Overview

The Message Transfer Part (MTP) Level 3 provides the functions and procedures related to message routing and network management. MTP Level 3 handles these functions, assuming that signaling points are connected with signaling links as described in MTP Level 1 and Level 2. Figure 4.1 shows how the MTP Level 3 compares to Layer 3 of the OSI 7-Layer Model.

Figure 4.1: MTP Level 3 of the SS7 Layers

The MTP Level 3 functions can be divided into two basic categories: signaling message handling functions, and signaling network management functions. Figure 4.2 shows how SS7 message handling functions are made up of message routing, discrimination and distribution; these functions are performed at each signaling point in the signaling network.

Figure 4.2: Signaling Network Functions

The signaling network management functions provide the actions and procedures required to activate and maintain signaling service, and to restore normal signaling conditions in the event of disruption in the signaling network, either in signaling links or at signaling points.

Signaling Message Handling

Signaling message handling is based on the routing label contained in the signaling information field (SIF) of the message signal units. Link status and fill-in signal unit messages only travel between two signaling points, therefore neither of these messages contains routing labels.

Figure 4.3 shows a routing label that is contained within the SIF field of the MSU. The routing label identifies the originating and destination address for the message. In some cases, the service information octet (SIO) is used...