating to the concerned traffic flow and routed to the
- remote signalling point via the alternative signalling link have been received.
- The remote signalling point will send the changeback acknowledgement to the
- signalling point initiating changeback in response to the changeback declaration;
- any available signalling route between the two signalling points can be used to
- carry the changeback acknowledgement.
- 6.3.3 The changeback declaration and changeback acknowledgement are signalling
- network management messages and contain:
- - the label, indicating the destination and originating signalling
- points, and the identity of the signalling link to which traffic will
- be diverted;
- - the changeback-declaration (or changeback-acknowledgement) signal, and
- - the changeback code.
- Formats and codes of the changeback declaration and changeback
- acknowledgement appear in S 15.
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- 6.3.4 A particular configuration of the changeback code is autonomously assigned
- to the changeback declaration by the signalling point initiating changeback; the
- same configuration is included in the changeback acknowledgement by the
- acknowledging signalling point. This allows discrimination between different
- changeback declarations and acknowledgements when more than one sequence control
- procedures are initiated in parallel, as follows.
- 6.3.5 In the case that a signalling point intends to initiate changeback in
- parallel from more than one alternative signalling link, a sequence control
- procedure is accomplished for each involved signalling link, and a changeback
- declaration is sent on each of them; each changeback declaration is assigned a
- different configuration of the changeback code. Stopped traffic is stored in one
- or more changeback buffers (in the latter case, a changeback buffer is provided
- for each alternative signalling link). When the changeback acknowledgement
- relating to that alternative signalling link is received, traffic being diverted
- from a given alternative signalling link can be restarted on the signalling link
- made available, starting with the content of the changeback buffer;
- discrimination between the different changeback acknowledgements is made by the
- changeback code configuration, which is the same as that sent in the changeback
- declaration.
- This procedure allows either reopening the recovered signalling link to
- traffic in a selective manner (provided that different changeback buffers are
- used) as soon as each changeback acknowledgement is received, or only when all
- the changeback acknowledgements have been received.
- 6.4 Time-controlled diversion procedure
- 6.4.1 The time-controlled diversion procedure is used at the end of the
- signalling point restart procedure (see S 9) when an adjacent signalling point
- becomes available, as well as for the reasons given in S 6.2.5. An example of
- such a use appears in Figure 12/Q.704.
- Figure 12/Q.704 - CCITT 35860
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- In this example, on failure of signalling link AB, traffic towards the
- destination D was directed to signalling link AC. When AB becomes available, the
- point A considers itself as the neighbour of a point which restarts and applies
- the signalling point restart procedure (see S 9).
- 6.4.2 When changeback is initiated after the signalling point restart procedure,
- the adjacent signalling point of the point which is restarting stops traffic to
- be directed from the alternative signalling link(s) for a time T3, after which it
- starts traffic on the signalling link(s) made available. The time delay minimizes
- the probability of out-of-sequence delivery to the destination point(s).
- 6.5 Procedures in abnormal conditions
- 6.5.1 If a changeback acknowledgement is received by a signalling point which
- has not previously sent a changeback declaration, no action is taken.
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- 6.5.2 If a changeback declaration is received after the completion of the
- changeback procedure, a changeback acknowledgement is sent in response, without
- taking any further action. This corresponds to the normal action described in S
- 6.3.2 above.
- 6.5.3 If no changeback acknowledgement is received in response to a changeback
- declaration within a time T4 (see S 16.8), the changeback declaration is repeated
- and a new timer T5 (see S 16.8), is started. If no changeback acknowledgement is
- received before the expiry of T5, the maintenance functions are alerted and
- traffic on the link made available is started. The changeback code contained in
- the changeback acknowledgement message makes it possible to determine, in the
- case of parallel changebacks from more than one reserve path, which changeback
- declaration is unacknowledged and has therefore to be repeated.
- 7 Forced rerouting
- 7.1 General
- 7.1.1 The objective of the forced rerouting procedure is to restore, as quickly
- as possible, the signalling capability between two signalling points towards a
- particular destination, in such a way as to minimize the consequences of a
- failure. However, since the unavailability of a signalling route is, in general,
- caused by the fact that the concerned destination has become inaccessible to a
- signalling transfer point, a probability of message loss exists (see S 5.3.3).
- Therefore, the structure of the signalling network should be such as to reduce
- the probability of signalling route unavailability to limits compatible with the
- overall dependability requirements (see Recommendation Q.706).
- 7.1.2 Forced rerouting is the basic procedure to be used in the case where a
- signalling route towards a given destination becomes unavailable (due to, for
- example, remote failures in the signalling network) to divert signalling traffic
- towards that destination to an alternative signalling route outgoing from the
- concerned signalling point. Signalling links pertaining to the alternative
- signalling route can be carrying their own signalling traffic (relating to
- different signalling routes), and this is not interrupted by the forced rerouting
- procedure.
- 7.2 Forced rerouting initiation and actions
- 7.2.1 Forced rerouting is initiated at a signalling point when a
- transfer-prohibited message, indicating a signalling route unavailability is
- received.
- The following actions are then performed:
- a) transmission of signalling traffic towards the concerned destination on
- the link set(s) pertaining to the unavailable route is immediately
- stopped; such traffic is stored in a forced rerouting buffer;
- b) the alternative route is determined according to the rules specified in
- S 4;
- c) as soon as action b) is completed, the concerned signalling traffic is
- restarted on a link set pertaining to the alternative route, starting
- with the content of the forced rerouting buffer;
- d) if appropriate, a transfer-prohibited procedure is performed (see S
- 13.2.2).
- 7.2.2 In the case when there is no signalling traffic to be diverted from the
- unavailable route, action b) and d) apply.
- 7.2.3 If no alternative route exists for signalling traffic towards the
- concerned destination, that destination is declared inaccessible, and the actions
- specified in S 5.3.3 apply.
- 8 Controlled rerouting
- 8.1 General
- 8.1.1 The objective of the controlled rerouting procedure is to restore the
- optimal signalling routing and to minimize mis-sequencing of messages. Therefore,
- controlled rerouting includes a time-controlled traffic diversion procedure,
- which is the same as that used in some cases of changeback (see S 6.4).
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- 8.1.2 Controlled rerouting is the basic procedure to be used in the following
- two cases:
- a) when a signalling route towards a given destination becomes available
- (due to, for example, recovery of previous remote failures in the
- signalling network), to divert back signalling traffic towards that
- destination from the alternative to the normal signalling route
- outgoing from the concerned signalling point;
- b) when a transfer-restricted message is received, after signalling
- traffic management has decided that alternative routing is appropriate
- (e.g., because it would be more efficient than routing via the link set
- over which the transfer-restricted message was received).
- Signalling links pertaining to the alternative signalling route can be
- carrying their own signalling traffic (relating to different routes) and this is
- not interrupted by the controlled rerouting procedure.
