Session, Mobility and Location Management

Before MS can use any type of GPRS services it must ``attach'' to network. Attach involves registration and authentication with network. Similarly it performs ``detach'' when it no longer needs the service. Before the Mobile station can talk to any external PDN it must get a PDP context after an attach. PDP context contains the IP address of MS, requested QoS and address of serving GGSN. PDP can be static (IP address assigned permanently by PLMN) or dynamic (address allocated by visited network). The figure 2.3 below shows the PDP activation procedure.

Figure 2.3: PDP Context Activation (Source: Bettstetter et. all)

The MS requests to SGSN with message ``activate PDP context request''. The SGSN after the authentication forewords this request to affected GGSN. The GGSN responds with ``create PDP context response'' message to SGSN which updates its PDP context table and confirms to MS with ``activate PDP context accept'' message.

The purpose of location management is to keep track of MS so that the incoming packets can be routed to it without ``paging''. But there is trade off of battery power Vs the frequency of location update and a compromise is the good solution. For this reason the MS maintains it state in one of the three states as shown in figure 2.4. The location update frequency is dependent upon the state of MS.

Figure 2.4: Mobile Station States in GPRS (Source: Bettstetter et. all)

In IDLE state the MS is not reachable and no location update is performed. After an ``attach'' the MS gets into READY state and may perform ``detach'' to go back to IDLE state. The STANDBY state is reached when the MS does not send any packets for a long time, and READY timer expires.

In READY state the MS sends Location update information to SGSN every movement to a new cell. For location management in STANDBY state GSM location area is divided in several Routing Areas (RA) consisting of several cells. Whenever a MS moves from one RA to another, it sends a ''routing area update'' to the associated SGSN. Inter SGSN and Intra SGSN RA updates are possible. Sometimes RA is combined with GSM LA.