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- All drawings appearing in this Fascicle have been done in Autocad.
- Recommendation Q.704
- SIGNALLING NETWORK FUNCTIONS AND MESSAGES
- 1 Introduction
- 1.1 General characteristics of the signalling network functions
- 1.1.1 This Recommendation describes the functions and procedures for and
- relating to the transfer of messages between the signalling points, which are the
- nodes of the signalling network. Such functions and procedures are performed by
- the Message Transfer Part at level 3, and therefore they assume that the
- signalling points are connected by signalling links, incorporating the functions
- described in Recommendations Q.702 and Q.703. The signalling network functions
- must ensure a reliable transfer of the signalling messages, according to the
- requirements specified in Recommendation Q.706, even in the case of the failure
- of signalling links and signalling transfer points; therefore, they include the
- appropriate functions and procedures necessary both to inform the remote parts of
- the signalling network of the consequences of a fault, and to appropriately
- reconfigure the routing of messages through the signalling network.
- 1.1.2 According to these principles, the signalling network functions can be
- divided into two basic categories, namely:
- - signalling message handling, and
- - signalling network management.
- The signalling message handling functions are briefly summarized in S 1.2,
- the signalling network management functions in S 1.3. The functional
- interrelations between these functions are indicated in Figure 1/Q.704.
- 1.2 Signalling message handling
- 1.2.1 The purpose of the signalling message handling functions is to ensure that
- the signalling messages originated by a particular User Part at a signalling
- point (originating point) are delivered to the same User Part at the destination
- point indicated by the sending User Part.
- Depending on the particular circumstances, this delivery may be made
- through a signalling link directly interconnecting the originating and
- destination points, or via one or more intermediate signalling transfer points.
- 1.2.2 The signalling message handling functions are based on the label contained
- in the messages which explicitly identifies the destination and originating
- points.
- The label part used for signalling message handling by the Message
- Transfer Part is called the routing label; its characteristics are described in S
- 2.
- 1.2.3 As illustrated in Figure 1/Q.704, the signalling message handling
- functions are divided into:
- - the message routing function, used at each signalling poin
- to determine the outgoing signalling link on which a
- message has to be sent towards its destination point;
- - the message discrimination function, used at a signalling
- point to determine whether or not a received message is
- destined to the point itself. When the signalling point has
- the transfer capability and a message is not destined to
- it, that message has to be transferred to the message
- routing function;
- - the message distribution function, used at each signalling
- point to deliver the received messages (destined to the
- point itself) to the appropriate User Part.
- The characteristics of the message routing, discrimination and
- distribution functions are described in S 2.
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- Fascicle VI.7 - Rec. Q.704 PAGE1
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- Figure 1/Q.704 - CCITT 35730
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- 1.3 Signalling network management
- 1.3.1 The purpose of the signalling network management functions is to provide
- reconfiguration of the signalling network in the case of failures and to control
- traffic in case of congestion. Such a reconfiguration is effected by use of
- appropriate procedures to change the routing of signalling traffic in order to
- bypass the faulty links or signalling points; this requires communication between
- signalling points (and, in particular, the signalling transfer points) concerning
- the occurrence of the failures. Moreover, in some circumstances it is necessary
- to activate and align new signalling links, in order to restore the required
- signalling traffic capacity between two signalling points. When the faulty link
- or signalling point is restored, the opposite actions and procedures take place,
- in order to reestablish the normal configuration of the signalling network.
- 1.3.2 As illustrated in Figure 1/Q.704, the signalling network management
- functions are divided into:
- - signalling traffic management,
- - signalling link management, and
- - signalling route management.
- These functions are used whenever an event (such as the failure or
- restoration of a signalling link) occurs in the signalling network; the list of
- the possible events and the general criteria used in relation to each signalling
- network management function are specified in S 3.
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- 1.3.3 SS 4 to 11 specify the procedures pertaining to signalling traffic
- management. In particular, the rules to be followed for the modification of
- signalling routing appear in S 4. The diversion of traffic according to these
- rules is made, depending on the particular circumstances, by means of one of the
- following procedures: changeover, changeback, forced rerouting, controlled
- rerouting and signalling point restart. They are specified in SS 5 to 9
- respectively. A signalling link may be made unavailable to User Part generated
- traffic by means of the management inhibiting procedure described in S 10.
- Moreover, in the case of congestion at signalling points, the signalling traffic
- management may need to slow down signalling traffic on certain routes by using
- the signalling traffic flow control procedure specified in S 11.
- 1.3.4 The different procedures pertaining to signalling link management are:
- restoration, activation and inactivation of a signalling link, link set
- activation and automatic allocation of signalling terminals and signalling data
- links. These procedures are specified in S 12.
- 1.3.5 The different procedures pertaining to signalling route management are:
- the transfer-prohibited, transfer-allowed, transfer-restricted1),
- transfer-controlled, signalling-route-set-test and signalling-route
- -set-congestion-testprocedures specified in S 13.
- 1.3.6 The format characteristics, common to all message signal units which are
- relevant to the Message Transfer Part, level 3, are specified in S 14.
- 1.3.7 Labelling, formatting and coding of the signalling network management
- messages are specified in S 15.
- 1.3.8 The description of signalling network functions in the form of state
- transition diagrams according to the CCITT Specification and Description Language
- (SDL) is given in S 16.
- 2 Signalling message handling
- 2.1 General
- 2.1.1 Signalling message handling comprises message routing, discrimination and
- distribution functions which are performed at each signalling point in the
- signalling network.
- Message routing is a function concerning the messages to be sent, while
- message distribution is a function concerning the received messages. The
- functional relations between message routing and distribution appear in
- Figure 2/Q.704.
- Figure 2/Q.704 - CCITT 35740
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- Fascicle VI.7 - Rec. Q.704 PAGE1
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- 2.1.2 When a message comes from level 4 (or is originated at level 3, in the
- case of Message Transfer Part level 3 messages), the choice of the particular
- signalling link on which it has to be sent is made by the message routing
- function. When two or more links are used at the same time to carry traffic
- having a given destination, this traffic is distributed among them by the load
- sharing function, which is a part of the message routing function.
- 2.1.3 When a message comes from level 2, the discrimination function is
- activated, in order to determine whether it is destined to another signalling
- point. When the signalling point has the transfer capability and the received
- message is not destined to it, the message has to be transmitted on an outgoing
- link according to the routing function.
- 2.1.4 In the case that the message is destined to the receiving signalling
- point, the message distribution function is activated in order to deliver it to
- the appropriate User Part (or to the local Message Transfer Part level 3
- functions).
