SS7 LINK SELECTION

To maximize the use of all available equipment, it is important to distribute the transmission of outbound MSUs over all of a signaling point's links. MTP Level 3 is responsible for routing outbound messages over available links, and determines on which link a given MSU will be sent.

When SCCP sends an outbound MSU to MTP Level 3, the link that MTP Level 3 selects depends on the value located in the Signaling Information Field (SIF).

When it receives an outbound MSU from SCCP, MTP Level 3 checks the Signaling Link Selection (SLS) field, which is contained in the SIF. MTP Level 3 then checks a "map" of available links that contains both a Signaling Link Code (SLC) for each link and a cross-reference for all possible SLS values. Based on this map, MTP Level 3 selects a link for sending the outbound MSU.

A primary purpose of the SLS field, then, is to help MTP Level 3 equally distribute message traffic among a signaling point's links. Theoretically, if a user part sends messages to SCCP at regular intervals, and SCCP increments the SLS value each time, then the traffic load should be evenly distributed among the links.

Note: In some cases, SCCP will not increment an SLS value for an MSU. Instead, SCCP uses the existing one found in the SLS field. This depends on the class of service designated for an MSU. Because multiple user parts existing at a signaling point are all sending MSUs to SCCP, the distribution of MSUs over the available links is not significantly affected. (More detail on SCCP's classes of service is provided in the "SCCP Services " section of this tutorial.)

Link Selection Example

When it receives an MSU from a TCAP user, SCCP checks the class of service. If the class of service allows SCCP to change the SLS, SCCP increments the value from the last one used. For example, if the last value used was 5, SCCP sets the SLS value to 6. The MSU is then sent to MTP Level 3, which refers to its map of SLS -SLC values while checking the SLS field. Since the value is 6, MTP Level 3 determines that link 2 will be used to send the outbound MSU.

The SLS-SLC map used by MTP Level 3 changes according to link availability. If a link becomes unavailable (e.g., for servicing), MTP Level 2 notifies MTP Level 3. In response, MTP Level 3 redraws the SLS-SLC map to take the unavailable link out of rotation for outbound messages.

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