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12.8 SIGNALING BETWEEN MOBILE AND NETWORK 373slot 0, and that the current frame is one of the five listed above. The MS now initializesits (eight-step) time-slot counter and obtains time-slot synchronization.The MS then reads the Synchronization Channel (SCH), whose bursts occur 8BPs after the FCCH bursts. SCH information indicates which one of the aboveframes is the current CCCH frame. The MS then initializes its (51-step) CCCHframe counter and is now synchronized with CCCH frames. Since BCCH appearsin predetermined frames (2–5) of CCCH, the MS is now also synchronized withBCCH and begins to read the System Information messages that are transmittedperiodically on BCCH. There are messages that indicate the frames used forPAGCH and RACH. The MS stores this information and is now synchronizedwith all common-control channels in the cell.SCH information also indicates the current frame number (0–25) in the multiframefor TACHs. This enables the MS to initialize its (26-step) frame counter,for synchronization with the TACHs.12.8.3 Idle MSIdle mobiles can move from cell to cell. In order to remain in contact with thenetwork, a MS periodically compares the signal strength of the RF carrier withthe CCCH in its cell to RF “CCCH” carriers in adjacent cells. The MS knows thechannel numbers of these carriers from the system information messages.If one or more of these RF carriers is stronger than the current carrier, the MSretunes to the strongest one and then reinitializes itself as described above.12.8.4 Location UpdatingGSM “location updating” is the counterpart of “registration” in AMPS and IS-54systems. GSM networks keep track of the location area where the MS is operating[22]. When receiving an incoming call, the MS is paged in all cells of its currentlocation area. GSM mobiles do a location update when entering a new locationarea, and at periodic intervals. In addition, some MSs are able to do a locationupdate when being activated or deactivated by their users.Updating on Entering a New Location Area. The location area identity (LAI)is broadcast in System Information messages on BCCH and stored in MS memory.When a new received LAI does not match the previously stored LAI, the MS does alocation update. This happens when an idle MS has tuned to a new BCCH carrier in adifferent location area.Periodic Updating. Whenever a MS does a location update, it resets a timerT. The timer has a timeout value of several hours. When the timer expires, theMS does a location update.Updating on Deactivation and Activation. Mobiles equipped to do theseupdates send an IMSI DETACH message when being deactivated. The network