e Source: From para. 11.1:3, ITU-T Rec.Q.703 (Ref. 7). The service information octet (SIO) is divided into a service indicator and a subservice field. The service indicator is used to associate signaling information for a particular user part and is present only in MSUs. Each is 4 bits long. For example, a service indicator with a value 0100 relates to the telephone user part, and 0101 relates to the ISDN user part. The subservice field portion of the SIO contains two network indicator bits and two spare bits. The network indicator discriminates between international and national signaling messages. It can also be used to discriminate between two national signaling networks, each having a different routing label structure. This is accomplished when the network indicator is set to 10 or 11. The signaling information field (SIF) consists of an integral number of octets greater than or equal to 2 and less than or equal to 62. In national signaling networks it may consist of up to 272 octets. Of these 272 octets, information blocks of up to 256 octets in length may be accommodated, accompanied by a label and other possible house- keeping information that may, for example, be used by layer 4 to link such information blocks together. The link status signal unit (LSSU) provides link status information between signaling points. The status field can be made up of one or two octets. CCITT Rec. Q.703 indicates application of the one-octet field in which the first three bits (from right to left) are used (bits A, B, and C) and the remaining five bits are spare. The values of the first three bits are given in Table 14.1. 14.7 SIGNALING NETWORK FUNCTIONS AND MESSAGES (LAYER 3) 14.7.1 Introduction In this section we describe the functions and procedures relating to the transfer of messages between signaling points (i.e., signaling network nodes). These nodes are connected by signaling links involving layers 1 and 2 described in Sections 14.5 and 14.6. Another important function of layer 3 is to inform the appropriate entities o