nitor and analyse network performance through protocol analysers. Some of the main events are studied that occur in a GSM network and that may encounter quality problems. Connection Establishment A circuit switched connection is established when either a terminated or originated call is launched. Before going in detail into the mobile originated call (MOC) scenario, a block diagram (Figure 2.61) is given that summarises the base scenarios. As illustrated in the figure, the following procedures occur in order: Paging takes place only for mobile terminated calls (MTC) on the PCH. A control channel is established between the MS and BSC for identification and service request (RACH from the MS side). An SCCP connection request is made from the NSS by the BSS (assignment of the SDCCH). Authentication is asked for by the NSS. IMEI checking may also take place if activated by the operator. Ciphering takes place on the air interface once authentication occurs. There is exchange of additional information between the MS and MSC. This is the last step of a successful LU. There is a TCH assignment on the A and air interface and waiting till an end-to-end connection is established (ringing or alerting at the called terminal). The called party answers; the connection is then established and charging starts. The MS and MSC release all resources after the conversation ends. This figure can be split into two cases: the mobile originated call (MOC) and the mobile terminated call (MTC) (see Figure 2.62). Location Update When a mobile first switches on, it camps on the appropriate network and registers to a given VLR. Then the SIM card in the mobile station stores the LAC and starts to systematically compare it to the LAI broadcasted in the BCCH layer of the serving cell. When the MS detects a difference, it initiates a location update procedure. If implemented in the network, an MS can be asked to perform a location update initiated by the network at periodic times (typically every 4 hours). The location update uses signalling only;