e one circuit (speech circuit in the TUP case) among those directly interconnecting the destination and originating points. For the OPC and DPC, unambiguous identification of signaling points is carried out by means of an allocated code. Separate code plans are used for the international and national networks. The CIC, as shown in Figure 14.10, is applicable only to the TUP. ITU-T (CCITT) Rec. Q.704 shows a signaling link selection (SLS) field fol- lowing (to the left) the OPC. The SLS is 4 bits long and is used for load sharing. The ISDN user part address structure is capable of handling E.164 addresses in the calling and in the called number and is also capable of redirecting address informa- tion elements. 14.12.2 Telephone User Part (TUP) The core of the signaling information is carried in the SIF (see Figure 14.10). The TUP label was described briefly in Section 14.7.2.1. Several signal message formats and codes are described in the paragraphs below. These follow the TUP label. One typical message of the TUP is the initial address message (IAM). Its format is shown in Figure 14.11. A brief description is given of each subfield, providing further insight on how SS No. 7 operates. Common to all signaling messages are the subfields HO and H1. These are the heading codes, each consisting of 4 bits, giving 16 code possibilities in pure binary coding. HO identifies the specific message group to follow. "Message group" means the type of message. Some samples of message groups are as follows: Message Group Type HO Code Forward address messages Forward setup messages Backward setup messages Unsuccessful backward setup messages Call supervision messages Node-to-node messages 14.12 USER PARTS FEDCBA Heading Calling party Heading Label category First bit transmitted LKJIHGFEDCBA Number of Address signals address Message indicators signals n x 8 Figure 14.11 Initial address message format. [From CCITT Rec. Q.723, Figure 3/Q.723, page 23 (Ref. 14).] H1 contains a signal code or identifies the format of more complex messag