Mobile Initialization
There are three main goals of the mobile initialization procedure:
1. Frequency synchronization
2. Timing synchronization
3. Overhead information acquisition
Frequency Synchronization. As the terminal is switched on, it scans over the available GSM RF channels and takes several readings of their RF levels to obtain an accurate estimate of the signal strengths. Starting with the channel with the highest level, the terminal searches for the frequency correction burst on the BCCH. If no frequency correction burst is detected, it then moves to the next highest level signal and repeats the process until it is successful. In this event, the terminal will then synchronize its local oscillator with the frequency reference of the base station transceiver.
Timing Synchronization. After frequency synchronization has been achieved, the terminal will search for the synchronization burst for the timing information present on the SCH. If it is not successful, it then moves to the next highest level signal and repeats the process starting from the frequency synchronization procedure until it is successful. In this event, it moves to the BCCH to acquire overhead system information.
Overhead Information Acquisition. After timing synchronization has been achieved, the terminal will search for overhead information on the BCCH. If the BCCH information does not include the current BCCH number, it will restart the mobile initialization procedure. In a successful event, the terminal will have acquired, from the BCCH and through the system information message present on the BCCH, the following main information:
- Country code
- Network code
- Location area code
- Cell identity
- Adjacent cell list
- BCCH location
- Minimum received signal strength
- The terminal checks if the acquired identification codes coincide with those in the SIM card. In a successful event, it will maintain the link and monitor the PCH. Otherwise, it will start a location update procedure.
Location Update
A location update procedure is carried out in one of the following events:
The terminal is switched on and verifies that the identification codes present on the current BCCH do not coincide with those in the SIM card.
The terminal moves into a location area different from that within which it is currently registered.
There has been no activity for a pre-established amount of time. As part of the process used to speed the paging procedure, location reports are used. These location reports are periodic reports used to update the location of the terminal so that, in the event of a page, the latest reported location is used as an initial guess to locate the terminal. The time span between location reports constitutes a system parameter whose value is indicated on the BCCH, varying in accordance with the network loading.
The location update procedure starts with the uplink channel request message on the RACH. The network answers with an immediate assignment message on the AGCH indicating the SDCCH number to be used throughout the location update procedure. The terminal moves to this SDCCH and sends a location updating request message with its identification (IMSI or, preferably, TMSI). An authentication procedure is then carried out. In case the authentication is unsuccessful, the procedure is aborted. In a successful event, the ciphering procedure is performed. The network then uses the location updating accept message to assign a new TMSI to the terminal. The terminal stores its TMSI and responds with a TMSI allocation complete message. The location update is concluded with a channel release message from the network to the terminal. The terminal then resumes its PCH monitoring procedure.
Authentication
An authentication procedure may be required at the location update procedure or at the request of a new service. The authentication procedure starts with the network sending an authentication request message to the terminal; the message conveys a 128-bit random number (RAND). The terminal uses the RAND, the secret key, Ki, stored at SIM, and the encryption algorithm, referred to as A3, to compute a 32-bit number, referred to as a signed response (SRES). Another 64-bit key, the ciphering key, Kc, is computed using another encryption algorithm, referred to as A8. The Kc parameter is later used in the ciphering procedure. After these computations, the terminal responds with an authentication response message, which contains the SRES. The network uses the same parameters and the same algorithm to compute another SRES. The terminal SRES and the network SRES are then compared with each other. If a match occurs, the network accepts the user as an authorized subscriber. Otherwise, the authentication is rejected.
Ciphering
Ciphering (or encryption) is usually required for user transactions over the RF link after authentication has been successful. The network transmits a ciphering mode message to the terminal indicating whether or not encryption is to be applied. In case ciphering is to be performed, the secret key Kc (64 bits), which was generated previously in the authentication procedure, the frame number (22 bits), and an encryption algorithm, referred to as A5, are used to compute a 114-bit encryption mask. This mask is modulo-2 added to the 2 × 57 = 114 bits of the data fields, in the bursts. Deciphering is obtained at the base station by performing the same procedure. The terminal answers with a ciphering mode acknowledgment message. Note that the ciphering to be used is continuously changing (on a frame-by-frame basis), because it depends on the current frame number.