- 2.1.5 Message routing, discrimination and distribution are based on the part of
- the label called the routing label, on the service indicator and, in national
- networks, also on the network indicator. They can also be influenced by different
- factors, such as a request (automatic or manual) obtained from a management
- system.
- 2.1.6 The position and coding of the service indicator and of the network
- indicator are described in S 14.2. The characteristics of the label of the
- messages pertaining to the various User Parts are described in the specification
- of each separate User Part and in S 15 for the signalling network management
- messages. The label used for signalling network management messages is also used
- for testing and maintenance messages (see Recommendation Q.707). Moreover, the
- general characteristics of the routing label are described in S 2.2.
- A description of the detailed characteristics of the message routing
- function, including load sharing, appears in S 2.3; principles concerning the
- number of load-shared links appear in Recommendation Q.705.
- A description of the detailed characteristics of the message
- discrimination and distribution functions appears in S 2.4.
- 2.1.7 In addition to the normal signalling message handling procedures it may,
- as an option, be possible to prevent the unauthorized use of the message transfer
- capability of a node. The procedures to be used are implementation-dependent and
- further information is given in Recommendation Q.705, S 8.
- 2.2 Routing label
- 2.2.1 The label contained in a signalling message, and used by the relevant User
- Part to identify the particular task to which the message refers (e.g. a
- telephone circuit), is also used by the Message Transfer Part to route the
- message towards its destination point.
- The part of the message label that is used for routing is called the
- routing label and it contains the information necessary to deliver the message to
- its destination point.
- Normally the routing label is common to all the services and applications
- in a given signalling network, national or international (however, if this is not
- the case, the particular routing label of a message is determined by means of the
- service indicator).
- The standard routing label is specified in the following. This label
- should be used in the international signalling network and is applicable also in
- national applications.
- Note - There may be applications using a modified label having the same
- order and function, but possibly different sizes, of sub-fields as the standard
- routing label.
- 2.2.2 The standard routing label has a length of 32 bits and is placed at the
- beginning of the Signalling Information Field. Its structure appears in Figure
- 3/Q.704.
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- Figure 3/Q.704 - CCITT 35750
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- 2.2.3 The destination point code (DPC) indic s the destination
- point of the message. The originating point code (OPC) indicates
- the originating point of the message. The coding of these codes is
- pure binary. Within each field, the least significant bit occupies
- the first position and is transmitted first.
- A unique numbering scheme for the coding of the fields will be
- used for the signalling points of the international network,
- irrespective of the User Parts connected to each signalling point.
- 2.2.4 The signalling link selection (SLS) field is used, where
- appropriate, in performing load sharing (see S 2.3). This field
- exists in all types of messages and always in the same position.
- The only exception to this rule is some Message Transfer Part level
- 3 messages (e.g., the changeover order), for which the message
- routing function in the signalling point of origin of the message
- is not dependent on the field: in this particular case the field
- does not exist as such, but it is replaced by other information
- (e.g., in the case of the changeover order, the identity of the
- faulty link).
- In the case of circuit related messages of the TUP, the field
- contains the least significant bits of the circuit identification
- code (or bearer identification code, in the case of the Data User
- Part), and these bits are not repeated elsewhere. In the case of
- all other User Parts, the SLS is an independent field in accordance
- with the criteria stated in S 2.2.5.
- In the case of Message Transfer Part level 3 messages, the
- signalling link selection field exactly corresponds to the
- signalling link code (SLC) which indicates the signalling link
- between the destination point and originating point to which the
- message refers.
- 2.2.5 From the rule stated in S 2.2.4 above, it follows that the
- signalling link selection of messages generated by any User Parts
- will be used in the load sharing mechanism. As a consequence, in
- the case of User Parts which are not specified (e.g., transfer of
- charging information) but for which there is the requirement to
- maintain the order of transmission of the messages, the field
- should be coded with the same value for all messages belonging to
- the same transaction, sent in a given direction.
- 2.2.6 The above principles should also apply to modified label
- structures that may be used nationally.
- 2.3 Message routing function
- 2.3.1 The message routing function is based on information contained
- in the routing label, namely on the destination point code and on
- the signalling link selection field; moreover, in some
- circumstances the service indicator may also need to be used for
- routing purposes.
- Note - A possible case for the use of the service indicator is
- that which would arise from the use of messages supporting the
- signalling route management function (i.e. transfer-prohibited,
- transfer-allowed and signalling-route-set-messages) referring to a
- destination more restrictive than a single signalling point (e.g.,
- an individual User Part) (see S 13). Some specific routing may be
- required for the MTP Testing User Part (for further study).
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- The number of such cases should be kept to a minimum in order
- to apply the same routing criteria to as many User Parts as
- possible.
- Each signalling point will have routing information that
- allows it to determine the signalling link over which a message has
- to be sent on the basis of the destination point code and
- signalling link selection field and, in some cases, of the network
- indicator (see S 2.4.3). Typically the destination point code is
- associated with more than one signalling link that may be used to
- carry the message; the selection of the particular signalling link
- is made by means of the signalling link selection field, thus
- effecting load sharing.
- 2.3.2 Two basic cases of load sharing are defined, namely:
- a) load sharing between links belonging to the same link set,
- b) load sharing between links not belonging to the same link set.
- A load sharing collection of one or more link sets is called a combined
- link set.
- The capability to operate in load sharing according to both these cases is
- mandatory for any signalling point in the international network.
- In case a), the traffic flow carried by a link set is shared (on the basis
- of the signalling link selection field) between different signalling links
- belonging to the link set. An example of such a case is given by a link set
- directly interconnecting the originating and destination points in the associated
- mode of operation, such as represented in Figure 4/Q.704.
- Figure 4/Q.704 - CCITT 35760
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- In case b) traffic relating to a given destination is shared (on the basis
- of the signalling link selection field) between different signalling links not
- belonging to the same link set, such as represented in Figure 5/Q.704. The load
- sharing rule used for a particular signalling relation may or may not apply to
- all the signalling relations which use one of the signalling links involved (in
- the example, traffic destined to B is shared between signalling links DE and DF
- with a given signalling link selection field assignment, while that destined to C
- is sent only on link DF, due to the failure of link EC).
- As a result of the message routing function, in normal conditions all the
- messages having the same routing label (e.g., call set-up messages related to a
- given circuit) are routed via the same signalling links and signalling transfer
- points.