- 8.2 Controlled rerouting initiation and actions
- 8.2.1 Controlled rerouting is initiated at a signalling point when a
- transfer-allowed message, indicating that the signalling route has become
- available, is received; also when a transfer-restricted message12) is received.
- The following actions are then performed:
- a) transmission of signalling traffic towards the concerned destination on
- the link set belonging to the alternative route or the route over which
- the transfer-restricted12) message was received is stopped; such
- traffic is stored in a "controlled rerouting buffer"; a timer T6 (see S
- 16.8), is started;
- b) if the signalling point serves as a signalling transfer point, a
- transfer-prohibited procedure is performed for the route made available
- (or the alternative route in the case of reception of a
- transfer-restricted12) message, if the alternative route was not
- previously used), and a transfer-allowed procedure for the alternative
- one (or on the restricted route in the case of the reception of a
- transfer-restricted12) message) (see SS 13.2.2 and 13.3.2,
- respectively);
- c) at the expiry of T6, the concerned signalling traffic is restarted on
- an outgoing link set pertaining to the signalling route made available,
- or the alternative route in the case of reception of the
- transfer-restricted12) message, starting with the content of the
- controlled rerouting buffer; the aim of the time delay is to minimize
- the probability of out-of-sequence delivery to the destination
- point(s).
- 8.2.2 In the case when there is no signalling traffic to be diverted from the
- route made available, only action b) applies.
- 8.2.3 If the destination was inaccessible or restricted12), when the route is
- made available, then the destination is declared accessible and actions specified
- in SS 6.2.3 and 6.2.4 apply (if appropriate).
- 9 Signalling point restart
- This procedure uses the Traffic Restart Allowed message (TRA) which
- contains:
- - the label indicating the originating signalling point and adjacent
- destination signalling point;
- - the traffic restart allowed signal.
- The format and coding of this message appear in S 15.
- 9.1 Actions in a signalling point (having the transfer function) which
- restarts
- A signalling point restarts when it becomes available (see S 3.6.2.1). A
- signalling point which restarts starts a timer T18 and starts activating all its
- signalling links (see S 12).
- When the first signalling link of a signalling link set is available,
- message traffic terminating at the far end of the linkset is immediately
- restarted (see also S 9.5).
- The restarting signalling point takes into account any transfer
- prohibited, transfer restricted (see S 13) and traffic restart allowed messages
- received.
- When all signalling links are available T18 is stopped.
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- When T18 is stopped or expires, the following actions are taken:
- - the signalling point starts a timer T19 during which it expects to
- receive additional transfer prohibited, transfer restricted (see S 13)
- and traffic restart allowed messages;
- - when all traffic restart allowed messages are received T19 is stopped.
- When T19 is stopped or expires, the signalling point starts a timer T20
- during which:
- - it broadcasts eventually transfer prohibited and transfer restricted13
- messages (see S 13), taking into account signalling links which are not
- available a d any transfer prohibited and transfer restricted13)
- messages eventually received;
- - when all these operations are completed, timer T20 is stopped.
- When T20 is stopped or expires, the signalling point broadcasts traffic
- restart allowed messages to all adjacent signalling points and restarts the
- remaining traffic.
- 9.2 Actions in a restarting signalling point (having no transfer function)
- An SP which restarts starts a timer T21 and starts activating all its
- signalling links (see S 12).
- When the first signalling link of a signalling linkset is available,
- message traffic terminating at the far end of the linkset is immediately
- restarted (see also S 9.5).
- The restarting signalling point takes into account transfer prohibited and
- transfer restricted messages13) (see S 13). If a traffic restart allowed message
- is received T21 is stopped. When T21 is stopped or expires, the signalling point
- restarts the remaining traffic.
- 9.3 Actions in a signalling point X adjacent to a restarting signalling point
- Y
- Signalling point X knows that signalling point Y is restarting when
- signalling point Y becomes accessible (see S 3.6.2.2). There are three cases to
- consider:
- i) Signalling points X and Y have the transfer function
- a) When signalling point Y becomes accessible because a direct
- linkset becomes available, signalling point X takes the
- following action:
- - starts a timer T21
- - immediately restarts traffic terminating in signalling point Y
- (see also S 9.5)
- - sends any eventual transfer prohibited and transfer
- restricted13) messages to signalling point Y (see S 13)
- - sends a traffic restart allowed message to signalling point Y
- - takes into account the eventual transfer prohibited and
- transfer restricted13) messages received from SP Y (see S 13).
- When a traffic restart allowed message is received from
- signalling point Y, timer T21 is stopped. When T21 is stopped or
- expires, signalling point X restarts any remaining traffic to Y,
- and broadcasts transfer allowed messages concerning Y, and all SPs
- made accessible via Y.
- b) When signalling point Y becomes accessible on reception of a
- transfer allowed or transfer restricted13) message (s e S 13),
- signalling point X sends to signalling point Y any required
- transfer prohibited and transfer restricted messages1 ) on the
- available route.
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- ii) Signalling point X has a transfer function and signalling point Y has
- not
- a) When signalling point Y becomes accessible because a direct
- signalling linkset becomes available, signalling point X takes
- the following actions:
- - immediately restarts traffic terminating in signalling point Y
- (see also S 9.5)
- - eventually sends to signalling point Y any transfer prohibited
- and transfer restricted messages (see S 13)
- - broadcasts transfer allowed messages concerning signalling
- point Y and sends a traffic restart allowed message to it.
- b) When signalling point Y becomes accessible on reception of a
- transfer allowed or transfer restricted1 ) message, signalling
- point X sends to signalling point Y any required transfer
- prohibited and transfer restricted14) messages on t e available
- route.
- iii) Signalling point X does not have the transfer function and
- signalling point Y does or does not have the transfer function.
- Signalling point X takes the following action:
- - immediately restarts traffic terminating at signalling point Y
- (see also S 9.5)
- - starts a timer T21
- - takes into account any eventual transfer prohibited and transfer
- restricted14) message received.
- On the receipt of a traffic restart allowed message, timer T21 is
- stopped. When T21 is stopped or expires, signalling point X restarts
- any remaining traffic.
- 9.4 Actions in signalling point X on receipt of unexpected TRA message
- If X has no STP function, no further action is taken.
- If X has the STP function, then X sends to the adjacent point Y, from
- which the TRA message was received, the appropriate TFP and TFR messages. X then
- operates normally.
- 9.5 General rules
- When a signalling point restarts, it considers, at the beginning of the
- point restart procedure, all signalling routes to be allowed. A signalling route
- set test message received in a restarting signalling point (during the point
- restart procedure) is ignored.
- Signalling route set test messages received in a signalling point adjacent
- to a restarting signalling point (before T21 expires) are handled, but the
- replies consider that all signalling routes using the restarting point are
- prohibited. When T21 is stopped or expires these signalling routes are allowed
- unless a transfer prohibited or transfer restricted14) message has been received
- from the restarting signalling point during T21.
