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578 MOBILE APPLICATION PART19.1 INTRODUCTION TO IS-MAPThis section introduces some general MAP concepts and terms.19.1.1 DefinitionsFirst, a few definitions [1] are given.Home Network. This is the CMN selected for mobile communication services bythe owner of a MS.Roaming. A MS can receive service in its home CMN and in other CMNs. When aMS is being served by a CMN other than its home CMN, we say that the MS isroaming. A MS stores the system identification (SID) of its home CMN—seeSection 12.3. A MS knows that it is “at home” if the SID received in overhead parametermessages (Section 12.4.2) matches its stored SID. If the SIDs do not match,the MS is roaming.Visited Network. This is the CMN that is serving the MS when it is roaming.Home MSC. This is the MSC in the home CMN of the MS to which the PSTN deliverscalls to the MS.Serving MSC. This is the MSC that is currently serving the MS. Depending on theMS location, this can be the home MSC, another MSC in the home CMN, or a MSCin another CMN.19.1.2 Equipment Entities Involved in IS-MAP TransactionsThe MAP-related equipment entities in a cellular mobile network (CMN) are shownin Fig. 19.1-1.Mobile Switching Centers (MSCs). In this example, the CMN has three MSCs,each of which serves MSs in its part of the CMN service area.Home Location Register (HLR). This register stores semipermanent and temporarydata on mobiles for which the CMN is the home network.Visitor Location Register (VLR). A VLR is associated with one or more MSCs.In Fig. 19.1-1, VLR-P is associated with MSC-A and MSC-B, and VLR-Q is associatedwith MSC-C. A VLR stores information on roaming mobiles that are currentlybeing served by an associated MSC.Authentication Center (AUC). The AUC stores the authentication and voiceprivacy information of mobiles for which CMN is the home network and whichhave authentication and voice privacy service (Sections 12.6.7 and 12.6.8).