- Principles relating to the number of load-shared links appear in
- Recommendation Q.705.
- Figure 5/Q.704 - CCITT 35770
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- 2.3.3 The routing information mentioned in S 2.3.1 should be appropriately
- updated when some event happens in the signalling network, which is relevant to
- the concerned signalling point (e.g., failure of a signalling link or
- unavailability of a signalling route). The updating of the routing information is
- made according to the particular event (see S 3) and to the signalling routing
- modification rules specified in S 4. If a signalling transfer point receives a
- message for destination point code which according to the routing information
- does not exist, the message is discarded and an indication is given to a
- management system.
- 2.3.4 Handling of level 3 messages
- 2.3.4.1 Messages not related to a signalling link have the signalling link code
- 0000 (e.g., transfer prohibited and transfer allowed). They are handled in
- accordance with the normal routing function, where the signalling link code (SLC)
- is used in the same way as SLS for load sharing.
- 2.3.4.2 Messages related to a signalling link should be subdivided into 2
- groups:
- a) Messages that are to be transmitted over a specific signalling link
- (e.g., changeback declaration (see S 6) and signalling link test
- messages (Recommendation Q.707)), where a special routing function must
- ensure that these messages are transmitted exclusively over a
- particular signalling link.
- b) Messages that must not be transmitted over a specific signalling link
- (e.g., changeover messages and emergency changeover messages (see S
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- 5)), whose transmission over the signalling link defined by the SLC
- contained in the label must be avoided.
- 2.3.5 Handling of messages under signalling link congestion
- 2.3.5.1 In the international signalling network, congestion priorities of
- messages are only assigned and the decision to discard under congestion is only
- made within each User Part. Message discard will only occur in the MTP should
- there be an extreme resource limitation (for the MTP there is no congestion
- priority).
- In national signalling networks, each message may be assigned by its
- generating User Part a congestion priority. This is used by the MTP to determine
- whether or not a message should be discarded under signalling link congestion. N
- + 1 levels of congestion priority (0 N 3) levels are accommodated in the
- signalling network, with 0 being the lowest and N the highest.
- In national signalling networks using more than one congestion priority,
- the highest priority is assigned to signalling network management messages.
- 2.3.5.2 In national signalling networks using multiple congestion priorities
- When a signalling link has been selected for transmitting a message,
- comparison of the congestion priority of the message is made with the congestion
- status of the selected signalling link (see S 3.8). If the congestion priority is
- not less than the signalling link congestion status, that message is transmitted
- using the selected signalling link.
- Otherwise, a transfer-controlled message is sent in response as specified
- in S 13.7. In this case, the disposition of the concerned message is determined
- according to the following criteria:
- i) If the congestion priority of the message is greater than or equal to
- the signalling link discard status, the message is transmitted.
- ii) If the congestion priority of the message is less than the signalling
- link discard status, the message is discarded.
- 2.4 Message discrimination and distribution functions
- 2.4.1 The routing criteria and load sharing method described in S 2.3 imply that
- a signalling point, sending messages pertaining to a given signalling transaction
- on a given link, should be able to receive and process messages pertaining to
- that transaction, e.g., in response to the sent ones, coming from any (but only
- one) link.
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- The destination point code field of the received message is examined by
- the discrimination function in order to determine whether or not it is destined
- to the receiving signalling point. When the receiving signalling point has the
- transfer capability and the message is not destined to it, that message has to be
- directed to the routing function, as described in the previous sections, in order
- to be sent on the appropriate outgoing link towards the message destination
- point.
- When a signalling transfer point detects that a received message cannot be
- delivered to its destination point, it sends in response a transfer-prohibited
- message as specified in S 13.2.
- 2.4.2 If the destination point code of the message identifies the receiving
- signalling point, the service indicator is examined by the message distribution
- function and the message is delivered to the corresponding User Part (or to the
- Message Transfer Part level 3).
- Should a User become unavailable (User unavailability is an implementation
- dependent notion), this is detected by the MTP. Whether the distribution marked
- accordingly is implementation dependent.
- When the distribution function detects that a received message cannot be
- delivered to the required User (implementation dependent criteria), a User Part
- Unavailable message should be returned to the originating end on a response
- basis. In the originating signalling point, the relevant User Part should be
- informed via an MTP-STATUS primitive. A mandatory parameter Cause is included in
- the MTP status indication with two possible values:
- - Signalling Network Congestion,
- - User Part Unavailability.
- The User Part should reduce its traffic in an appropriate manner and take
- specific actions.
- 2.4.3 In the case of a signalling point handling both international and national
- signalling traffic (e.g., an international gateway exchange), the network
- indicator is also examined in order to determine the relevant numbering scheme
- (international or national) and possibly the label structure. Moreover, within a
- national network, the network indicator may be examined to discriminate between
- different label structures or between different signalling point numbering if
- dependent on the network levels (see S 14.2).
- 3 Signalling network management
- 3.1 General
- 3.1.1 The signalling network management functions provide the actions and
- procedures required to maintain signalling service, and to restore normal
- signalling conditions in the event of disruption in the signalling network,
- either in signalling links or at signalling points. The disruption may be in the
- form of complete loss of a signalling link or a signalling point, or in reduced
- accessibility due to congestion. For example, in the case of a link failure, the
- traffic conveyed over the faulty link should be diverted to one or more
- alternative links. The link failure may also result in unavailable signalling
- routes and this, in turn, may cause diversion of traffic at other signalling
- points in the signalling network (i.e., signalling points to which no faulty
- links are connected).
- 3.1.2 The occurrence of, or recovery from failures or congestion generally
- results in a change of the status of the affected signalling link(s) and
- route(s). A signalling link may be considered by level 3, either as "available"
- or "unavailable" to carry signalling traffic; in particular, an available
- signalling link becomes unavailable if it is recognized as "failed",
- "deactivated" "blocked2)" or "inhibited", and it becomes once again available if
- it is recognized as "restored", "activated", "unblocked" or "uninhibited"
- respectively. A signalling route may be considered by level 3 as "available",
- "restricted" or "unavailable" too. A signalling point may be "available" or
- "unavailable". A signalling route set may be "congested" or "uncongested". The
- detailed criteria for the determination of the changes in the status of
- signalling links, routes and points are described in SS 3.2, 3.4 and 3.6
- respectively.
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- depend on a failure in the link itself, but on other causes, such as a "processor
- outage" condition in a signalling point.