- The procedure includes the general rule that late events [e.g.,
- restoration of a link after T18 expires, transfer prohibited or transfer
- restricted14) messages received after T19 expires, etc.] are treated outside the
- restart procedure.
- All messages concerning another destination received in a restarting
- signalling point are treated normally during the point restart procedure. All
- messages concerning a local MTP user received in a restarting signalling point
- (Service Indicator != 0000) are treated normally. All messages received with
- Service Indicator = 0000 in a restarting signalling point, for the signalling
- point itself, are treated as described in the signalling point restart procedure;
- those messages not described elsewhere in the procedure are discarded and no
- action is taken (message groups CHM, ECM, FCM, RSM, MIM and DLM).
- 10 Management inhibiting
- 10.1 General
- Signalling link management inhibiting is requested by management when it
- becomes necessary e.g., for maintenance or testing purposes (for example, if the
- link experiences too many changeovers and changebacks in a short time, or there
- is a significant link error rate), to make or keep a signalling link unavailable
- to User Part-generated signalling traffic. Management inhibiting is a signalling
- traffic management action, and does not cause any link status changes at level 2.
- A signalling link is marked "inhibited" under the management inhibiting
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- procedure. In particular, a signalling link that was active and in service prior
- to being inhibited will remain so, and will thus be able to transmit maintenance
- and test messages.
- Inhibiting of a signalling link may be requested by management functions
- at either end of the link. The request is granted, provided that the inhibiting
- action does not cause any previously accessible destinations to become
- inaccessible at either end of the signalling link. The request may also be
- refused under certain circumstances such as congestion.
- A signalling link normally remains inhibited until uninhibiting is invoked
- in the signalling point at which inhibiting was initiated. Uninhibiting is
- initiated either at the request of a management function or by routing functions
- at either end of the signalling link when it is found that a destination has
- become inaccessible for signalling traffic and the link sets associated with
- routes to that destination contain inhibited links. Unless unavailable for other
- reasons, uninhibiting causes the signalling link to enter the available state and
- changeback to be initiated.
- Periodic tests are made on the inhibit status of inhibited links. Such
- periodic tests should not add significantly to the traffic load on the signalling
- network, and remove the need for a signalling point to perform inhibit tests at
- signalling point restart.
- If a test on the inhibit status of a link reveals discrepancies between
- the signalling points at each end of the link, the link is either uninhibited or
- force uninhibited as appropriate, to align the inhibit status at each end of the
- link.
- 10.2 Inhibiting initiation and actions
- When at signalling point "X" a request is received from a management
- function to inhibit a signalling link to signalling point "Y", the following
- actions take place:
- a) A check is performed at signalling point "X" to determine whether, in
- the case of an available link, inhibiting will result in a destination
- becoming inaccessible, or in the case of an unavailable link,
- signalling point "Y" is inaccessible. If either is the case, management
- is informed that the inhibiting request is denied.
- b) If inhibiting is permitted, signalling point "X" sends an inhibit
- message to signalling point "Y" indicating that it wishes to inhibit
- the signalling link identified in the message.
- c) Signalling point "Y", on receiving the inhibit message from "X", checks
- whether, in the case of an available link, inhibiting will result in a
- destination becoming inaccessible and, if so, an inhibit denied message
- is returned to signalling point "X". The latter then informs the
- management function which requested inhibiting that the request cannot
- be granted.
- d) If the signalling point "Y" finds that inhibiting of the concerned link
- is permissible, it sends an inhibit acknowledgement to signalling point
- "X" and marks the link remotely inhibited.
- If the link concerned is currently carrying traffic, signalling point
- "Y" sends the inhibit acknowledgement via that link and diverts
- subsequent traffic for it, using the time controlled changeover
- procedure. "Y" then starts inhibit test timer T23.
- e) On receiving an inhibit acknowledgement message, signalling point "X"
- marks the link locally inhibited and informs management that the link
- is inhibited.
- If the link concerned is currently carrying traffic, signalling point
- "X" diverts subsequent traffic for that link, using the time-controlled
- changeover procedure. "X" then starts inhibit test timer T22.
- f) When changeover has been completed, the link while inhibited, will be
- unavailable for the transfer of user-generated traffic but still
- permits the exchange of test messages.
- g) If, for any reason, the inhibit acknowledgement message is not
- received, a timer T14 expires and the procedure is restarted including
- inspection of the status of the destination of the inhibit message. If
- the destination is not available, management is informed.
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- At most two consecutive automatic attempts may be made to inhibit a
- particular signalling link.
- A signalling point may not transmit an inhibit message for a particular
- signalling link if it has already transmitted an uninhibit message for that link,
- and neither an acknowledgement for that uninhibit message has been received nor
- has the uninhibit procedure finally timed out.
- 10.3 Uninhibiting initiation and actions
- Signalling link uninhibiting is initiated at the signalling point which
- originally caused the link to be inhibited, upon receipt of an uninhibit or
- forced uninhibit request.
- In a given signalling point, an uninhibit request may be initiated for a
- locally inhibited link by the management or signalling routing control function,
- while a forced uninhibit request may be initiated for a remotely inhibited link
- by the signalling routing control function only.
- Signalling routing control will initiate signalling link uninhibit if an
- inhibited link is found to be a member of a link set in a route to a destination
- which has become inaccessible.
- If such signalling routing control uninhibiting were unsuccessful because
- of a failed or blocked inhibited link, and if that link later recovers or becomes
- unblocked with the destination still unavailable, uninhibiting is re-attempted.
- A signalling point may not transmit an uninhibit message for a particular
- signalling link if it has already transmitted an inhibit message for that link,
- and neither an acknowledgement for that inhibit message has been received nor has
- the inhibit procedure finally timed out.
- 10.3.1 Management-initiated uninhibiting
- Upon receipt of an uninhibiting request from the management function of
- signalling point "X" regarding an inhibited link to signalling point "Y", the
- following actions take place:
- a) A check is performed at signalling point "X" to determine whether an
- uninhibit message can be sent to signalling point "Y", either over an
- available route, or if all routes to signalling point "Y" are
- unavailable, over the concerned inhibited link. If all routes to
- signalling point "Y" are unavailable and the concerned inhibited link
- is marked failed or processor outage, management is informed that
- uninhibiting is not possible.
- b) If uninhibiting is possible, signalling point "X" sends an uninhibit
- signalling link message to signalling point "Y" indicating that the
- link identified in the message should be uninhibited.
- c) Upon receipt of the uninhibit link message, signalling point "Y"
- returns an uninhibit acknowledgement message to signalling point "X"
- and cancels the remote inhibit indication. If no local inhibited,
- failed or blocked condition exists on the link, it is put in the
- available state and changeback is initiated.
- d) On receipt of the uninhibit acknowledgement message, signalling point
- "X" cancels the local inhibit indication and informs management that
- the link has been uninhibited. If no remote inhibited, failed or
- blocked condition exists on the link, it is put in the available state
- and changeback is initiated.
