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162 INTRODUCTION TO SIGNALING SYSTEM NO. 7Sprint, etc.). For example, a value of U identifies the East Coast area of AT&T’slong-distance network, and V identifies an ISC in that area of the network.ITU-T has allocated the range U ¼ 020–059 in zone Z ¼ 3 to the United States.7.2.4 National Point Codes in Other CountriesCountries other than the United States use 14-bit national point codes. The codeassignments are made by the individual national telecoms.7.2.5 Identification of TrunksA trunk group with TUP or ISUP signaling is identified uniquely in a nationalnetwork (or in the international network) by the point codes of the exchanges thatare interconnected by it.The circuit identification code (CIC) identifies a trunk within a trunk group. TheCIC field has a length of 12 bits and thus can identify trunks in groups of up to 4095trunks.7.3 SS7 SIGNAL UNITS AND PRIMITIVESSS7 signal units (SUs) have different lengths but always occupy an integral numberof octets (groupings of eight bits). A SU consists of an ordered set of fields withparameters. In the documentation of the early SS7 parts (MTP and TUP), the SUsare shown as in Fig. 7.3-1(a). Par.1, Par.2, ...denote the first, second, ...parameterfields of the SU. In the more recently defined parts of SS7, a SU is shown as a stackof octets—see Fig. 7.3-1(b). Bits in the octets are numbered from right to left, and bit1 of octet 1 is the first bit sent out. For uniformity, this representation is usedthroughout this book.The lengths of SS7 parameter fields are not limited to integral multiples of octets.For example, Par.2 and Par.3 in Fig. 7.3-1(b) have a length of 12 bits.A working signaling link carries a continuous SU stream in each direction.7.3.1 Signal Unit TypesIn addition to the message signal unit, SS7 includes two other SU types [6]. The linkstatus signal units (LSSUs) and fill-in signal units (FISUs) originate in the MTP2 atone end of the signaling link and are processed by the MTP2 at the other end (theyare the SS7 equivalents of the “link” SUs described in Section 5.2). LSSUs are usedfor the control of a signaling link, and FISUs are sent when no MSUs or LSSUs arewaiting to be sent out.Figure 7.3-2 shows a MSU in some detail. It consists of a MTP3 message, surroundedby MTP2 data. Length indicator (LI) has a dual role. In the first place, itindicates the number of octets, measured from octet 4 through octet (n 2 2). Inaddition, the value of LI implies the SU type. In MSUs, LI exceeds 2, LSSUshave LI ¼ 1 or 2, and FISUs have LI ¼ 0.