ervices and the short message Each mobile user is provided with a Subscriber Identity Module service. (SIM) which contains the IMSI, the individual subscriber authenti- The BSC to MSC A-interface, and the various MSC to Register cation key (Ki) and the authentication algorithm (A3). After the interfaces employ CCITT No.7 signalling using the Message Trans- mobile user has made an access and service request, the network fer Part (MTP), Signalling Connection Control Part (SCCP), Trans- checks the identity of the user by sending a random number action Capabilities Part (TCAP) and Mobile Application Part (RAND) to the mobile. The mobile uses the RAND, Ki and A3 (MAP). algorithm to produce a signed response SRES. This response is An example of the signalling messaging for establishing a mobile compared with a similar response calculated by the network, and originated call is shown in Figure 47.13. The key events are: access only continues if the two responses match. The SIM also contains a cipher key generating algorithm (A8). 1. Request and assignment of a channel, between MS and BSS. The MS uses the RAND and A8 to calculate a ciphering key (Kc) 2. A service request procedure which accesses the VLR. which is used to encrypt and decrypt signalling and user data 3. An authentication and ciphering exchange which validates the information. mobile user and sets the encryption cipher. The authentication centre (AUC) is responsible for all security 4. Call set up which includes sending of dialled digits and estab- aspects and its function is closely linked with the HLR. The AUC lishing the connection. generates the Ki's and associates them with IMSIs, and provides the HLR with sets of RAND, SRES and Kc for each IMSI. The HLR Location updating is shown in Figure 47.14 An update request then provides the appropriate VLR with these sets and it is the VLR indicated by the mobile and passed to the VLR in the new location which carries out the authentication check. Authentication of area. The new VLR requests the IMSI from the ol