Mobile Station Termination
After the mobile initialization procedure, the terminal camps on the PCH. It eventually detects a paging request message conveying its TMSI. This impels the terminal to access the RACH to transmit a channel request message. An immediate assignment with the SDCCH number is sent by the network on the AGCH. The terminal moves to SDCCH and the following occurs. The terminal transmits a paging response message indicating the reason for the specific message (response to a paging). An authentication procedure is carried out, as already described. In a successful event, a ciphering procedure is accomplished, as already described. The base station then sends a setup message. The terminal responds with a call confirmed message followed by an alerting message to indicate that the subscriber is being alerted. At the subscriber’s call acceptance, the terminal sends a connect message and removes the alerting tone. The network responds with an assignment command message indicating the traffic channel number to be used for the conversation. The subscriber, still on the SDCCH, responds with an assignment acknowledgment message and moves to the traffic channel that has been assigned. The network confirms the acceptance of the call by the other party by means of a connect acknowledgment message on the FACCH of the assigned TCH. And the conversation proceeds on the TCH.
Mobile Station Origination
The terminal detects a user-originated call. It then accesses the RACH to send a channel request message. An immediate assignment with the SDCCH number is sent by the network on the AGCH. The terminal moves to this channel and the following occurs. The terminal transmits a paging response message indicating the reason for the specific message (call setup). The base station responds with an unnumbered acknowledgment message. An authentication procedure is carried out, as already described. In a successful event, a ciphering procedure is performed, as already described. The terminal then sends a setup message. The base station responds with a call confirmed message followed by an alerting message in which case the terminal applies the ring-back tone. At the called party’s call acceptance, the network sends an assignment command message informing the traffic channel number to be used for the conversation. The subscriber, still on the SDCCH, responds with an assignment acknowledgment message and moves to the traffic channel that has been assigned. The network confirms the acceptance of the call by the other party by means of a connect acknowledgment message on the FACCH of the assigned TCH. And the conversation proceeds on the TCH.
Handover
The handover process in a GSM network has the mobile terminal as an integral part of the procedure. The whole process is named mobile-assisted handover (MAHO). While making use of the traffic channel, the mobile monitors the signal levels of its own channel, of the other channels of the same cell, and of the channels of six surrounding cells. The measurements are then reported to the base on an SACCH. Concerning the control of the process, handovers may occur:
- Within the same BTS or between BTSs controlled by the same BSC
- Between different BSCs controlled by the same MSC
- Between different BSCs controlled by different MSCs
- Between different BSCs controlled by different MSCs belonging to different PLMNs
In addition, there are two modes of handovers: synchronous or asynchronous. In the synchronous mode, the origin cell and the destination cell are synchronized. By measuring the time difference between their respective time slots, the mobile itself may compute the timing advance. This is used to adjust its transmissions on the new channel, therefore, speeding up the handover process. In the asynchronous mode, the origin cell and the destination cell are unsynchronized. The timing advance, in this case, must be acquired by means of a procedure involving the terminal and the new BTS, as follows. The mobile terminal sends a series of access bursts with a zero timing advance through several handover access messages. The BTS then computes the required timing advance using a round-trip time delay of the messages. On the average, the handover processing time in the synchronous mode (200 ms) is twice as long as that of the synchronous mode (100 ms).
Next a simple asynchronous handover procedure occurring between BTSs of the same BSC is described. While in conversation on a TCH, the terminal monitors the signal levels of several channels. These measurements are reported to the base station on a periodic basis by means of the measurement report message running on the SACCH. Whenever suitable, the base sends a handover command message on the FACCH, indicating that a handover is to take place. The number of the new TCH is included within the message. The terminal then moves to this new channel and sends a series of handover access messages so that the base may compute the timing advance to be transmitted to the terminal. This is done in the physical information message transmitted to the terminal on the FACCH. The timing adjustment is carried out and the terminal responds with a handover complete message.
Call Clearing
The call clearing process may be initiated either by the network or by the mobile. In either case, the channel used for the exchange of information is the BCCH. Assuming the network initiates the clearing, the base sends a disconnect message to the terminal. The terminal responds with a release message. The base replies with a release complete message. If the terminal initiates the clearing, then the same messages flow, but in the opposite direction.
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