Message Format The MTP3 portion of an SS7 message consists of two fields: the Signaling Information Field (SIF) and the Service Information Octet (SIO). The SIF contains routing information and the actual payload data being transported by the MTP3 service. | Message Format The MTP3 portion of an SS7 message consists of two fields the Signaling Information Field SIF and the Service Information Octet SIO . The SIF contains routing information and the actual payload data being transported by the MTP3 service. The SIO contains general message characteristics for identifying the network type prioritizing messages ANSI only and delivering them to the appropriate MTP3 user. When an SS7 node receives messages Signaling Message Handling SMH uses the SIO and the portion of the SIF that contains routing information to perform discrimination routing and distribution. SMH functions are discussed in the Signaling Message Handling section later in this chapter. Service Information Octet As shown in Figure 7-4 the SIO is a one-octet field composed of the Service Indicator SI and the Subservice Field SSF . While the SI occupies the four least significant bits of the SIO the SSF occupies the four most significant bits. Figure 7-4. SIO Fields The Service Indicator designates the type of MTP payload contained in the Signaling Information Field. MTP3 uses the SI to deliver the message payload to the appropriate MTP3 user using the message distribution function discussed later in the Signaling Message Handling section. The message is delivered to MTP3 for SI values of 0-2 the message is delivered to the appropriate User Part for SI values of 3 and higher. For example all ISUP messages used in setting up phone calls would use a Service Indicator of 5. Table 7-1 lists the values for the Service Indicator. Table 7-1. Service Indicator Values Binary Value Type of Payload 0000 Signaling Network Management Messages 0001 Signaling Network Testing and Maintenance Messages 0010 Signaling Network Testing and Maintenance Special Messages ANSI or Spare ITU-T 0011 SCCP 0100 Telephone User Part 0101 ISDN User Part 0110 Data User Part call and circuit-related messages 0111 Data User Part facility registration and cancellation messages 1000 Reserved for .
