Subscriber Data in GSM

Besides data of the address type, which is the most important subscriber data of any communication network, a whole series of other service- and contract-specific data exists in GSM networks. Addresses serve to identify, authenticate, and localize subscribers, or switch connections to subscribers. Service-specific data is used to parameterize and personalize supplementary services. Finally, contracts with subscribers can define different service levels, e.g. booking of special supplementary services or subscriptions to data or teleservices. The contents of such contracts are stored in appropriate data structures in order to enable correct realization or provision of these services.

The association of the most important identifiers and their storage locations is summarized in Figure 3.7. Subscriber-related addresses are stored on the SIM and in the HLR and VLR as well. These data (IMSI, MSISDN, TMSI, MSRN) serve to address, identify, and localize a subscriber or a mobile station. Whereas IMSI and MSISDN are permanent

Imsi Msisdn Sim
Figure 3.7: Overview of addresses and pertinent databases

data items, TMSI and MSRN are temporary values, which change according to the current location of the subscriber. Of the other data items defined for user or network equipment elements (like IMEI, LAI, or SPCs), only some are used (LAI, SPC) for localizing or routing. IMEI and BSIC/CI hold a special position by being used only for identification of network elements.

Security-relevant subscriber data is stored in the AUC, which also calculates identifiers and keys for cryptographic processing functions. Each set of data in the AUC contains the IMSI of the subscriber as a search key. For identification and authentication of a subscriber, the AUC stores the subscriber's secret key Ki from which a pair of keys RAND/SRES are precalculated and stored. Once an authentication request occurs, this pair of keys is queried by the VLR to conduct the identification/authentication process properly. The key Kc for user data encryption on the radio channel is also calculated in advance in the AUC from the secret key Ki and is requested by the VLR at connection setup.

Table 3.1: Mobile subscriber data in the HLR

Subscriber and subscription data

Tracking and routing information

International Mobile Subscriber Identity (IMSI)

International Mobile Subscriber ISDN Number (MSISDN)

Bearer and teleservice subscriptions

Service restrictions, e.g. roaming restrictions

Parameters for additional services

Information on the subscriber's equipment (if available)

Authentication data (subject to implementation)

Mobile Station Roaming Number (MSRN) Current VLR address (if available)

Current MSC address (if available)

Local Mobile Subscriber Identity (LMSI) (if available)

Further data about the subscriber and his or her contractual agreement with the service provider are presented in Tables 3.1 and 3.2. Above all, the HLR contains the permanent data about the subscriber's contractual relationship, e.g. information about subscribed bearer and teleservices (data, fax, etc.), service restrictions, and parameters for supplementary services. Beyond that, the registers also contain information about equipment used by the subscriber (IMEI). Depending on the implementation of the authentication center AUC and the security mechanisms, data and keys used for subscriber authentication and encryption can also be stored there.

The search keys used for retrieving subscriber information (such as IMSI, MSISDN, MSRN, TMSI and LMSI), from a register are indicated either in boldface (Figure 3.7) or in italics (Tables 3.1 and 3.2).

Table 3.2: Mobile subscriber data in the VLR

Subscriber and subscription data

Tracking and routing information

International Mobile Subscriber Identity (IMSI) Mobile Station Roaming Number (MSRN)

International Mobile Subscriber ISDN Number Temporary Mobile Station Identity (TMSI) (MSISDN)

Parameters for supplementary services

Information on subscriber-used equipment (if available)

Authentication data (subject to implementation)

Local Mobile Subscriber Identity (LMSI) (if available)

Local Area Identity (LAI) of LA, where MS was registered (used for paging and call setup)

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