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12.8 SIGNALING BETWEEN MOBILE AND NETWORK 375message (Section 12.9.1) on PAGCH, identifying the SDCCH. From this point on,all messages are on SDCCH.The MS sends a LOCATION UPDATING REQUEST message (Section 12.9.2),which includes information about its transmission characteristics, its identity, andindicating whether it is equipped for enciphering (data encryption).If the SIM in the MS has been used before, its memory stores the most recentlocation area identity (LAI) and TMSI, and the MS identifies itself with theseparameters. Otherwise, the MS uses its IMSI (stored in semipermanent SIMmemory). The network checks the MS identification and—if valid—starts the MSauthentication.The GSM authentication algorithm has two inputs: a random number RAND (128bits) and an individual subscriber authentication key Ki (128 bits). The key is storedin the SIM of the mobile and in the network. It is never transmitted on the Um (radio)interface. The MS and the network execute the algorithm, using the same RAND andtheir respective Ki values. The results of the algorithm are the signed result SRES(32 bits) and the cipher key) Kc (64 bits).The network starts the authentication by sending an AUTHENTICATIONREQUEST (Section 12.9.2), which includes a RAND value. The mobile executesalgorithm and returns an AUTHENTICATION RESPONSE (Section 12.9.2) thatincludes SRES. The network compares the received SRES with the SRES it has calculated.If they match, the network considers the MS as authenticated and continuesthe location updating procedure.If the MS has indicated that it can handle ciphered (encrypted) information, thenetwork then sends a CIPHER MODE COMMAND message (Section 12.9.1), andthe MS responds with a CIPHER MODE COMPLETE (Section 12.9.1) message.From this point on, the MS and BSS encrypt their information on SDCCH, usingthe value of Kc to form an encryption/decryption mask. If the MS has indicatedthat it cannot handle ciphering, this step is omitted.The network may decide to allocate a new TMSI to the mobile. In this case, itsends a TMSI REALLOCATION COMMAND (Section 12.9.2) that includes thenew TMSI. After entering the TMSI in its SIM, the MS sends a TMSI REALLOCA-TION COMPLETE message (Section 12.9.2), and the network sends a CHANNELRELEASE (Section 12.9.1). It then releases SDCCH, and the MS also releases thechannel.If neither the authentication nor TMSI reallocation is done, the network sends aLOCATION UPDATING ACCEPT message (Section 12.9.2) to inform the MS andthen sends the CHANNEL RELEASE.12.8.5 Setup of a Call Originated by MSWhen the user of a MS originates a call, he first enters the called number and.possiblyadditional information with the MS keypad, and then depresses the “send”button Figure 12.8-2 show the signaling for the setup of the call.The initial signaling is very Similar to the initial signaling for location updating.The MS first requests a SDCCH with a CHANNEL REQUEST message