- e) If, for any reason, the uninhibit acknowledgement message is not
- received, a timer T12 expires. If this is the first expiry of T12 for
- this uninhibition attempt on this link, the procedure is restarted
- including inspection of the status of the destination of the unhibit
- message. If the destination is not available, or T12 has expired for
- the second time during the uninhibition attempt on this link,
- management is informed, and the uninhibition is abandoned.
- 10.3.2 Signalling routing control initiated uninhibiting
- Upon receipt of an uninhibit request from signalling routing control at
- signalling point "X" regarding an inhibited link to signalling point "Y", the
- following actions take place:
- a) A check is performed at signalling point "X" to determine whether the
- concerned inhibited link is marked failed or blocked. If it is, then
- signalling point "X" is unable to transmit an uninhibit message to
- signalling point "Y", uninhibiting is therefore not possible, and the
- uninhibiting attempt is abandoned.
- b) If uninhibiting is possible, a further check is performed by signalling
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- point "X" to determine whether inhibiting initiated by "X" (local
- inhibiting) or inhibiting initiated by "Y" (remote inhibiting) is in
- effect.
- c) If local inhibiting is in effect, then the actions described in SS
- 10.3.1 b), c), d) and e) take place. If uninhibition is abandoned, step
- f) below is taken.
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- d) If remote inhibiting is in effect, then signalling point "X" requests
- forced uninhibiting of the signalling link by sending a force uninhibit
- signalling link message to signalling point "Y", which will then
- initiate uninhibiting in accordance with the description given in SS
- 10.3.1 b), c), d) and e).
- The force uninhibit signalling link message is transmitted down the link
- to be uninhibited.
- e) If, for any reason, an uninhibit signalling link message is not
- received in response to the force uninhibit message, a timer T13
- expires. If this is the first expiry of T13 for this uninhibition
- attempt on this link, the procedure is restarted including inspection
- of the status of the inhibited link. If the link is marked failed or
- blocked, or timer T13 has expired for the second time during
- uninhibition of this link, management is informed and the uninhibition
- is abandoned.
- f) If an attempt to uninhibit a signalling link is abandoned, signalling
- routing control attempts to uninhibit the next inhibited link to
- signalling point "Y", starting from a) above. The search continues
- until either a link is successfully uninhibited or all possible links
- to "Y" in the routing table have been exhausted, or the destination has
- become available for other reasons.
- 10.4 Receipt of unexpected management inhibition messages
- a) An inhibit signalling link message concerning an inhibited signalling
- link is answered with an inhibit acknowledgement message without taking
- any further action.
- b) An uninhibit signalling link message concerning an uninhibited
- signalling link is answered with an uninhibit acknowledgement message
- without taking any further action.
- c) A force uninhibit signalling link message concerning an uninhibited
- link is answered with an uninhibit signalling link message without
- taking any further action.
- d) If an inhibit acknowledgement message is received and no inhibit
- signalling link message is outstanding for the concerned link, no
- action is taken.
- e) If an uninhibit acknowledgement message is received and no uninhibit
- signalling link message is outstanding for the concerned link, no
- action is taken.
- 10.5 Management inhibited link status and processor recovery
- a) After a local processor recovery that involves loss of inhibit status
- information, the signalling point will mark all links as uninhibited,
- and message traffic will be restarted.
- b) If messages for Level 4 are received on an inhibited signalling link,
- the messages will be discriminated and distributed.
- 10.6 Inhibit test procedure
- When a signalling link becomes management inhibited, periodic tests are
- started to guard the inhibition status at each end of the link.
- 10.6.1 A local inhibit test is performed when timer T22 expires at signalling
- point X and the concerned link is marked locally inhibited. In this case a local
- inhibit test message is sent to the signalling point Y at the other end of the
- link, and timer T22 is restarted.
- Reception of a local inhibit test message causes:
- i) no action, if the concerned link is marked remotely inhibited at the
- receiving signalling point Y or:
- ii) the force uninhibit procedure to be invoked at the receiving signalling
- point Y, if the concerned link is not marked remotely inhibited at Y.
- This procedure causes the locally inhibited status of the link at X to
- be cancelled.
- If a timer T22 expires and the concerned link is not locally inhibited, no
- further action is taken.
- 10.6.2 A remote inhibit test is performed when timer T23 expires at signalling
- point Y and the concerned link is marked remotely inhibited. In this case a
- remote inhibit test message is sent to signalling point X at the other end of the
- link, and timer T23 is restarted.
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- Reception of a remote inhibit test message causes:
- i) no action, if the concerned link is marked locally inhibited at the
- receiving signalling point X or:
- ii) the uninhibit procedure to be invoked at the receiving signalling point
- X, if the concerned link is not marked locally inhibited at X. This
- procedure causes the remotely inhibited status of the link at Y to be
- cancelled.
- If a timer T23 expires and the concerned link is not remotely inhibited,
- no further action is taken.
- 11 Signalling traffic flow control
- 11.1 General
- The purpose of the signalling traffic flow control function is to limit
- signalling traffic at its source in the case when the signalling network is not
- capable of transferring all signalling traffic offered by the user because of
- network failures or congestion situations.
- Flow control action may be taken as a consequence of a number of events;
- the following cases have been identified:
- - Failure in the signalling network (signalling links or signalling
- points) has resulted in routeset unavailability. In this situation,
- flow control may provide a short term remedy until more appropriate
- actions can be taken.
- - Congestion of a signalling link or signalling point has resulted in a
- situation where reconfiguration is not appropriate.
- - Failure of a part has made it impossible for the user to handle
- messages delivered by the Message Transfer Part.
- When the normal transfer capability is restored, the flow control
- functions initiate resumption of the normal traffic flow.
- 11.2 Flow control indications
- The need for the following indications has been identified.
- 11.2.1 Signalling route set unavailability
- In the case when no signalling route is available for traffic towards a
- particular destination (see SS 5.3.3 and 7.2.3) an indication is given from the
- Message Transfer Part to the local user parts informing them that signalling
- messages destined to the particular signalling point cannot be transferred via
- the signalling network. Each user then takes appropriate actions in order to stop
- generation of signalling information destined for the inaccessible signalling
- point.
- 11.2.2 Signalling route set availability
- In the case when a signalling route becomes available for traffic to a
- previously unavailable destination (see SS 6.2.3 and 8.2.3), an indication is
- given from the Message Transfer Part to the local user parts informing them that
- signalling messages destined to the particular signalling point can be
- transferred via the signalling network. Each user then takes appropriate actions
- in order to start generation of signalling information destined for the now
- accessible signalling point.
- 11.2.3 Signalling route set congestion (International signalling network)
- 11.2.3.1 When the congestion status of a signalling route set changes to
- congested, the following actions will be taken:
- i) When a message signal unit from a local User Part is received for a
- congested route set the following actions are performed:
- a) The MSU is passed to level 2 for transmission.
- b) A congestion indication primitive will be returned to each level
- 4 User Part, for the initial message and for at least every n
- messages (n = 8) received for the congested destination. The
- congestion indication primitive contains as a parameter the DPC
- of the affected destination.
