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A mobile device and the BTS have to transmit in specific time slots or bursts. If they transmit too early or too late then they will cause interference to the previous or following call. In an ideal situation all users would be exactly the same distance from the BTS and would transmit at the beginning of the allocated slot. In practice, however, mobile devices are at different distances from the BTS and also each mobile device is free to move closer or further away. Due to the propagation delay over the air interface, mobile devices which are further away actually start to transmit before their allocated time slot. This is known as timing advance and is directly related to the distance from the BTS. The initial timing advance measurement is estimated by monitoring the received signal from a mobile device when it initially sends a burst on the RACH. As the mobile device moves away or towards the BTS, the network informs it of its new timing advance value on the SACCH. Cell sizes in rural areas (also in urban areas if a cell hierarchy is used) can be as large as 35 km in normal circumstances and up to 120 km for GSM400. Note that the timing advance can deal with mobile devices moving at speeds of up to 500 kmph.

3.5

INITIAL CONNECTION PROCEDURE

When the mobile device is switched on, it tries to register with a mobile network. The subscriber’s home network will be stored in the SIM module and this will be checked first. If this network is available then the mobile device will request a connection. If the subscriber’s home network is not available, the mobile device will try to attach to the last network to which it was connected prior to being switched off. This information is also stored in the SIM module. If neither of the above networks is available, the mobile device begins searching through all of the frequencies in the band to try to find a suitable network. This is the case, for example, when a subscriber arrives at an international airport from a foreign country.

While searching through the various frequencies, the mobile device is looking for a strong BCCH signal. This signal includes a number of channels, including the FCCH and the SCH. The FCCH simply emits a sine wave carrier to enable the mobile device to synchronize its frequency reference with the base station. The SCH contains the base station identity code and a frame number. The BCCH also gives the mobile device infor- mation about the network, such as where it is, which LA it falls under and who the operator is. On selecting a strong BCCH, the mobile will try to attach to this network. It does this by sending a request on the RACH channel, to which the base station listens continuously for mobiles wishing to register themselves. The RACH is a shared channel (referred to as a common channel) which works on the slotted-Aloha protocol. If many subscribers try to connect to the network simultaneously their requests will cause inter- ference to each other. This may result in the network receiving the requests in error and discarding them. The mobile devices will wait for a random amount of time before trying to register again. The mobile networks covering an international airport are put under a great deal of strain when a flight arrives as many hundreds of users attempt to make an initial connection.

MS Channel Request (RACH) _BTS Immediate Assign (AGCH)

Paging Request (PCH)

Time Paging Response (SDCCH) Authentication and Cipering (SDCCH)

Figure 3.18 Initial access

Figure 3.18 illustrates how a mobile device attaches to the network. In this example the mobile device has been paged by the network as is the case for a mobile terminated (MT) call. The mobile device continuously monitors the paging channel for such requests and replies on the RACH for a dedicated channel. Once a request is received by the network a response is sent on the access grant channel (AGCH) channel. This response will indicate a dedicated signalling channel which the mobile device should now use to continue its negotiations with the network. A standalone dedicated control channel (SDCCH) is used for this purpose. This channel has a much lower bit rate than a dedicated traffic channel and therefore is more efficient for the small amount of data to be transferred for signalling purposes. The mobile device can now continue with the attach request. It will send its IMSI to the MSC where it will be processed. The MSC will connect to the HLR/AuC of the mobile subscriber’s home network to authenticate the SIM module. Authentication triplets will be sent back to the MSC. These include a random number (RAND), a key (Kc) and a result (SRES). The random number is passed to the SIM in the mobile device, which will use its authentication system to also produce a result (SRES). This result is passed back to the MSC, which will compare it to the result from the AuC. If the results are the same then the SIM is authenticated. The authentication algorithm is rather complex and so an invalid mobile device will reply with a wrong result. The key (Kc) is used to encrypt the data between the mobile station and the BTS. Figure 3.19 illustrates the authentication and encryption procedure.

Once the SIM is authenticated, the MSC may now request the IMEI. Once received from the mobile device this may be checked against the EIR, to see whether or not the mobile device is on a stolen list, not type approved, etc. If it is on such a list then it may not be allowed to register with the network. Once the IMSI and IMEI have been successfully checked, the MSC requests information about the subscriber from the HLR, which will include services available and other details. The MSC will now register the mobile device in the VLR, which will in turn inform the HLR of the current location of this mobile device. The MSC also provides the mobile device with a temporary identifier (TMSI) which is used in any future transactions with the mobile device. Using a temporary identifier, TMSI increases overall security since the IMSI of the mobile device is not sent frequently over the air. The TMSI also consists of a smaller number of digits (4 bytes)

BSC BTS Abis BSS Mobile Station SIM ME Encrypted using Kc Home-PLMN HLRAuC EIR NSS MSC/VLR TRAU SRES' RAND RAND , Kc and SRES IMSI Figure 3.19 GSM authentication

than the IMSI and thus also increases efficiency. The initial signalling procedure is now complete. The mobile device is now assigned an SDCCH or a TCH and its call proceeds.

