An MS or a subscriber can be localized within the network by using identity numbers. An active GSM MS has multiple identity numbers.

1. Mobile Station ISDN Number (MS ISDN)

The MS ISDN is the unique phone number of the subscriber in the public telephone network. The MS ISDN consists of Country Code (CC), the National Destination Code (NDC), which defines the regular GSM provider of the subscriber, and the subscriber number. The MS ISDN should not be longer than 15 digits.

2. International Mobile Subscriber Identity (IMSI)

The IMSI is another unique identification for the subscriber. In contrast to the MS ISDN, which is used as the phone number of the subscriber within the GSM network and the normal public phone network, the IMSI is only used for subscriber identification in the GSM network. It is used by the Subscriber Identity Module (SIM), which we explain later, the HLR and the VLR. It consists of three parts: the Mobile Country Code (MCC, three digits), the Mobile Network Code (MNC, two digits), and the Mobile Subscriber Identification Number (MSIN, up to ten digits).

3. Mobile Station Roaming Number (MSRN)

The MSRN is a temporary identification that is associated with a mobile if it is not in the area of its HLR. This number is then used for routing of connections. The number consists again of a CC, MNC, and a TMSI, which is given to the subscriber by the GSM network (s)he is roaming into.

4. International Mobile Station Equipment Identity (IMEI)

The IMEI is a means of identifying hardware - i.e., the actual mobile device. It is to be noted that the three identity numbers described are either permanently or temporarily associated with the subscriber. In contrast, the IMEI identifies the actual MS used. It consists of 15 digits: six are used for the Type Approval Code (TAC), which is specified by a central GSM entity; two are used as the Final Assembly Code (FAC), which represents the manufacturer; and six are used as a Serial Number (SN), which identifies every MS uniquely for a given TAC and FAC.