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- ii) When a message signal unit is received at an STP for a congested route
- set, the following actions take place:
- a) The MSU is passed to level 2 for transmission.
- b) A transfer controlled message is sent to the originating point
- for the initial message and for every n messages (n = 8)
- received from any originating point for the congested route set
- or for every link of the congested route set or for every
- linkset of the congested route set.
- 11.2.3.2 After the reception of a transfer controlled message, the receiving
- signalling point informs each level 4 User Part of the affected destination by
- means of a congestion indication primitive specified in S 11.2.3.1 i).
- 11.2.3.3 When the status of a signalling route set changes to uncongested,
- normal operation is resumed. Resumption of message transmission towards the
- concerned destination is the responsibility of the level 4 User Parts.
- 11.2.4 Signalling route set congestion (National option with congestion
- priorities)
- In the case when the congestion status of a signalling route set changes
- as a result of either the receipt of a transfer controlled message relating to a
- particular destination (see S 13.7) or an indication of local signalling link
- congestion, or due to the signalling route-set-congestion-test procedure (see S
- 13.9) an indication is given from the Message Transfer Part to the local level 4
- informing it about the current congestion status of the signalling route set.
- Each user then takes appropriate actions in order to stop generation of
- signalling messages destined for the affected signalling point with congestion
- priorities lower than the specified congestion status. Messages received from the
- local level 4 with congestion priorities lower than the current signalling route
- set congestion status are discarded by the Message Transfer Part.
- 11.2.5 Signalling route set congestion (National options without congestion
- priorities)
- For national signalling networks using multiple signalling link congestion
- states without congestion priority, S + 1(1 ú S ú 3) levels of route set
- congestion status are provided.
- The procedure is the same as that specified in S 11.2.3, except that the
- congestion indication primitive contains the congestion status as a parameter in
- addition to the DPC of the affected destination.
- 11.2.6 Signalling point/signalling transfer point congestion
- The detection of congestion onset and abatement in a signalling point or
- signalling transfer point should, if required, be implementation dependent. Any
- resulting action taken, and messages and primitives sent, should align with those
- procedures, messages and primitives specified for signalling route set
- congestion.
- 11.2.7 MTP user flow control
- If the Message Transfer Part is unable to distribute a received message to
- a local User Part because that User Part is unavailable, (User Part
- unavailability is an implementation dependent notion), the Message Transfer Part
- sends a User Part Unavailable (UPU) message to the Message Transfer Part at the
- originating signalling point.
- When the originating signalling point's Message Transfer Part receives a
- User Part Unavailable message, it:
- a) informs the management process,
- b) sends an indication (MTP-STATUS with the appropriate parameters) to the
- affected local User Part informing it that that User Part at the
- particular remote signalling point is unavailable.
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- The user should then take appropriate action in order to stop generation
- of signalling information for the unavailable User Part.
- The User Part Unavailable message contains:
- - the label, indicating the destination and originating points;
- - the user part unavailable signal;
- - the identity of the unavailable user part.
- The format and coding of this message appear in S 15.
- When the Message Transfer Part is again able to distribute received
- messages to a previously unavailable local User Part, that Message Transfer Part
- delivers the received messages to that User Part.
- 11.2.8 User part congestion
- User part congestion procedures in the MTP are for further study.
- 12 Signalling link management
- 12.1 General
- 12.1.1 The signalling link management function is used to control the locally
- connected signalling links. The function provides means for establishing and
- maintaining a certain predetermined capability of a link set. Thus, in the event
- of signalling link failures the signalling link management function controls
- actions aimed at restoring the capability of the link set.
- Three sets of signalling link management procedures are specified in the
- following sections. Each set corresponds to a certain level of automation as
- regards allocation and reconfiguration of signalling equipment. The basic set of
- signalling link management procedures (see S 12.2) provides no automatic means
- for allocation and reconfiguration of signalling equipment. The basic set
- includes the minimum number of functions which must be provided for international
- application of the signalling system.
- Two alternative sets of signalling link management procedures are provided
- as options and include functions allowing for a more efficient use of signalling
- equipment in the case when signalling terminal devices have switched access to
- signalling data links.
- 12.1.2 A signalling link set consists of one or more signalling links having a
- certain order of priority as regards the signalling traffic conveyed by the link
- set (see S 4). Each signalling link in operation is assigned a signalling data
- link and a signalling terminal at each end of the signalling data link.
- The signalling link identity is independent of the identities of the
- constituent signalling data link and signalling terminals. Thus, the identity
- referred to by the Signalling Link Code (SLC) included in the label of messages
- originated at Message Transfer Part level 3 is the signalling link identity and
- not the signalling data link identity or the signalling terminal identity.
- Depending on the level of automation in an application of the signalling
- system, allocation of signalling data link and signalling terminals to a
- signalling link may be made manually or automatically.
- In the first case, applicable for the basic signalling link management
- procedures, a signalling link includes predetermined signalling terminals and a
- predetermined signalling data link. To replace a signalling terminal or
- signalling data link, a manual intervention is required. The signalling data link
- to be included in a particular signalling link is determined by bilateral
- agreement (see also Recommendation Q.702).
- In the second case for a given signalling point, a signalling link
- includes any of the signalling terminals and any of the signalling data links
- applicable to a link group. As a result of, for example, signalling link failure,
- the signalling terminal and signalling data link included in a signalling link,
- may be replaced automatically. The criteria and procedures for automatic
- allocation of signalling terminals and signalling data links are specified in SS
- 12.5 and 12.6 respectively. The implementation of these functions requires that
- for a given link group any signalling terminal can be connected to any signalling
- data link.
- Note - A link group is a group of identical signalling links directly
- connecting two signalling points. A link set may include one or more link groups.
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- 12.1.3 When a link set is to be brought into service, actions are taken to
- establish a predetermined number of signalling links. This is done by connecting
- signalling terminals to signalling data links and for each signalling link
- performing an initial alignment procedure (see Recommendation Q.703, S 7.3). The
- process of making a signalling link ready to carry signalling traffic is defined
- as signalling link activation.
- Activation of a signalling link may also be applicable, for example when a
- link set is to be extended or when a persisting failure makes another signalling
- link in the link set unavailable for signalling traffic.
- In the case of signalling link failure, actions should be taken to restore
- the faulty signalling link, i.e. to make it available for signalling again. The
- restoration process may include replacement of a faulty signalling data link or
- signalling terminal.
- A link set or single signalling link is taken out of service by means of a
- procedure defined as signalling link deactivation.
- The procedures for activation, restoration and deactivation are initiated
- and performed in different ways depending on the level of automation applicable
- for a particular implementation of the signalling system. In the following,
- procedures are specified for the cases when:
- a) no automatic functions are provided for allocation of signalling
- terminals and signalling data links (see S 12.2).
- b) an automatic function is provided for allocation of signalling
- terminals (see S 12.3).