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- 3.1.3 Whenever a change in the status of a signalling link, route or point
- occurs, the three different signalling network management functions (i.e.,
- signalling traffic management, link management and route management) are
- activated, when appropriate, as follows:
- a) The signalling traffic management function is used to divert signalling
- traffic from a link or route to one or more different links or routes,
- to restart a signalling point, or to temporarily slow down signalling
- traffic in the case of congestion at a signalling point; it comprises
- the following procedures:
- - changeover (see S 5),
- - changeback (see S 6),
- - forced rerouting (see S 7),
- - controlled rerouting (see S 8),
- - signalling point restart (see S 9),
- - management inhibiting (see S 10),
- - signalling traffic flow control (see S 11).
- b) The signalling link management function is used to restore failed
- signalling links, to activate idle (not yet aligned) links and to
- deactivate aligned signalling links; it comprises the following
- procedures (see S 12):
- - signalling link activation, restoration and deactivation,
- - link set activation,
- - automatic allocation of signalling terminals and signalling data
- links.
- c) The signalling route management function is used to distribute
- information about the signalling network status, in order to block or
- unblock signalling routes; it comprises the following procedures:
- - transfer-controlled procedure (see SS 13.6, 13.7 and 13.8),
- - transfer-prohibited procedure (see S 13.2),
- - transfer-allowed procedure (see S 13.3),
- - transfer-restricted procedure (see S 13.4),
- - signalling-route-set-test procedure (see S 13.5),
- - signalling-route-set-congestion test procedure (see S 13.9).
- 3.1.4 An overview of the use of the procedures relating to the different
- management functions on occurrence of the link, route and point status changes is
- given in SS 3.3, 3.5 and 3.7 respectively.
- 3.2 Status of signalling links
- 3.2.1 A signalling link is always considered by level 3 in one of two possible
- major states: available and unavailable. Depending on the cause of
- unavailability, the unavailable state can be subdivided into seven possible cases
- as follows (see also Figure 6/Q.704):
- - unavailable, failed or inactive,
- - unavailable, blocked,
- - unavailable (failed or inactive) and blocked,
- - unavailable, inhibited,
- - unavailable, inhibited and (failed or inactive),
- - unavailable, inhibited and blocked,
- - unavailable, (failed or inactive), blocked and inhibited.
- The concerned link can be used to carry signalling traffic only if it is
- available except possibly for certain classes of test and management messages.
- Eight possible events can change the status of a link: signalling link failure,
- restoration, deactivation, activation, blocking, unblocking, inhibiting and
- uninhibiting; they are described in SS 3.2.2 to 3.2.9.
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- 3.2.2 Signalling link failure
- A signalling link (in service or blocked, see S 3.2.6) is recognized by
- level 3 as failed when:
- a) A link failure indication is obtained from level 2. The indication may
- be caused by:
- - intolerably high signal unit error rate (see Recommendation Q.703,
- S 10);
- - excessive length of the realignment period (see Recommendation
- Q.703, SS 4.1 and 7);
- - excessive delay of acknowledgements (see Recommendation Q.703, SS
- 5.3 and 6.3);
- - failure of signalling terminal equipment;
- - two out of three unreasonable backward sequence numbers or forward
- indicator bits (see Recommendation Q.703, SS 5.3 and 6.3);
- - reception of consecutive link status signal units indicating out of
- alignment, out of service, normal or emergency terminal status (see
- Recommendation Q.703, S 1.7);
- - excessive periods of level 2 congestion (see Recommendation Q.703,
- S 9).
- The first two conditions are detected by the signal unit error rate
- monitor (see Recommendation Q.703, S 10).
- b) A request (automatic or manual) is obtained from a management or
- maintenance system.
- Moreover a signalling link which is available (not blocked) is recognized
- by level 3 as failed when a changeover order is received.
- 3.2.3 Signalling link restoration
- A signalling link previously failed is restored when both ends of the
- signalling link have successfully completed an initial alignment procedure (see
- Recommendation Q.703, S 7).
- 3.2.4 Signalling link deactivation
- A signalling link (in service, failed or blocked) is recognized by level 3
- as deactivated (i.e., removed from operation) when:
- a) a request is obtained from the signalling link management function (see
- S 12);
- b) a request (automatic or manual) is obtained from an external management
- or maintenance system.
- 3.2.5 Signalling link activation
- A signalling link previously inactive is recognized by level 3 as
- activated when both ends of the signalling link have successfully completed an
- initial alignment procedure (see Recommendation Q.703, S 7).
- 3.2.6 Signalling link blocking
- A signalling link (in service, failed or inactive) is recognized as
- blocked when an indication is obtained from the signalling terminal that a
- processor outage condition exists at the remote terminal (i.e., link status
- signal units with processor outage indication are received, see Recommendation
- Q.703, S 8).
- Note - A link becomes unavailable when it is failed or deactivated or
- [(failed or deactivated) and blocked] or inhibited. See Figure 6/Q.704.
- 3.2.7 Signalling link unblocking
- A signalling link previously blocked is unblocked when an indication is
- obtained from the signalling terminal that the processor outage condition has
- ceased at the remote terminal. (Applies in the case when the processor outage
- condition was initiated by the remote terminal.)
- Note - A link becomes available when it is restored or activated or
- unblocked, or [(restored or activated) and (unblocked)] or uninhibited. See
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- 3.2.8 Signalling link inhibiting
- A signalling link is recognized as inhibited when:
- a) an acknowledgement is received from a remote signalling point in
- response to an inhibit request sent to the remote end by the local
- signalling link management. Level 3 has marked the link locally
- inhibited;
- b) upon receipt of a request from a remote signalling point to inhibit a
- link and successful determination that no destination will become
- inaccessible by inhibiting the link, the link has been marked remotely
- inhibited by level 3.
- 3.2.9 Signalling link uninhibiting
- A signalling link previously inhibited is uninhibited when:
- a) a request is received to uninhibit the link from a remote end or from a
- local routing function;
- b) an acknowledgement is received from a remote signalling point in
- response to an uninhibit request sent to the remote end by the local
- signalling link management.
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- 3.3 Procedures used in connection with link status changes
- In S 3.3, the procedures relating to each signalling management function,
- which are applied in connection with link status changes, are listed. See also
- Figures 6/Q.704, 7/Q.704 and 8/Q.704. Typical examples of the application of the
- procedures to the particular network cases appear in Recommendation Q.705.
