d VLR and then mobile users can be carried out on call set up, both mobile originated signals the new location to the HLR. The HLR provides the sub- and mobile terminated, on location updating, and on activation of 47/14 Cellular radio systems Channel request Immediate assign Service request Process access request Authenticate request (RAND) Authenticate Authenticate response (SRES) Authenticate result Access request accepted Cypher mode command Set cypher mode Cypher mode complete Cypher mode ack Set up Send infor for O/G call Complete call Call proceeding IFAM (initial and final address message) Alert ACM (address complete message) Connect Answer message Connect ack CONVERSATION Figure 47.13 Mobile originating call supplementary services. As the authentication sets are used up in the can be displayed on the mobile and stored in the SIM for future VLR, further sets are requested from the HLR. reference. A related service is cell broadcast which allows mess- An additional security feature of GSM is the equipment identity ages of up to 93 characters to be sent to all mobiles within a specific register (EIR). This enables monitoring of the mobile equipment geographical area, for example to deliver traffic or weather reports. IMEI (International Mobile Equipment Identity) which is used to validate mobile equipments thus preventing non-approved, faulty or 47.5.7 Roaming stolen equipment from using the system. This range of security features provide a high degree of protection to the user and the Naturally, with a pan-European system, roaming of subscribers network operator. between networks is specified by GSM. When a mobile first swit- ches on in a foreign PLMN (Public Land Mobile Network), the local 47.5.6 GSM services and features MSC/VLR will determine the identity of the home PLMN from the mobile network code which is part of the IMSI. The home HLR will In addition to speech, GSM offers a wide range of data bearer be interrogated to establish whether roaming is permitted and for services up to 9.6kbit/s suitable