Signaling network management

Signaling network management provides the functions necessary to maintain signaling service during failures and congestion, and restore normal signaling service after recovery from these conditions. MTP 3 supports all required ANSI and ITU-T network management procedures without intervention from the user parts or applications. MTP 3 notifies applications of significant network events that might impact their operation, such as changes in the accessibility status of remote signaling points, the onset and abatement of congestion, and signaling point restarts.

This topic discusses:

Changeover and changeback

MTP 3 automatically initiates changeover procedures whenever a link fails, is deactivated, is remotely blocked, or is inhibited. No user part or application action is required. MTP 3 does not notify the user part or application when the changeover occurs.

When a signaling link is restored, unblocked, or uninhibited, MTP 3 automatically performs the changeback function without interacting with the user parts or applications.

Forced and controlled rerouting

MTP 3 also handles forced and controlled rerouting upon receipt of the transfer prohibited and transfer allowed or transfer restricted messages. On receipt of a transfer prohibited (TFP) message, MTP 3 attempts to redirect all traffic for the prohibited destination to an alternate route. If no alternate routes are available, the destination is declared inaccessible and each user part or application is notified with a StatPaused status indication for the concerned destination. Destinations can also be declared inaccessible for other reasons such as signaling link or signaling point failures, which result in similar StatPaused indications to the user parts.

Traffic routing over an unavailable or restricted route is automatically restored upon receipt of the transfer allowed (TFA) or transfer restricted (TFR) message for that route. If the TFA/TFR make a previously inaccessible destination accessible, each user part is notified with a StatResumed indication for that destination.

MTP restart

MTP 3 can be configured with or without the MTP restart capability. The MTP restart function allows a signaling point just becoming available (such as after a failure) to bring up sufficient links to handle the expected traffic load before receiving new traffic.

If configured to do so, MTP 3 performs the restart function when the first signaling link becomes active (such as at system startup or after a total failure affecting all links), or on command from a management primitive. At the beginning of an MTP restart, each user part or application is notified with a StatRestart indication. Any new traffic requests generated by user parts during the restart are discarded. When the restart is complete and the MTP 3 layer is ready for traffic, each user part or application receives a StatRestartEnds indication.

Signaling link management

MTP 3 provides the basic link management functions and optionally the signaling link management procedures based on automatic allocation of signaling terminals described in the ANSI and ITU-T MTP standards.

Each linkset has a minNmbActLnk attribute that determines the normal number of active links in the linkset. Typically this number includes all links in the linkset, but you can configure extra alternate links that are activated only in the presence of failures of other links.

When a linkset is activated (such as at system startup time), MTP 3 attempts to activate the minNmbActLnk highest priority links in that linkset. If a link fails or cannot be aligned successfully, MTP 3 periodically attempts to restore the link until successful or until the link is manually disabled through a management function.

If the current number of active links in a linkset drops below the minNmbActLnk threshold, MTP 3 attempts to activate the highest priority inactive link not currently inhibited, remotely blocked, or manually disabled. If no other links in the linkset meet this criteria, MTP 3 attempts to uninhibit the highest priority inhibited link in the linkset.

If the current number of active links in a linkset goes above the minNmbActLnk threshold due to a link restoration, MTP 3 attempts to deactivate the lowest priority active link to return the number of active links in the linkset to its normal condition.

Automatic activation or deactivation of links is normally performed without interacting with the user parts or applications, unless it results in one or more destinations becoming accessible or inaccessible. In this case, the user parts are notified with a StatResumed or StatPaused indication.

If no automatic activation or deactivation of links is desired, then the minNmbActLnk threshold can be set to the actual number of links in the linkset, or greater than the number of links in the linkset.

Signaling link test

MTP 3 requires a successful signaling link test (SLTM generated and SLTA response expected) as part of link activation before considering a signaling link active. Then the signaling link test is performed periodically on each active signaling link, at a configurable period (link timer T34, corresponding to ANSI T1.111.7/ITU-T Q.707 timer T2). Signaling link testing is performed with no user part or application interaction.

The SLTM/SLTA exchange is not used in Japanese variants.  In this case, successful alignment at MTP layer 2 is considered successful alignment at MTP layer 3.

Signaling route management

When configured as an STP, MTP 3 implements the signaling route management procedures transfer prohibited, transfer allowed, transfer restricted, signaling route set test, and signaling route set congestion test described in the ANSI and ITU-T MTP standards. MTP 3 performs these procedures without interacting with the user parts or applications.