- Figure 7/Q.704 - CCITT 35811
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- 3.3.1 Signalling link failed
- 3.3.1.1 ignalling traffic management: the changeover procedure (see S 5) is
- applied, if required, to divert signalling traffic from the unavailable link to
- one or more alternative links with the objective of avoiding message loss,
- repetition or mis-sequencing; it includes determination of the alternative link
- or links where the affected traffic can be transferred and procedures to retrieve
- messages sent over the failed link but not received by the far end.
- 3.3.1.2 ignalling link management: the procedures described in S 12 are used to
- restore a signalling link and to make it available for signalling. Moreover,
- depending on the link set status, the procedures can also be used to activate
- another signalling link in the same link set to which the unavailable link
- belongs and to make it available for signalling.
- 3.3.1.3 ignalling route management: in the case when the failure of a
- signalling link causes a signalling route set to become unavailable or
- restricted3), the signalling transfer point which can no longer route the
- concerned signalling traffic applies the transfer-prohibited procedures or
- transfer-restricted procedures described in S 13.
- 3.3.2 Signalling link restored
- 3.3.2.1 Signalling traffic management: the changeback procedure (see S 6) is
- applied, if required, to divert signalling traffic from one or more links to a
- link which has become available; it includes determination of the traffic to be
- diverted and procedures for maintaining the correct message sequence.
- 3.3.2.2 ignalling link management: the signalling link deactivation procedure
- (see S 12) is used if, during the signalling link failure, another signalling
- link of the same link set was activated; it is used to assure that the link set
- status is returned to the same state as before the failure. This requires that
- the active link activated during the link failure is deactivated and considered
- no longer available for signalling.
- 3.3.2.3 ignalling route management: in the case when the restoration of a
- signalling link causes a signalling route set to become available, the signalling
- transfer point which can once again route the concerned signalling traffic
- applies the transfer-allowed procedures described in S 13.
- 3.3.3 Signalling link deactivated
- 3.3.3.1 Signalling traffic management: as specified in S 3.3.1.1.
- Note - The signalling traffic has normally already been removed when
- signalling link deactivation is initiated.
- 3.3.3.2 Signalling link management: if the number of active signalling links in
- the link set to which the deactivated signalling link belongs has become less
- than the normal number of active signalling links in that link set, the
- procedures described in S 12 may be used to activate another signalling link in
- the link set.
- 3.3.3.3 Signalling route management: as specified in S 3.3.1.3.
- 3.3.4 Signalling link activated
- 3.3.4.1 Signalling traffic management: as specified in S 3.3.2.1.
- 3.3.4.2 Signalling link management: if the number of active signalling links in
- the link set to which the activated signalling link belongs has become greater
- than the normal number of active signalling links in that link set, the
- procedures described in S 12 may be used to deactivate another signalling link in
- the link set.
- 3.3.4.3 Signalling route management: as specified in S 3.3.2.3.
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- 3.3.5 Signalling link blocked
- 3.3.5.1 Signalling traffic management: as specified in S 3.3.1.1.
- As a national option, local processor outage may also be applied to the
- affected signalling link before commencement of the appropriate signalling
- traffic management option. On completion of that signalling traffic management
- action, local processor outage is removed from the affected signalling link. No
- further signalling traffic management will be performed on that affected
- signalling link until a timer T24 (see S 16.8) has expired or been cancelled,
- thus allowing time for indications from the remote end to stabilize as it carries
- out any signalling traffic management of its own.
- 3.3.5.2 Signalling route management: if the blocking of the link causes a
- signalling route set to become unavailable or restricted4), the signalling
- transfer point which can no longer route the concerned signalling traffic applies
- the transfer-prohibited or transfer-restricted procedures described in S 13.
- 3.3.6 Signalling link unblocked
- 3.3.6.1 Signalling traffic management: the actions will be the same as in S
- 3.3.2.1.
- 3.3.6.2 Signalling route management: if the link unblocked causes a signalling
- route set to become available, the signalling transfer point which can once again
- route the signalling traffic in that route set applies the transfer-allowed
- procedures described in S 13.
- 3.3.7 Signalling link inhibited
- 3.3.7.1 Signalling traffic management: as specified in S 3.3.1.1.
- 3.3.7.2 Signalling link management: as specified in S 3.3.3.2.
- 3.3.8 Signalling link uninhibited
- 3.3.8.1 Signalling traffic management: as specified in S 3.3.2.1.
- 3.3.8.2 Signalling link management: as specified in S 3.3.4.2.
- 3.3.8.3 Signalling route management: if the link uninhibited causes a
- signalling route set to become available, the signalling transfer point which can
- once again route the signalling traffic in that route set applies the
- transfer-allowed procedures described in S 13.
- 3.4 Status of signalling routes
- A signalling route can be in three states for signalling traffic having
- the concerned destination; these are available, restricted4), unavailable (see
- also Figure 6/Q.704).
- 3.4.1 Signalling route unavailability
- A signalling route becomes unavailable when a transfer-prohibited message,
- indicating that signalling traffic towards a particular destination cannot be
- transferred via the signalling transfer point sending the concerned message, is
- received (see S 13).
- 3.4.2 Signalling route availability
- A signalling route becomes available when a transfer-allowed message,
- indicating that signalling traffic towards a particular destination can be
- transferred via the signalling transfer point sending the concerned message, is
- received (see S 13).
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- 3.4.3 Signalling route restricted 5)
- A signalling route becomes restricted when a transfer-restricted message,
- indicating that the signalling traffic towards a particular destination is being
- transferred with some difficulty via the signalling transfer point sending the
- concerned message is received (see S 13).
- 3.5 Procedures used in connection with route status changes
- In S 3.5 the procedures relating to each signalling management function,
- which in general are applied in connection with route status changes, are listed.
- See also Figures 6/Q.704 and 8/Q.704. Typical examples of the application of the
- procedures to particular network cases appear in Recommendation Q.705.
- 3.5.1 Signalling route unavailable
- 3.5.1.1 Signalling traffic management: the forced rerouting procedure (see S 7)
- is applied; it is used to transfer signalling traffic to the concerned
- destination from the link set, belonging to the unavailable route, to an
- alternative link set which terminates in another signalling transfer point. It
- includes actions to determine the alternative route.
- 3.5.1.2 Signalling route management: because of the unavailability of the
- signalling route, the network is reconfigured; in the case that a signalling
- transfer point can no longer route the concerned signalling traffic, it applies
- the procedures described in S 13.
- 3.5.2 Signalling route available
- 3.5.2.1 Signalling traffic management: the controlled rerouting procedure (see
- S 8) is applied; it is used to transfer signalling traffic to the concerned
- destination from a signalling link or link set belonging to an available route,
- to another link set which terminates in another signalling transfer point. It
- includes the determination of which traffic should be diverted and procedures for
- maintaining the correct message sequence.
