Figure 19.6-2. Format of IMSI. (From GSM.09.02-Version 4.9.0. Courtesy of ETSI.)

Figure 19.6-2. Format of IMSI. (From GSM.09.02-Version 4.9.0. Courtesy of ETSI.)

identity that identifies a mobile in that PLMN—see Fig. 19.6-2. IMSI is an international number that uniquely identifies a mobile worldwide. In its home country, a MS can be uniquely identified by a national mobile station identity (NMSI):


MSISDN, IMSI, and NMSI permanently identify a MS. In addition, the combination of a location area identity (LAI) and temporary mobile station identity (TMSI) identifies a MS during the time it is served by the VLR that covers the location area. The format of LAI is


where MCC-MNC identifies a particular PLMN, and the local area code (LAC) represents a location in the PLMN.

TMSI is a 32-bit binary number that identifies the MS while it is operating in a particular location area.

Because of the multiple MS identities and numbering plans, GSM-MAP requires a number of operations that are not encountered in IS-MAP.

19.6.5 Information in SIM, HLR, VLR, EIR, and AUC

The most important parameters stored in the subscriber identity module (SIM) of a MS (Section 12.7.4), and in the various GSM network entities, are listed below [9]. Semipermanent and temporary parameters are denoted by (S) and (T).

Information in SIM

• Ki (S), authentication key (Section 12.8.4) . Kc (T), cipher key (Section 12.8.4)

A MS updates its temporary parameters on command from the serving MSC.

Information in HLR. The HLR of a PLMN has records for all MSs whose home network is PLMN.

• Originating and terminating service profile of MS (S)

• Address of the VLR associated with the MSC that is currently serving MS (T)

Information in VLR. This register has a record for each MS currently served by one of its associated MSCs.

• Originating and terminating service profile of MS (T) . IMSI (T)

• TMSI in the current location area (T)

• MSRN that is currently assigned to the MS (T)

Information in AUC. The AUC in a PLMN has records with authentication and privacy information for all mobiles whose home network is PLMN.

19.6.6 SCCP Addresses

GSM-MAP messages are transferred by the message transfer part (MTP) and the signaling connection control part (SCCP) of the SS7 signaling networks that also transfer messages for fixed-network applications.

GSM-MAP uses the connectionless services of SCCP (Section 15.3.3). We now explore the SCCP addresses of GSM-MAP entities. The application service entities (ASE) at a signaling point are addressed by subsystem numbers (SSNs):

0000 0110 Home location register

0000 0111 Visitor location register 0000 1000 Mobile switching center

0000 1001 Equipment identity register

Figure 19.6-3. Format and derivation of E.214 global title. (From GSM.09.02-Version 4.9.0. Courtesy of ETSI.)

In SCCP called (CDA) and calling (CGA) addresses, SSN is always present. In addition, the addresses include a point code (PC) and/or a global title (GT). This depends on locations of the entities involved in the transaction:

• If both entities are in the same PLMN, the address is a point code.

• If both entities are in the same country but in different PLMNs, the address is a global title with the format of an E.164 number, according to the numbering plan for the fixed network in that country.

• If the entities are in different countries, the address is a global title with a format defined in ITU-T Rec. E.214 [10].

The format is shown in Fig. 19.6-3(a). It consists of an E.164 part (country code CC and network code NC) and an E.212 part (MSINT), which represents the leading digits of MSIN.

The derivation of this address from the IMSI of a mobile is shown in Fig. 19.6-3(b). Country code CC is derived from mobile country code MCC. Network code NC is derived from mobile network code MNC and the leading digits of MSIN. MSINT is truncated if necessary, in order to limit the length of the address to 15 digits.

19.6.7 Derivation of Called Address

The entity that initiates a transaction has to derive the address of the called entity from an input datum. We explore the address derivations in a number of situations that frequently occur in GSM-MAP.

A VLR initiates a transaction with the HLR of a mobile identified by IMSI. VLR derives the called address from IMSI.

A gateway MSC initiates a transaction with the HLR of a mobile identified by MSISDN. In this situation, the GMSC, mobile, and HLR belong to the same PLMN. Most PLMNs have one HLR, and the called address (a point code) is fixed. If a PLMN has several HLRs, GMSC derives the address from MSISDN.

VLR1 initiates a transaction with VLR2, which covers a particular location area (LAI). In this case, VLR1 derives the address of VLR2 from LAI. A VLR has no access to a VLR in another country.

A VLR initiates a transaction with its associated MSC, or vice versa. The interface between these two entities is internal (B interface), and addressing is not required.

A HLR initiates a transaction with the VLR serving a MS. This situation occurs only if the VLR initiated a previous transaction concerning MS with HLR. During the previous transaction, HLR has stored the VLR address in its record on MS and uses this address when it initiates its transaction.

19.6.8 GSM-MAP Transactions and Operations

As in IS-MAP, the GSM-MAP transactions consist of one operation and require two messages. GSM-MAP uses the terms of ITU-T TCAP: a Begin message (with one invoke) initiates the transaction, and an End message (with a return-result or return-error) ends the transaction.

In the sections that follow, the messages of a transaction are shown as in the example of Fig. 19.6-4, where ASE-A requests ASE-B to provide a roaming number for the mobile identified by IMSI. The Begin message is denoted by MAP_ SEND_ ROAMING_NUMBER, which indicates the invoked operation. The End message is denoted by the same character string, followed by ack [9].

In what follows, the most important parameters in invokes and return-results are shown in parentheses, below the respective messages.

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