Before a mobile station can be called or gets access to services, the subscriber has to register with the mobile network (PLMN). This is usually the home network where the subscriber has a service contract. However, the subscriber can equally register with a foreign network provider in whose service area he or she is currently visiting, provided there is a roaming agreement between the two network operators. Registration is only required if there is a change of networks, and therefore a VLR of the current network has not yet issued a TMSI to the subscriber. This means the subscriber has to report to the current network with his IMSI and receives a new TMSI by executing a Location Registration procedure. This TMSI is stored by the MS in its nonvolatile SIM storage, such that even after a powerdown and subsequent power-up only a normal Location Updating procedure is required.
The sequence of operations for registration is presented schematically in Figure 8.1. After a subscriber has requested registration at his or her current location by sending a location update request with his or her IMSI and the current location area (LAI), first the MSC instructs the VLR with a MAP message update location area to register the MS with its current LAI. In order for this registration to be valid, the identity of the subscriber has to be checked first, i.e. the authentication procedure is executed. For this purpose, the authentication parameters have to be requested from the AUC through the HLR. The precalculated sets of security parameters (Kc, RAND, SRES) are usually not transmitted individually to the respective VLR. In most cases, several complete sets are kept at hand for several authentications. Each set of parameters, however, can only be used once, i.e. the VLR must continually update its supply of security parameters (authentication parameter request).
After successful authentication (see Section 6.3.2), the subscriber is assigned a new MSRN, which is stored with the LAI in the HLR, and a new TMSI is also reserved for this subscriber; this is TMSI Reallocation (see Figure 7.25). To encrypt the user data, the base station needs the ciphering key Kc, which it receives from the VLR by way of the MSC with the command start ciphering. After ciphering of the user data has begun, the TMSI is sent in encrypted form to the mobile station. Simultaneously with the TMSI assignment, the correct and successful registration into the PLMN is acknowledged (loca-pdate accept). Finally, the mobile station acknowledges the correct reception of the TMSI (tmsi reallocation complete, see Figure 7.26).
While the location information is being updated, the VLR is obtaining additional information about the subscriber, e.g. the MS category or configuration parameters for supplementary services. For this purpose, the Insert Subscriber Data Procedure is defined (insert subscriber data message in Figure 8.1). It is used for registration or location updating in the HLR to transmit the current data of the subscriber profile to the VLR. In general, this MAP procedure can always be used when the profile parameters are changed, e.g. if the subscriber reconfigures a supplementary service such as unconditional forwarding. The changes are communicated immediately to the VLR with the Insert Subscriber Data Procedure.
The location update procedure is executed, if the mobile station recognizes by reading the LAI broadcast on the BCCH that it is in a new location area, which leads to updating the
location information in the HLR record. Alternatively, the location update can also occur periodically, independent of the current location. For this purpose, a time interval value is broadcast on the BCCH, which prescribes the time between location updates. The main objective of this location update is to know the current location for incoming calls or short messages, so that the call or message can be directed to the current location of the mobile station. The difference between the location update procedure and the location registration procedure is that in the first case the mobile station has already been assigned a TMSI. The
TMSI is unique only in connection with an LAI, and both are kept together in the nonvolatile storage of the SIM card. With a valid TMSI, the MS also keeps a current ciphering key Kc for encryption of user data (Figure 8.2), although this key is renewed during the location update procedure. This key is recalculated by the MS based on the random number RAND used for authentication, whereas on the network side it is calculated in the AUC and made available in the VLR.
Corresponding to the location update procedure, there is an MM procedure at the air interface of the MM-category specific. Besides the location updating proper, it contains three blocks which are realized at the air interface by three procedures of the category common (see Figure 7.26): the identification of the subscriber, the authentication, and the start of ciphering on the radio channel. In the course of location updating, the mobile station also receives a new TMSI, and the current location is updated in the HLR. Figure 8.2 illustrates the standard case of a location update. The MS has entered a new LA, or the timer for periodic location updating has expired, and the MS requests to update its location information. It is assumed that the new LA still belongs to the same VLR as the previous one, so only a new TMSI needs to be assigned. This is the most frequent case. But if its not quite so crucial to keep the subscriber identity confidential, it is possible to avoid assigning a new TMSI. In this case, only the location information is updated in the HLR/VLR.
The new TMSI is transmitted to the MS in enciphered form together with the acknowledgement of the successful location update. The location update is complete after acknowledgement by the mobile station. After execution of the authentication, the VLR can complete its database and replace the "consumed" 3-tuple (RAND, SRES, Kc) by another one requested from the HLR/AUC.
If location change involves both LA and VLR, the location update procedure is somewhat more complicated (Figure 8.3). In this case, the new VLR has to request the identification and security data for the MS from the old VLR and store them locally. Only in emergency cases, if the old VLR cannot be determined from the old LAI or if the old TMSI is not known in the VLR, the new VLR may request the IMSI directly from the MS (identification procedure). Only after a mobile station has been identified through the IMSI from the old VLR and after the security parameters are available in the new VLR, is it possible for the mobile station to be authenticated and registered in the new VLR, for a new TMSI to be assigned, and for the location information in the HLR to be actualized. After successful registration in the new VLR (location update accept) the HLR instructs the old VLR to cancel the invalid location data in the old VLR (cancel location).
In the examples shown (Figures 8.1-8.3), the location information is stored as MSRN in the HLR. The MSRN contains the routing information for incoming calls and this infor mation is used to route incoming calls to the current MSC. In this case, the HLR receives the routing information already at the time of the location update. Alternatively, at location update time, the HLR may just store the current MSC and/or VLR number in connection with an LMSI, such that routing information is only determined at the time of an incoming call.
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