The functionality of the message signal unit lies in the actual content of the service information octet and the signaling information field (see Figure 8).

The service information octet is an 8-bit field (as might be inferred from its name) that contains three types of information as follows:

1. four bits are used to indicate the type of information contained in the signaling information field; they are referred to as the service indicator; the values most commonly used in American networks are outlined in Table 1

   Value	Function
   0	signaling network management
   1	signaling network testing and maintenance
   3	signaling connection control part (SCCP)
   5	ISDN user part (ISUP)

   
   Table 1. Common Signaling Indicator Values

2. two bits are used to indicate whether the message is intended (and coded) for use in a national or international network; they are generally coded with a value of 2, national network
3. the remaining 2 bits are used (in American networks) to identify a message priority, from 0 to 3, with 3 being the highest priority; message priorities do not control the order in which messages are transmitted; they are only used in cases of signaling network congestion; in that case, they indicate whether a message has sufficient priority to merit transmission during an instance of congestion or whether it can be discarded en route to a destination

The format of the contents of the signaling information field is determined by the service indicator. (Within user parts, there are further distinctions in message formats, but the service indicator provides the first piece of information necessary for routing or decoding the message.)

The first portion of the signaling information field is identical for all MSUs currently in use. It is referred to as the routing label. Simply stated, the routing label identifies the message originator, the intended destination of the message, and a field referred to as the signaling-link selection field which is used to distribute message traffic over the set of possible links and routes. The routing label consists of 7 octets that are outlined below in Table 2 (in order of transmission).

Octet Group	Function	Number of Octets Involved
destination point code (DPC)	contains the address of the node to which the message is being sent	3 octets
originating point code (OPC)	contains the address of message originator	3 octets
signaling link selection (SLS)	distributes load among redundant routes	1 octet

Table 2. Routing Label

Point codes consist of the three-part identifier (network number, cluster number, and member number), which uniquely identifies a signaling point.