- 3.5.2.2 Signalling route management: because of the restored availability of
- the signalling route, the network is reconfigured; in the case that a signalling
- transfer point can once again route the concerned signalling traffic, it applies
- the procedures described in S 13.
- 3.5.3 Signalling route restricted 5)
- 3.5.3.1 Signalling traffic management: the controlled rerouting procedure (see
- S 8) is applied; it is used to transfer signalling traffic to the concerned
- destination from the link set belonging to the restricted route, to an
- alternative link set if one is available to give more, if possible, efficient
- routing. It includes actions to determine the alternative route.
- 3.5.3.2 Signalling route management: because of restricted availability of the
- signalling route, the network routing is, if possible, reconfigured; procedures
- described in S 13 are used to advise adjacent signalling points.
- 3.6 Status of signalling points
- A signalling point can be in two states; available or unavailable (see
- Figure 6/Q.704). However, implementation dependent congestion states may exist.
- 3.6.1 Signalling point unavailability
- 3.6.1.1 Unavailability of a signalling point itself: A signalling point becomes
- unavailable when all connected signalling links are unavailable.
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- 3.6.1.2 Unavailability of an adjacent signalling point: A signalling point
- considers that an adjacent signalling point becomes unavailable when:
- - all signalling links connected to the adjacent signalling point are
- unavailable and
- - the adjacent signalling point is inaccessible.
- 3.6.2 Signalling point availability
- 3.6.2.1 Availability of a signalling point itself: A signalling point becomes
- available when at least one signalling link connected to this signalling point
- becomes available.
- 3.6.2.2 Availability of an adjacent signalling point: A signalling point
- considers that an adjacent signalling point becomes available when:
- - at least one signalling link connected to the adjacent signalling point
- becomes available and that signalling point has restarted, or
- - the adjacent signalling point becomes accessible on the reception of a
- transfer allowed message or a transfer restricted6) message (see S
- 13.4).
- 3.7 Procedure used in connection with point status changes
- 3.7.1 Signalling point unavailable
- There is no specific procedure used when a signalling point becomes
- unavailable. The transfer prohibited procedure is used to update the status of
- the recovered routes in all nodes of the signalling network (see S 13.2).
- 3.7.2 Signalling point available
- 3.7.2.1 Signalling traffic management: the signalling point restart procedure
- (see S 9) is applied; it is used to restart the traffic between the signalling
- network and the signalling point which becomes available. This restart is based
- on the following criteria:
- - avoid loss of messages
- - limit the level 3 load due to the restart of a signalling point
- - restart, as much as possible, simultaneously in both directions of the
- signalling relations.
- 3.7.2.2 Signalling link management: The first step of the signalling point
- restart procedure attempts to restore the signalling links of the point which
- becomes available; the signalling link restoration procedure is used (see S 12);
- 3.7.2.3 Signalling route management: The second step of the signalling point
- restart procedure consists of updating the signalling route states before
- carrying traffic to the point which becomes available and in all adjacent points;
- the transfer prohibited and transfer restricted procedures are used (see S 13).
- 3.7.3 Signalling point congested: (implementation-dependent option, see S
- 11.2.6).
- 3.8 Signalling network congestion
- 3.8.1 General
- In S 3.8, criteria for the determination of signalling link congestion
- status and signalling route set congestion status are specified. The procedures
- relating to each signalling network management function, which in general are
- applied in connection with congestion status changes, are listed.
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- 3.8.2 Congestion status of signalling links
- 3.8.2.1 When predetermined levels of MSU fill in the transmission or
- retransmission buffer are crossed, an indication is given to level 3 advising of
- congestion/congestion abatement. The location and setting of the congestion
- thresholds are considered to be implementation-dependent.
- Note - The criterion for setting the congestion thresholds is based on:
- (1) the proportion of the total (transmit and retransmit) buffer capacity that is
- occupied, and/or (2) the total number of messages in the transmit and retransmit
- buffers. (The buffer capacity below the threshold should be sufficient to
- overcome load peaks due to signalling network management functions and the
- remaining buffer capacity should allow User Parts time to react to congestion
- indications before message discard occurs.) The monitoring may be performed in
- different ways depending on the relative sizes of the transmit and retransmit
- buffers. In the case of a relatively small retransmit buffer, monitoring of the
- transmit buffer may be sufficient. In the case of a relatively large retransmit
- buffer, both the transmit buffer and retransmit buffer occupancies may need to be
- monitored.
- a) In the international signalling network, one congestion onset and one
- congestion abatement threshold are provided. The congestion abatement
- threshold should be placed lower than the congestion onset threshold in
- order to provide hysteresis during the process of recovering from
- congestion.
- b) In national signalling networks, with multiple congestion thresholds,
- N(1 N 3) separate thresholds are provided for detecting the onset of
- congestion. They are called congestion onset thresholds and are
- numbered 1, . . ., N, respectively. N separate thresholds are provided
- for monitoring the abatement of congestion. They are called congestion
- abatement thresholds and are numbered 1, . . ., N, respectively.
- 3.8.2.2 In national signalling networks with multiple congestion thresholds N
- separate thresholds are provided for determining whether, under congestion
- conditions, a message should be discarded or transmitted using the signalling
- link. They are called congestion discard thresholds and are numbered 1, . . ., N,
- respectively.
- Congestion discard threshold n(n = 1, . . ., N) is placed higher than
- congestion onset threshold n in order to minimize message loss under congestion
- conditions.
- Congestion discard threshold n(n = 1, . . ., N - 1) should be placed at or
- lower than congestion onset threshold n + 1 in order to make congestion control
- effective.
- When the current buffer occupancy does not exceed congestion discard
- threshold 1, the current signalling link discard status is assigned the zero
- value.
- Each congestion abatement threshold should be placed lower than the
- corresponding congestion onset threshold in order to provide hysteresis during
- the process of recovering from congestion.
- In national signalling networks with N > 1, the congestion abatement
- threshold n(n = 2, . . ., N) should be placed higher than the congestion onset
- threshold n - 1 so as to allow for a precise determination of signalling link
- congestion status.
- Congestion abatement threshold 1 should be placed higher than the normally
- engineered buffer occupancy of a signalling link.
- Under normal operation, when the signalling link is uncongested, the
- signalling link congestion status is assigned the zero value.