3.6

PROTOCOLS AND SIGNALLING

GSM has been designed with open interfaces in mind and a simplified diagram of the protocols used over these interfaces is illustrated in Figure 3.20. It can be seen that the air interface consists of the GSM time slots and frequency bands as denoted by TDMA/FDMA. Above this is the point-to-point link access protocol D (LAPD) channel protocol, which is the link layer for traditional ISDN signalling. A modified version of this, LAPDm (TS04.06), is used over the air interface between the BTS TRX and the mobile device. It is modified since the GSM air interface layer 1 already has an FEC mechanism and thus the LAPD CRC error detection at the datalink layer is not required. Also LAPD messages begin and end with an 8-bit synchronization flag, which is not required due to the GSM timing relationship between data and the time slot over the air. The address field includes the 3-bit service access point identifier (SAPI). SAPI 0 is used for call control, mobility management and radio resource signalling. SAPI 3 is used for SMS. All other values are currently reserved for future standardization. This datalink layer also provides the ability to assign three levels of priority, high, normal and low, to messages that are transferred in dedicated mode on SAPI 0. Priority is generally given to radio resource management (RR) messages over both mobility management (MM) and connection management (CM).

The RR layer is used to establish, maintain and release RR connections which allow a point-to-point dialogue between the mobile device and the network. This connection is used for data and user signalling. The procedures include cell selection and reselection as well as the handover procedures and reception of the BCCH and CCCH when no RR connection is established. Figure 3.21 shows a sample of RRM messages.

BSC BTS Mobile Station BSS A Abis Um MSC SCCP MTP MM BSSAP LAPD TDMA/FDMA MM RR CM CM SCCP MTP BSSAP LAPD TDMA/FDMA RR Figure 3.20 GSM protocols Type Messages RR INITIALISATION REQUEST IMMEDIATE ASSIGNMENT Channel establishment PACKET ASSIGNMENT

Ciphering CIPHERING MODE COMMAND CIPHERING MODE COMPLETE

ASSIGNMENT COMMAND ASSIGNMENT COMPLETE

Handover

HANDOVER COMMAND HANDOVER COMPLETE

PAGING REQUEST(1-3) PAGING RESPONSE

Paging and notification

INTER SYSTEM TO UTRAN HANDOVER

Figure 3.21 Example RRM messages

The MM layer is required to support the mobility of the mobile device. This includes informing the network of its present location and providing user authentication. It also provides connection management services to the CM layer. Figure 3.22 shows a sample of MM messages.

The CM layer is functionally split into a number of different entities. These include call control (CC), short message service support (SMS), supplementary services support (SS)

Type Messages IMSI DETACH INDICATION

Registration

LOCATION UPDATING REQUEST LOCATION UPDATING ACCEPT AUTHENTICATION REQUEST AUTHENTICATION RESPONSE Security

IDENTITY REQUEST IDENTITY RESPONSE CM SERVICE REQUEST CM SERVICE ACCEPT Connection

management CM RE-ESTABLISHMENT REQUEST

Figure 3.22 Example MM messages

Type Messages ALERTING

SETUP PROGRESS

AUTHENTICATION REQUEST AUTHENTICATION RESPONSE Call establishment

CONNECT CONNECT ACKNOWLEDGE RELEASE RELEASE COMPLETE Call clearing

DISCONNECT

HOLD HOLD ACKNOWLEDGE

Supplementary service

control RETRIEVE RETRIEVE ACKNOWLEDGE

Figure 3.23 Example CM messages

and location services support (LCS). The CC procedures have been closely modelled on the ITU-T ISDN Q.931 recommendation. Differences between the two are compared in the 3GPP specification TS24.008 annex E. The elementary procedures for CC are grouped into the following classes:

• call establishment procedures

• call clearing procedures

• call information phase procedures

The CC for GSM differs somewhat from that of Q.931 in one large respect which is that of mobility, i.e. routing to a mobile subscriber. An example of how this routing is performed for a mobile terminated call is highlighted in Section 3.7.4.

The supplementary services include such things as call forwarding, line identification, call waiting, call barring, multiparty calls and closed user groups.

The following protocols are also present in Figure 3.20: BSSAP and SCCP, and are described in Section 3.7.2. Figure 3.23 shows a sample of CM messages.