- c) automatic functions are provided for allocation of signalling terminals
- and signalling data links (see S 12.4).
- 12.2 Basic signalling link management procedures
- 12.2.1 Signalling link activation
- 12.2.1.1 In the absence of failures, a link set contains a certain predetermined
- number of active (i.e. aligned) signalling links. In addition, the link set may
- contain a number of inactive signalling links, i.e. signalling links which have
- not been put into operation. Predetermined signalling terminals and a signalling
- data link are associated with each inactive signalling link.
- The number of active and inactive signalling links in the absence of
- failures, and the priority order for the signalling links in a link set, should
- be identical at both ends of the link set.
- Note - In the typical case, all signalling links in a link set are active
- in the absence of failures.
- 12.2.1.2 When a decision is taken to activate an inactive signalling link,
- initial alignment starts. If the initial alignment procedure is successful, the
- signalling link is active and a signalling link test is started. If the
- signalling link test is successful the link becomes ready to convey signalling
- traffic. In the case when initial alignment is not possible, as determined at
- Message Transfer Part level 2 (see Recommendation Q.703, S 7), new initial
- alignment procedures are started on the same signalling link after a time T17
- (delay to avoid the oscillation of initial alignment failure and link restart.
- The value of T17 should be greater than the loop delay and less than timer T2,
- see Recommendation Q.703, S 7.3). If the signalling link test fails, link
- restoration starts until the signalling link is activated or a manual
- intervention is made.
- 12.2.2 Signalling link restoration
- After a signalling link failure is detected, signalling link initial
- alignment will take place. In the case when the initial alignment procedure is
- successful, a signalling link test is started. If the signalling link test is
- successful the link becomes restored and thus available for signalling.
- If initial alignment is not possible, as determined at Message Transfer
- Part level 2 (see Recommendation Q.703, S 7), new initial alignment procedures
- may be started on the same signalling link after a time T17 until the signalling
- link is restored or a manual intervention is made e.g. to replace the signalling
- data link or the signalling terminal.
- If the signalling link test fails, the restoration procedure is repeated
- until the link is restored or a manual intervention made.
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- 12.2.3 Signalling link deactivation
- An active signalling link may be made inactive by means of a deactivation
- procedure, provided that no signalling traffic is carried on that signalling
- link. When a decision has been taken to deactivate a signalling link the
- signalling terminal of the signalling link is taken out of service.
- 12.2.4 Link set activation
- A signalling link set not having any signalling links in service is
- started by means of a link set activation procedure. Two alternative link set
- activation procedures are defined:
- - link set normal activation,
- - link set emergency restart.
- 12.2.4.1 Link set normal activation
- Link set normal activation is applicable when a link set is to be put into
- service for the first time (link set initial activation) or when a link set is to
- be restarted (link set normal restart); the latter is applicable for example in
- the case when:
- - all signalling links in a link set are faulty,
- - a processor restart in a signalling point makes it necessary to
- re-establish a link set,
- - a signalling point recognizes other irregularities concerning the
- interworking between the two signalling points,
- provided that none of the above events create an emergency situation.
- When link set normal activation is initiated, signalling link activation
- starts on as many signalling links as possible. (All signalling links in the link
- set are regarded as being inactive at the start of the procedure.)
- The signalling link activation procedures are performed on each signalling
- link in parallel as specified in S 12.2.1 until the signalling links are made
- active.
- Signalling traffic may, however, commence when one signalling link is
- successfully activated.
- 12.2.4.2 Link set emergency restart
- Link set emergency restart is applicable when an immediate reestablishment
- of the signalling capability of a link set is required, (i.e. in a situation when
- the link set normal restart procedure is not fast enough). The precise criteria
- for initiating link set emergency restart instead of normal restart may vary
- between different applications of the signalling system. Possible situations for
- emergency restart are, for example:
- - when signalling traffic that may be conveyed over the link set to be
- restarted is blocked,
- - when it is not possible to communicate with the signalling point at the
- remote end of the link set.
- When link set emergency restart is initiated, signalling link activation
- starts on as many signalling links as possible, in accordance with the principles
- specified for normal link set activation. In this case, the signalling terminals
- will have emergency status (see Recommendation Q.703, S 7) resulting in the
- sending of status indications of type "E" when applicable. Furthermore, the
- signalling terminals employ the emergency proving procedure and short time-out
- values in order to accelerate the procedure.
- When the emergency situation ceases, a transition from emergency to normal
- signalling terminal status takes place resulting in the employment of the normal
- proving procedure and normal time-out values.
- 12.2.4.3 Time-out values
- The initial alignment procedure (specified in Recommendation Q.703, S 7.3)
- includes time-outs the expiry of which indicates the failure of an activation or
- restoration attempt.
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- 12.3 Signalling link management procedures based on automati
- allocation of signalling terminals
- 12.3.1 Signalling link activation
- 12.3.1.1 In the absence of failures a link set contains a certain predetermined
- number of active (i.e. aligned) signalling links. The link set may also contain a
- number of inactive signalling links.
- An inactive signalling link is a signalling link not in operation. A
- predetermined signalling data link is associated with each inactive signalling
- link; however, signalling terminals may not yet be allocated.
- The number of active and inactive signalling links in the absence of
- failures, and the priority order for the signalling links in a link set, should
- be identical at both ends of the link set.
- 12.3.1.2 Whenever the number of active signalling links is below the value
- specified for the link set, actions to activate new inactive signalling links
- should be taken automatically. This is applicable, for example, when a link set
- is to be brought into service for the first time (see S 12.3.4) or when a link
- failure occurs. In the latter case, activation starts when the restoration
- attemps on the faulty link are considered unsuccessful (see S 12.3.2).
- The signalling link(s) to activate is the inactive link(s) having the
- highest priority in the link set.
- Generally, if it is not possible to activate a signalling link, an attempt
- to activate the next inactive signalling link (in priority order) is made. In the
- case when an activation attempt performed on the last signalling link in the link
- set is unsuccessful, the "next" signalling link is the first inactive signalling
- link in the link set (i.e. there is a cyclic assignment).
- Activation of a signalling link may also be initiated manually.
- Activation shall not be initiated automatically for a signalling link
- previously deactivated by means of a manual intervention.
- 12.3.1.3 When a decision is taken to activate a signalling link, the signalling
- terminal to be employed has to be allocated at each end.
- The signalling terminal is allocated automatically by means of the
- function defined in S 12.5.
- In the case when the automatic allocation function cannot provide a
- signalling terminal the activation attempt is aborted.
- The predetermined signalling data link which may be utilized for other
- purposes when not connected to a signalling terminal must be removed from its
- alternative use (e.g. as a speech circuit) before signalling link activation can
- start.
- 12.3.1.4 The chosen signalling terminal is then connected to the signalling data
- link and initial alignment starts (see Recommendation Q.703, S 7).
