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202 TELEPHONE USER PARTpart (ISUP). In Japan, a national version of ISUP has replaced multifrequencysignaling.Sections 9.1 through 9.5 primarily describe TUP as specified in ITU-T recommendations.Section 9.6 briefly discusses some aspects of national versions.9.1 MESSAGES AND PRIMITIVESThis section outlines the structure of TUP messages and reviews the primitives thatinvolve TUP.9.1.1 General Message StructureThe general TUP message format is shown in Fig. 9.1-1 [2,3]. Octet (a) is the serviceinformation octet (SIO) (Section 8.8.3), consisting of service indicator (SI) andsubservice field (SSF). The value SI ¼ 0100 indicates a TUP message.The routing label (RL) is in octets b through e and contains the originating anddestination point codes (OPC, DPC) and the signaling link selector (SLS) (Section8.8.2). The trunk for which the message is intended is identified by the combinationof OPC, DPC, and CIC (circuit identification code).9.1.2 Signaling Link Selector and Circuit Identification CodeBits 8 to 5 of octet e have a dual function: they represent both the SLS and the fourlow-order bits (CIC L ) of the circuit identification code. The high-order bits (CIC H )are in octet f. The value of SLS thus equals the value of CIC, modulo 16.As a result, each trunk has an associated signaling route. This is one of therequirements for in-sequence message delivery (Section 8.8.5).Figure 9.1-1. General format of TUP messages and signals. SIO, Service information octet; SIF,signaling information field.