- At the onset of congestion, when the buffer occupancy is increasing, the
- signalling link congestion status is determined by the highest congestion onset
- threshold exceeded by the buffer occupancy. That is, if congestion onset
- threshold n(n = 1, . . ., N) is the highest congestion onset threshold exceeded
- by the current buffer occupancy, the current signalling link congestion status is
- assigned the value n (see Figure 8a/Q.704).
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- At the abatement of congestion, when the buffer occupancy is decreasing,
- the signalling link congestion status is determined by the lowest congestion
- abatement threshold below which the buffer occupancy has dropped. That is, if
- congestion abatement threshold n(n = 1, . . ., N) is the lowest congestion
- abatement threshold below which the current buffer occupancy has dropped, the
- current signalling link congestion status is assigned the value n - 1 (see Figure
- 8b/Q.704).
- The use of the signalling link congestion status is specified in S
- 2.3.5.2.
- Figure 8b/Q.704 - CCITT 73520
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- When the current buffer occupancy exceeds congestion discard threshold n(n
- = 1, . . ., N - 1), but does not exceed congestion discard threshold n + 1, the
- current signalling link discard status is assigned the value n (see
- Figure 8c/Q.704).
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- When the current buffer occupancy exceeds congestion discard threshold N,
- the current signalling discard status is assigned the value N.
- The use of the signalling link discard status is specified in S 2.3.5.2.
- 3.8.2.3 In national signalling networks using multiple signalling link
- congestion states without congestion priority, S + 1(1 S 3) levels of
- signalling link congestion status are accommodated in the signalling network, 0
- being the lowest and S the highest.
- The signalling link congestion status is determined by a timing mechanism
- after the buffer occupancy exceeds the congestion onset threshold, or drops below
- the congestion abatement threshold. Under normal operation, when the signalling
- link is uncongested, the signalling link congestion status is assigned the zero
- value.
- At the onset of congestion, when the buffer occupancy exceeds the
- congestion onset threshold, the first signalling link congestion status is
- assigned a value s, predetermined in the signalling network.
- If the signalling link congestion status is set to s(s = 1, . . ., S - 1)
- and the buffer occupancy continues to be above the congestion onset threshold
- during Tx, the signalling link congestion status is updated by the new value s +
- 1.
- If the signalling link congestion status is set to s(s = 1, . . ., S) and
- the buffer occupancy continues to be below the abatement threshold during Ty, the
- signalling link congestion status is updated by the new value s - 1.
- Otherwise, the current signalling link congestion status is maintained
- (see Figure 8d/Q.704).
- The congestion abatement threshold should be placed lower than the
- congestion onset threshold.
- 3.8.3 Procedures used in connection with link congestion status changes
- In S 3.8.3, the procedures relating to each signalling network management
- function, which in general are applied in connection with link congestion status
- changes, are listed.
- Signalling route management: in the case when the congestion of a
- signalling link causes a signalling route set to become congested, the
- transfer-controlled procedure (see SS 13.6 and 13.7) is used, if required, to
- notify originating signalling points that they should reduce the concerned
- signalling traffic towards the affected destination.
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- 3.8.4 Congestion status of signalling route sets
- At each originating signalling point, there is associated with each
- signalling route set a congestion status, which indicates the degree of
- congestion in the signalling route set.
- a) In the international signalling network, two states are provided,
- congested and uncongested.
- If a link in a signalling route towards a given destination becomes
- congested, the congestion status of the signalling route set towards
- the affected destination is changed to congested.
- When a transfer controlled message relating to a given destination is
- received, the congestion status of the signalling route set towards the
- affected destination is indicated to the level 4 User Parts in
- accordance with the transfer-controlled procedure specified in S 13.6.
- The congestion status is not retained by level 3 at the receiving
- signalling point.
- b) In national signalling networks with multiple congestion levels7)
- corresponding to the N + 1 levels of signalling link congestion, there
- are N + 1 values of signalling route set congestion status, with 0
- being the lowest and N the highest.
- Normally the congestion status of a signalling route set is assigned
- the zero value, indicating that the signalling route set is
- uncongested.
- If a signalling link in the signalling route set to a given destination
- becomes congested, the congestion status of the signalling route set is
- assigned the value of the signalling link congestion status, if it is
- higher than the current signalling route set congestion status.
- When a transfer-controlled message relating to a given destination is
- received, the congestion status of the signalling route set towards
- that destination is updated, in accordance with the transfer-controlled
- procedure as specified in S 13.7.
- The congestion status of the signalling route set towards that
- destination may be decremented in accordance with the
- signalling-route-set-congestion-test procedure as specified in S 13.9.
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- c) In national signalling networks using multiple congestion levels8)
- without congestion priority, there are S + 1 values of signalling route
- set congestion states, with 0 being the lowest and S the highest.
- Normally the congestion status of a signalling route set is assigned
- the zero value, indicating that the signalling route set is
- uncongested.
- If a local signalling link in the signalling route set to a given
- destination becomes congested, the congested status of the signalling
- route set is assigned the value of the signalling link congestion
- status, if it is larger than the current signalling route set
- congestion status.
- When a transfer-controlled message relating to a given destination is
- received, the congestion status of the signalling route set towards
- that destination is updated in accordance with the transfer-controlled
- procedure as specified in S 13.8. The congestion status of the route
- set towards the congested destination is not retained by level 3 at the
- receiving signalling point.
- 3.8.5 Procedures used in connection with route set congestion status changes
- In S 3.8.5, the procedures relating to each signalling network management
- function, which in general are applied in connection with route set congestion
- status changes, are listed.
- 3.8.5.1 Signalling traffic management: the signalling traffic flow control
- procedure (see S 11) is applied; it is used to regulate the input of signalling
- traffic from User Parts to the concerned signalling route set.
- 3.8.5.2 Signalling route management: as a national option, the
- signalling-route-set-congestion-test procedure (see S 13.9) is applied; it is
- used to update the congestion status of the concerned signalling route set until
- the congestion status is reduced to the zero value.
- 4 Signalling traffic management
- 4.1 General
- 4.1.1 The signalling traffic management function is used, as indicated in S 3,
- to divert signalling traffic from signalling links or routes, or to temporarily
- reduce it in quantity in the case of congestion.