- If the initial alignment procedure is successful, the signalling link is
- active and a signalling link test is started. If the signalling link test is
- successful the link becomes ready to convey signalling traffic.
- If initial alignment is not possible, as determined at Message Transfer
- Part level 2 (see Recommendation Q.703, S 7), the activation is unsuccessful and
- activation of the next inactive signalling link (if any) after a time T17 is
- initiated. Successive initial alignment attempts may, however, continue on the
- previous (faulty) signalling link after a time T17 until it is restored or its
- signalling terminal is disconnected (see S 12.5).
- In view of the fact that if it is not possible to activate a signalling
- link an attempt is made to activate the next inactive signalling link in a link
- set, it may be that the two ends of a link set continuously attempt to activate
- different signalling links. By having different values of initial alignment time
- out T2 at the two ends of the link set (see S 12.3.4.3) it is ensured that
- eventually both ends of the link set will attempt to activate the same signalling
- link.
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- 12.3.2 Signalling link restoration
- 12.3.2.1 After a signalling link failure is recognized, signalling link initial
- alignment will take place (see Recommendation Q.703, S 7). In the case when the
- initial alignment is successful, a signalling link test is started. If the
- signalling link test is successful the link becomes restored and thus available
- for signalling. If the initial alignment is unsuccessful or the test fails, the
- signalling terminals and signalling link may be faulty and require replacement.
- 12.3.2.2 The signalling terminal may be automatically replaced in accordance
- with the principles defined for automatic allocation of signalling terminals (see
- S 12.5). After the new signalling terminal has been connected to the signalling
- data link, signalling link initial alignment starts. If successful, the
- signalling link is restored.
- If initial alignment is not possible or if no alternative signalling
- terminal is available for the faulty signalling link, activation of the next
- signalling link in the link set (if any) starts. In the case when it is not
- appropriate to replace the signalling terminal of the faulty signalling link
- (e.g. because it is assumed that the signalling data link is faulty) activation
- of the next inactive signalling link (if any) is also initiated. In both cases
- successive initial alignment attempts may continue on the faulty signalling link
- after a time T17 until a manual intervention is made or the signalling terminal
- is disconnected (see S 12.5).
- Note - In the case when a signalling terminal cannot be replaced,
- activation of the next signalling link is only initiated if the link set includes
- an alternative link group having access to signalling terminals other than the
- one used by the signalling link for which restoration is not possible.
- 12.3.3 Signalling link deactivation
- In the absence of failures a link set contains a specified number of
- active (i.e. aligned) signalling links. Whenever that number is exceeded (e.g. as
- a result of signalling link restoration), the active signalling link having the
- lowest priority in the link set is to be made inactive automatically provided
- that no signalling traffic is carried on that signalling link.
- Deactivation of a particular signalling link may also be initiated
- manually, for example in conjunction with manual maintenance activities.
- When a decision has been taken to deactivate a signalling link, the
- signalling terminal and signalling data link may be disconnected.
- After deactivation, the idle signalling terminal may become part of other
- signalling links (see S 12.5).
- 12.3.4 Link set activation
- A signalling link set not having any signalling links in service is
- started by means of a link set activation procedure. The objective of the
- procedure is to activate a specified number of signalling links for the link set.
- The activated signalling links should, if possible, be the signalling links
- having the highest priority in the link set. Two alternative link set activation
- procedures are defined:
- - link set normal activation,
- - link set emergency restart.
- 12.3.4.1 Link set normal activation
- Link set normal activation is applicable when a link set is to be put into
- service for the first time (link set initial activation) or when a link set is to
- be restarted (link set normal restart); the latter is applicable, for example, in
- the case when:
- - all signalling links in a link set are faulty;
- - a processor restart in a signalling point makes it necessary to
- re-establish a link set;
- - a signalling point recognizes other irregularities concerning the
- interworking between the two signalling points, e.g. that a certain
- signalling data link is associated with different signalling links at
- the two ends of the link set;
- provided that none of the above events create an emergency situation.
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- When link set normal activation is initiated, signalling link activation
- starts on as many signalling links as possible. (All signalling links in the link
- set are regarded as being inactive at the start of the procedure). If activation
- cannot take place on all signalling links in the link set (e.g., because a
- sufficient number of signalling terminals is not available), then the signalling
- links to activate are determined in accordance with the link priority order.
- Note - All idle signalling terminals may not necessarily be made available
- for link set activation. Thus making possible, for example, restoration of faulty
- signalling links in other link sets at the same time.
- The signalling link activation procedures are performed as specified in S
- 12.3.1.
- If the activation attempt for a signalling link is unsuccessful (i.e.
- initial alignment is not possible), activation of the next inactive signalling
- link, if any, in the priority order is initiated. (Inactive links exist in the
- case when the number of signalling terminals available is less than the number of
- signalling links defined for the link set). According to the principles for
- automatic allocation of signalling terminals defined in S 12.5, the signalling
- terminal connected to the unsuccessfully activated signalling link will typically
- be connected to the signalling data link of that signalling link for which the
- new activation attempt is to be made.
- When a signalling link is successfully activated, signalling traffic may
- commence.
- After the successful activation of one signalling link, the activation
- attempts on the remaining signalling links continue in accordance with the
- principles defined in S 12.3.1, in such a way that the signalling links having
- the highest priorities are made active. This is done in order to obtain, if
- possible, the normal configuration within the link set. Signalling link
- activation continues until the predetermined number of active signalling links is
- obtained.
- 12.3.4.2 Link set emergency restart
- Link set emergency restart is applicable in the case the link set normal
- restart procedure is not fast enough. Emergency restart is performed in the same
- way as link set normal activation except that, in the case of emergency restart,
- the emergency proving procedure and the short emergency time-out values (see
- Recommendation Q.703, S 7) are employed in order to accelerate the procedure (see
- further S 12.2.4.2).
- 12.3.4.3 Time-out values
- The values of the initial alignment time-out T2 (see Recommendation Q.703,
- S 7) will be different at the two ends of the link set, if automatic allocation
- of signalling terminals or signalling data links is applied at both ends of a
- signalling link set.
- 12.4 Signalling link manageme t procedures based on automatic
- allocation of signalling data links and signalling terminals
- 12.4.1 Signalling link activation
- 12.4.1.1 In the absence of failures a link set contains a certain predetermined
- number of active (i.e. aligned) signalling links. The link set may also contain a
- number of inactive signalling links.
- An inactive signalling link is a signalling link currently not in
- operation. It is not associated with any signalling terminal or signalling data
- link (i.e. the signalling link is only identified by its position in the link
- set).
- The number of active and inactive signalling links (in the absence of
- failures), and the priority order for the signalling links in a link set, should
- be identical at both ends of the link set.
- 12.4.1.2 Whenever the number of active signalling links is below the value
- specified for the link set, actions to activate new inactive signalling links
- should be taken automatically. This is, for example, applicable when a link set
- is to be brought into service for the first time (see S 12.4.4) or when a link
- failure occurs. In the latter case, activation starts when the restoration
- attempts on the faulty link are considered unsuccessful (see S 12.4.2).
