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19.6 INTRODUCTION TO GSM-MAP 609Authentication Center (AUC). This register stores authentication and voiceprivacy information about the mobiles for which PLMN is the home network.Equipment Identity Register (EIR). This register stores the internationalmobile equipment identity (IMEI) of all mobile equipment (Section 12.7.4) forwhich PLMN is the home network.19.6.3 InterfacesWe next explore the GSM-MAP interfaces between the equipment entities ofFig. 19.6-1 [6]. There are some differences from the IS-MAP interfaces.The interface between a MSC and its associated VLR is known as the “B” interface.ETSI considers the B interface as “internal” and does not define it. Themessage transfer does not involve the signaling network.The C interface is between a gateway MSC and the HLR in its PLMN. The Dinterface is between a VLR and a HLR, in the same or different PLMNs.The E interface is between adjacent MSCs in a PLMN and is used in handoveroperations.The F interface is an intra-PLMN interface, between a MSC and the EIR.The G interface is between VLRs, in the same or in different PLMNs. This interfacedoes not exist in IS-MAP.In GSM-MAP, the AUC is an integral part of HLR, and no interface is defined.19.6.4 Numbering PlansIn the United States mobile networks use the numbering plan that was already inplace for fixed networks. The MIN of a mobile has the format of nationalnumbers (Section 1.2.1).Nations with GSM mobile networks have a fixed network numbering plan and aseparate PLMN numbering plan.In the fixed (PSTN/ISDN) network, a mobile is identified by a MSISDN, whichhas the format defined in ITU-T Rec. E.164 [7]—see Section 1.2. MSISDN is aninternational number, consisting of country code CC and national number NN.The mobile station roaming number (MSRN), which is used by the fixed networkto extend the call setup to the MSC currently serving the called MS, is a national orinternational E.164 number.In the PLMN, a MS is idenfied by either an international mobile station identity(IMSI) or by the combination of a location area identity (LAI) and a temporarymobile station identity (TMSI) (Section 12.7.4).The IMSI format has been defined in ITU-T Rec. E.212 [8]:IMSI ¼ MCC-MNC-MSINwhere the combination of the mobile country code MCC and the mobile networkcode MNC identifies the home PLMN of MS, and MSIN is the mobile station