Signaling Message Handling Functions

The signaling message-handling function ensures that a signaling message originated by a particular user part at an originating signaling point is delivered to the same user part at the destination point as indicated by the sending user part. Depending on the particular circumstances, the delivery may be made through a signaling link directly interconnecting the originating and destination points or via one or more intermediate signaling transfer points (STPs).

The signaling message-handling functions are based on the label contained in the messages which explicitly identifies the destination and origination points. The label part used for signaling message handling by the MTP is called the routing label. As shown in Figure 14.3 (upper left portion), the signaling message-handling is divided up into the following:

• The message routing function, used at each signaling point to determine the outgoing signaling link on which a message is to be sent toward its destination point.

• The message discrimination function, used at a signaling point to determine whether or not a received message is destined to that point itself. When the signaling point has the transfer capability, and a message is not destined for it, that message is transferred to the message routing function.

• The message distribution function, used at each signaling point to deliver the received messages (destined to the point itself) to the appropriate user part. Routing Label. The label contained in a signaling message and used by the relevant user part to identify a particular task to which the message refers (e.g., a telephone circuit) is also used by the message transfer part to route the message toward its destination point. The part of the message 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 services and applications in a given signaling network, national and international. [However, if this is not the case, the particular routing label of a message is determined by means of the service indicator (SI).] The standard routing label should be used in the international signaling network and is applicable in national applications. The standard routing label is 32 bits long and is placed at the beginning of the signaling information field (SIO). Its structure is illustrated in Figure 14.6.

The destination point code (DPC) indicates the destination 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 fold, the least significant bit occupies the first position and is transmitted first.

A unique numbering scheme for the coding of the fields is used for the signaling points of the international network irrespective of the user parts connected to each signaling




Length n x 8 (bit) (n 2r 0)

4 14 14 Routing label


DPC Destination point code OPC Originating print code SLS Signaling link selection

DPC Destination point code OPC Originating print code SLS Signaling link selection

Figure 14.6 Routing label structure. [Based on Figure 3/Q.704, page 5, CCITT Rec. Q.704 (Ref. 8).]

point. The signaling link selection (SLS) field is used, where appropriate, in performing load sharing. 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 layer 3 messages (e.g., changeover order) for which the message routing function in the signaling point of origin of the message is not dependent on the field. In this particular case the field does not exist as such, but 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 the bearer identification code in the case of the data user part (DUP)], and these bits are not repeated elsewhere. In the case of all other user parts, the SLS is an independent field. In these cases it follows that the signaling link selection of messages generated by any user part will be used in the load-sharing mechanism. As a consequence, in the case of the user parts which are not specified (e.g., transfer of charging information) but for which there is a requirement to maintain order of transmission of messages, the field is coded with the same value for all messages belonging to the same transaction, sent in a given direction.

In the case of message transfer part layer 3 messages, the signaling link selection field exactly corresponds to the signaling link code (SLC) which indicates the signaling link between destination point and originating point to which the message refers.

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