- The signalling link(s) to activate is the inactive link(s) having the
- highest priority in the link set.
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- If it is not possible to activate a signalling link an attempt to activate
- the next inactive signalling link (in priority order) is made. In the case when
- an activation attempt performed on the last signalling link in the link set is
- unsuccessful, the "next" signalling link is the first inactive link in the link
- set (i.e. a cyclic assignment).
- Note - Activation of the next signalling link is only initiated if the
- link set includes an alternative link group, having access to other signalling
- terminals and/or other signalling data links than the signalling link for which
- activation is not possible.
- Activation of a particular signalling link may also be initiated upon
- receiving a request from the remote signalling point, or by a manual request.
- Activation shall not be initiated automatically for a signalling link
- previously inactivated by means of a manual intervention.
- 12.4.1.3 When a decision is taken to activate a signalling link, the signalling
- terminals and signalling data link to be employed have to be allocated.
- A signalling terminal is allocated automatically by means of the function
- defined in S 12.5.
- The signalling data link is allocated automatically by means of the
- function defined in S 12.6. However, in conjunction with link set activation the
- identity of the signalling data link to use may be predetermined (see further
- S 12.4.4). A signalling data link which is not connected to a signalling terminal
- may be utilized for other purposes, e.g. as a speech circuit. When the data link
- is to be employed for signalling, it must be removed from its alternative use.
- In the case when the automatic allocation functions cannot provide a
- signalling terminal or a signalling data link, the activation attempt is aborted.
- 12.4.1.4 When the signalling data link and signalling terminal to be used for a
- particular signalling link are determined, the signalling terminal is connected
- to the signalling data link and signalling link initial alignment starts (see
- Recommendation Q.703, S 7). If the initial alignment procedure is successful, the
- signalling link is active and a signalling link test is started. If the
- signalling link test is successful the link becomes ready to convey signalling
- traffic.
- If initial alignment is not possible, as determined at Message Transfer
- Part level 2 (see Recommendation Q.703, S 7), alternative signalling data links
- are automatically connected to the signalling terminal, until an initial
- alignment procedure is successfully completed. In the case when the function for
- automatic allocation of signalling data links cannot provide an alternative
- signalling data link, the activation is regarded as unsuccessful and activation
- of the next inactive signalling link (if any) is initiated (see, however, the
- Note to S 12.4.1.2 above). Successive initial alignment attempts may continue on
- the previous signalling link after a time T17 until it is activated or its
- signalling terminal is disconnected (see S 12.5).
- 12.4.2 Signalling link restoration
- 12.4.2.1 After a signalling link failure is recognized, signalling link initial
- alignment will take place (see Recommendation Q.703, S 7). In the case when the
- initial alignment is successful, a signalling link test is started. If the
- signalling link test is successful the link becomes restored and thus available
- for signalling.
- If the initial alignment is unsuccessful or if the test fails the
- signalling terminal and signalling data link may be faulty and require
- replacement.
- 12.4.2.2 The signalling data link may be automatically replaced by an
- alternative, in accordance with the principles defined in S 12.6. After the new
- signalling data link has been connected to the signalling terminal, signalling
- link initial alignment starts. If successful, the signalling link is restored. If
- not, alternative data links are connected to the signalling terminal, until an
- initial alignment procedure is successfully completed.
- If the automatic allocation function cannot provide a new signalling data
- link, activation of the next inactive signalling link (if any) is initiated (see,
- however, the Note to S 12.4.1.2). Successive initial alignment attempts may,
- however, continue on the previous (faulty) signalling link after a time T17 until
- it is restored or its signalling terminal is disconnected.
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- 12.4.2.3 The signalling terminal may be automatically replaced in accordance
- with the principles defined in S 12.5. After the new signalling terminal has been
- connected to the signalling data link, signalling link initial alignment starts.
- If successful, the signalling link is restored. If not, activation of the next
- signalling link in the link set (if any) starts (see, however, the Note to S
- 12.4.1.2).
- Successive initial alignment attempts may, however, continue on the
- previous (faulty) signalling link after a time T17 until it is restored or, for
- example, the signalling terminal or signalling data link is disconnected.
- Note - Activation of the next signalling link in the link set should not
- be initiated as long as one of the activities described in SS 12.4.2.2 and
- 12.4.2.3 above is taking place.
- 12.4.3 Signalling link deactivation
- In the absence of failures, a link set contains a specified number of
- active (i.e. aligned) signalling links. Whenever that number is exceeded (e.g. as
- a result of signalling link restoration) the active signalling link having the
- lowest priority in the link set is to be made inactive automatically, provided
- that no signalling traffic is carried on that signalling link.
- Deactivation of a particular signalling link may also be initiated
- manually, e.g. in conjunction with manual maintenance activities.
- When a decision has been taken to deactivate a signalling link, the
- signalling terminal and signalling data link may be disconnected. After
- deactivation the idle signalling terminal and signalling data link may become
- parts of other signalling links (see SS 12.5 and 12.6).
- 12.4.4 Link set activation
- Link set activation is applicable in the case when a link set not having
- any signalling links in service is to be started for the first time or after a
- failure (see S 12.3.4). The link set activation procedure is performed as
- specified in S 12.3.4, also as regards the allocation of signalling data links,
- i.e. signalling data links are allocated in accordance with predetermined list
- assigning a signalling data link to some or all of the signalling links in the
- link set. This is done in order to cater for the situation when it is not
- possible to communicate with the remote end of the link set (see S 12.6).
- However, when a signalling link has become active, signalling data link
- allocation may again be performed automatically (i.e. activation of a signalling
- link takes place as specified in S 12.4.1).
- 12.5 Automatic allocation of signalling terminals
- In conjunction with the signalling link activation and restoration
- procedures specified in SS 12.3 and 12.4, signalling terminals may be allocated
- automatically to a signalling link. A signalling terminal applicable to the link
- group is allocated in accordance with the following principles:
- a) an idle signalling terminal (i.e. a signalling terminal not connected
- to a signalling data link) is chosen if possible;
- b) if no idle signalling terminal is available, a signalling terminal is
- chosen which is connected to an unsuccessfully restored or activated
- signalling link.
- Note - Activation and restoration is regarded as unsuccessful when it is
- not possible to complete the initial alignment procedure successfully (see SS
- 12.3 and 12.4).
- Measures should be employed to ensure that signalling terminal to be
- allocated to signalling links are able to function correctly (see Recommendation
- Q.707).
- A link set may by assigned a certain number of signalling terminals. A
- signalling terminal may be transferred from a signalling link in one link set to
- a signalling link in another set [in accordance with b) above] only when the
- remaining number of signalling terminals in the link set is not below the
- specified value.
- Note - From a link set with a minimum number of signalling terminals, only
- one signalling terminal and signalling data link may be removed at a time (e.g.
- for testing, see Recommendation Q.707).
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