Section: 4.4.3 IMSI attach procedure
Content: The IMSI attach procedure is the complement of the IMSI detach procedure (see subclause 4.3.4). It is used to indicate the IMSI as active in the network. In A/Gb mode and GERAN Iu mode, the network indicates whether the IMSI attach/detach procedures are required by using the ATT flag which is broadcast in the L3-RR SYSTEM INFORMATION TYPE 3 message (see 3GPP TS 44.018[ None ]  [84] subclause 10.5.2.11). In UTRAN Iu mode, the network indicates whether the IMSI attach/detach procedures are required by using the ATT flag which is included in the CS domain specific system information element (see subclause 10.5.1.12.2). The IMSI attach procedure is invoked if the ATT flag indicates that the IMSI detach/attach procedures are required and an IMSI is activated in a mobile station (i.e. activation of a mobile station with plug-in SIM/USIM, insertion of a card in a card-operated mobile station etc.) within coverage area from the network or a mobile station with an IMSI activated outside the coverage area enters the coverage area. The IMSI attach procedure is used only if the update status is U1 UPDATED and if the stored location area identification is the same as the one which is actually broadcasted on the BCCH of the current serving cell. In a shared network, the MS shall choose one of the PLMN identities as specified in 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [14]. The MS shall use the IMSI attach procedure only if the update status is U1 UPDATED and the stored location area identification is equal to the combination of the chosen PLMN identity and the LAC received on the BCCH. Otherwise a normal location updating procedure (see subclause 4.4.1) is invoked independently of the ATT flag indication. The IMSI attach procedure is performed by using the location updating procedure (see subclause 4.4.1). The location updating type information element in the LOCATION UPDATING REQUEST message shall in this case indicate IMSI attach.