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.

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 MTP Testing User Part
1001	Broadband ISDN User Part
1010	Satellite ISDN User Part
1011 – 1111	Spare[*]

^[*] ANSI reserves values 1101 and 1110 for individual network use.

The SSF consists of two fields: the Network Indicator (NI) and Priority. The priority field is defined for ANSI networks and is an option that may be implemented in ITU-T national networks. The priority bits are spare bits in ITU-T networks when not used for Priority. The NI indicates whether the message is for a national or international network. A national network can also discriminate between different Point Code structures used by different countries and invoke the appropriate version of the message handling functions accordingly. Table 7-2 lists the values for the NI.

Table 7-2. Network Indicator Values

Binary Value	Message Type
0000	International
0001	International Spare
0010	National
0011	National Spare

Messages are usually routed using the national or international values. The spare values are often used for testing and for temporary use during Point Code conversions. The national spare value can also be used for creating an additional national network. For example, in some European countries, network operators have used the national spare network indicator for creating a national interconnect network. Using this method, the switches between operator networks have two Point Codes assigned: one for the interconnect network using the national network indicator, and the other for the operator network using the national spare network indicator. This allows the network operator to administer Point Codes as he chooses within his national network, while using the interconnect network to interface with other network operators.

The ITU-T defines the two least significant bits of the SSF as spare bits. These bits are used to define message priority in ANSI networks, but are unused in ITU-T networks. The ANSI message priority values are 0–3 with 3 being the highest priority. The node originating the message assigns the priority to allow message throttling during periods of network congestion. The use of the message priority field is discussed in the section, "Multiple Congestion Levels."

Signaling Information Field (SIF)

The SIF contains the actual user data being transported by MTP, such as telephone numbers, control signals, or maintenance messages. The Service Indicator designates the type of information contained within the SIF user data field. For example, a Service Indicator of 0 indicates that the SIF contains Signaling Network Maintenance data. A Service Indicator of 5 indicates that the SIF contains ISUP information. The beginning portion of the SIF also contains the Routing Label that is used for routing the message within the network. The Routing Label contains the following three components:

• Originating Point Code (OPC)— Identifies the node originating the message

• Destination Point Code (DPC)— Identifies the destination node

• Signaling Link Selector (SLS)— An identifier used for load sharing across linksets and links

Figure 7-5 shows the fields in the routing label.

When a node generates a message, it inserts its own Point Code into the OPC field. This Point Code identifies the node that originated the message to subsequent nodes. As previously discussed, the DPC field is populated based on the internal routing tables. The SLS code is used for load sharing MTP3 User Part messages across links and linksets. The originating node generates a bit pattern and places it in this field. The SLS code maps the message to a particular link among the linksets and links that are available for routing. It is generated in a manner that minimizes mis-sequencing of messages belonging to a particular transaction from the perspective of MTP users, while balancing the load across the links and linksets.

For more information about the use of the SLS code for load sharing, see "Routing" within the "Signaling Message Handling" section. The Signaling Link Code (SLC) for messages generated by MTP3 (e.g., SNM) replaces the SLS field. The "Message Load Sharing" section discusses the SLC code further.

The ITU-T and ANSI Routing Labels are similar in structure, but differ slightly in size and meaning. The following sections detail these differences.

ITU-T Routing Label

The ITU-T routing label consists of the following fields:

• DPC

• OPC

• SLS

The ITU-T point codes are 14 bits in length. For ITU-T national networks, all 14 bits are interpreted as a single identifier that is often referred to as a structureless Point Code. For international networks, an International Signaling Point Code (ISPC) is subdivided into hierarchical fields, shown in Figure 7-6.

The SLS is a four-bit field that identifies the link and/or linkset on which a message is transmitted.

ANSI Routing Label

The ANSI routing label consists of the following fields:

• DPC

• OPC

• SLS

The ANSI Point Code is 24 bits in length and is subdivided into three fields of one octet each, as shown in Figure 7-7. The three octets define the network, cluster, and member that uniquely identify the signaling node within the network hierarchy. The SLS field is an eight-bit field used for selecting the link and/or linkset for message transmission. This field was only five bits in earlier versions of the protocol, but was extended for better load sharing across signaling links in the 1996 version of the ANSI standards.