- 4.1.2 The diversion of traffic in the cases of unavailability or availability or
- restriction of signalling links and routes is typically made by means of the
- following basic procedures, included in the signalling traffic management
- function:
- - signalling link unavailability (failure, deactivation, blocking or
- inhibiting): the changeover procedure (see S 5) is used to divert
- signalling traffic to one or more alternative links (if any);
- - signalling link availability (restoration, activation, unblocking or
- uninhibiting): the changeback procedure (see S 6) is used to divert
- signalling traffic to the link made available;
- - signalling route unavailability: the forced rerouting procedure (see S
- 7) is used to divert signalling traffic to an alternative route (if
- any);
- - signalling route availability: the controlled rerouting procedure (see
- S 8) is used to divert signalling traffic to the route made available;
- - signalling route restricted8): the controlled rerouting procedure (see
- S 8) is used to divert signalling traffic to an alternative route (if
- any);
- - signalling point availability: the signalling point restart procedure
- (see S 9) is used to divert the signalling traffic to (or via) the
- point made available.
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- Each procedure includes different elements of procedure, the application
- of one or more of which depends on the particular circumstances, as indicated in
- the relevant sections. Moreover, these procedures include a modification of the
- signalling routing, which is made in a systematic way, as described in SS 4.2 to
- 4.7.
- 4.1.3 The signalling traffic flow control procedures are used in the case of
- congestion, in order to limit signalling traffic at its source. The procedures
- are specified in S 11.
- 4.2 Normal routing situation
- 4.2.1 Signalling traffic to be sent to a particular signalling point in the
- network, is normally routed to one or, in the case of load sharing between link
- sets in the international network, two link sets. A load sharing collection of
- one or more link sets is called a combined link set. Within a link set, a further
- routing may be performed in order to load share the traffic over the available
- signalling links (see S 2).
- To cater for the situations when signalling links or routes become
- unavailable, alternative routing data are defined.
- For each destination which may be reached from a signalling point, one or
- more alternative link sets (combined link sets) are allocated. An alternative
- combined link set may consist of one or more (or all) of the remaining available
- link sets, which may carry signalling traffic towards the concerned destination.
- The possible link set (combined link sets) appear in a certain priority order.
- The link set (combined link set) having the highest priority is used whenever it
- is available. It is defined that the normal link set (combined link set) for
- traffic to the concerned destination. The link set (combined link set) which is
- in use at a given time is called the current link set (combined link set). The
- current link set (combined link set) consists either of the normal link set
- (combined link set) or of an alternative link set (combined link set).
- For each signalling link, the remaining signalling links in the link set
- are alternative links. The signalling links of a link set are arranged in a
- certain priority order. Under normal conditions the signalling link (or links)
- having the highest priority is used to carry the signalling traffic.
- These signalling links are defined as normal signalling links, and each
- portion of load shared traffic has its own normal signalling link. Signalling
- links other than normal may be active signalling links (but not carrying any
- signalling traffic at the time) or inactive signalling links (see S 12).
- 4.2.2 Message routing (normal as well as alternative) is in principle
- independently defined at each signalling point. Thus, signalling traffic between
- two signalling points may be routed over different signalling links or paths in
- the two directions.
- 4.3 Signalling link unavailability
- 4.3.1 When a signalling link becomes unavailable (see S 3.2) signalling traffic
- carried by the link is transferred to one or more alternative links by means of a
- changeover procedure. The alternative link or links are determined in accordance
- with the following criteria.
- 4.3.2 In the case when there is one or more alternative signalling links
- available in the link set to which the unavailable link belongs, the signalling
- traffic is transferred within the link set to:
- a) an active and unblocked signalling link, currently not carrying any
- traffic. If no such signalling link exists, the signalling traffic is
- transferred to
- b) one or possibly more than one signalling link currently carrying
- traffic. In the case of transfer to one signalling link, the
- alternative signalling link is that having the highest priority of the
- signalling links in service.
- 4.3.3 In the case when there is no alternative signalling link within the link
- set to which the unavailable signalling link belongs, the signalling traffic is
- transferred to one or more alternative link sets (combined link sets) in
- accordance with the alternative routing defined for each destination. For a
- particular destination, the alternative link set (combined link set) is the link
- set (combined link set) in service having the highest priority.
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- Within a new link set, signalling traffic is distributed over the
- signalling links in accordance with the routing currently applicable for that
- link set; i.e., the transferred traffic is routed in the same way as the traffic
- already using the link set.
- 4.4 Signalling link availability
- 4.4.1 When a previously unavailable signalling link becomes available again (see
- S 3.2), signalling traffic may be transferred to the available signalling link by
- means of the changeback procedure. The traffic to be transferred is determined in
- accordance with the following criteria.
- 4.4.2 In the case when the link set, to which the available signalling link
- belongs, already carries signalling traffic on other signalling links in the link
- set, the traffic to be transferred is the traffic for which the available
- signalling link is the normal one.
- The traffic is transferred from one or more signalling links, depending on
- the criteria applied when the signalling link became unavailable (see S 4.3.2).
- 4.4.3 In the case when the link set (combined link set) to which the available
- signalling links belongs, does not carry any signalling traffic [i.e., a link set
- (combined link set) has become available], the traffic to be transferred is the
- traffic for which the available link set (combined link set) has higher priority
- than the link set (combined link set) currently used.
- The traffic is transferred from one or more link sets (combined link sets)
- and from one or more signalling links within each link set.
- 4.5 Signalling route unavailability
- When a signalling route becomes unavailable (see S 3.4) signalling traffic
- currently carried by the unavailable route is transferred to an alternative route
- by means of forced re-routing procedure. The alternative route (i.e. the
- alternative link set or link sets) is determined in accordance with the
- alternative routing defined for the concerned destination (see S 4.3.3).
- 4.6 Signalling route availability
- When a previously unavailable signalling route becomes available again
- (see S 3.4) signalling traffic may be transferred to the available route by means
- of a controlled rerouting procedure. This is applicable in the case when the
- available route (link set) has higher priority than the route (link set)
- currently used for traffic to the concerned destination (see S 4.4.3).
- The transferred traffic is distributed over the links of the new link set
- in accordance with the routing currently applicable for that link set.
- 4.7 Signalling route restriction9)
- When a signalling route becomes restricted (see S 3.4), signalling traffic
- carried by the restricted route is, if possible, transferred to an alternative
- route by means of the controlled rerouting procedure, if an equal priority
- alternative is available and not restricted. The alternative route is determined
- in accordance with alternate routing defined for the concerned destination (see S
- 4.3.3).
- 4.8 Signalling point availability
- When a previously unavailable signalling point becomes available (see S
- 3.6), signalling traffic may be transferred to the available point by means of a
- signalling point restart procedure (see S 9).
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