Section: 4.3.1 TMSI reallocation procedure 4.3.1.0 General
Content: The purpose of the TMSI reallocation procedure is to provide identity confidentiality, i.e. to protect a user against being identified and located by an intruder (see 3GPP TS 42.009[ Security aspects ]  [5], 3GPP TS 43.020[ Security related network functions ]  [13] and 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]). If the identity confidentiality service is applied for an IMSI, a Temporary Mobile Subscriber Identity (TMSI) is used for identification within the radio interface signalling procedures. In a network supporting the feature 'Intra domain connection of RAN nodes to multiple CN nodes' a TMSI shall be allocated to each IMSI attached mobile station. See 3GPP TS 23.236[ Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes ]  [94], subclause 4.3. The structure of the TMSI is specified in 3GPP TS 23.003[ Numbering, addressing and identification ]  [10]. The TMSI has significance only within a location area. Outside the location area it has to be combined with the Location Area Identifier (LAI) to provide for an unambiguous identity. Usually the TMSI reallocation is performed at least at each change of a location area. (Such choices are left to the network operator). The reallocation of a TMSI can be performed either by a unique procedure defined in this subclause or implicitly by a location updating procedure using the TMSI. The implicit reallocation of a TMSI is described together with that procedure. If a TMSI provided by a mobile station is unknown in the network e.g. due to a data base failure, the network may require the mobile station to provide its International Mobile Subscriber Identity (IMSI). In this case the identification procedure (see subclause 4.3.3) should be used before the TMSI reallocation procedure may be initiated. The TMSI reallocation can be initiated by the network at any time whilst a RR connection exists between the network and the mobile station. NOTE 1: Usually the TMSI reallocation is performed in ciphered mode. NOTE 2: Normally the TMSI reallocation will take place in conjunction with another procedure, e.g. at location updating or at call setup (see 3GPP TS 29.002[ Mobile Application Part (MAP) specification ]  [37]). NOTE 3: The explicit TMSI reallocation procedure is started by the network only if the mobile station is updated in the current location area or if a location updating procedure is ongoing for that particular mobile station, or if the network wishes to send a non-broadcast LAI according to 3GPP TS 23.236[ Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes ]  [94] to the mobile station.