## 2 Mobile Cellular Network Basics
### 3G Mobility Management for Data Communication  
In mobile networks, the mobile end point may change its location and hence the systems serving this mobile end point may change. For instance, as seen in Figure 2, when a mobile user connects to a mobile PS network and moves from one point to another without closing the connection, the serving system for the end point, such as the Node B, can change from NodeB-A to NodeB-B. In Global System for Mobile (GSM) networks, knowing the end point location is one of the most critical issues for the network devices of an operator. Therefore, in order to give better service, the mobile operator must know the exact location of the mobile end point. Packet Mobility Management (PMM) context is used to track a mobile end point's location and its serving systems such as the NodeB, the RNC and the SGSN. In the PS domain, the SGSN and the mobile end point perform this task by managing PMM contexts. However, GGSN is not directly  
Figure 1: Mobile Cellular Network. Figure 2: Serving NodeB changes. related to location tracking and does not need to know the mobile end point's PMM context. A PMM context may be different on a mobile end point and on a SGSN. Mobility Management (MM) is necessary for tracking the physical location of the UE and serving devices such as NodeB. After the establishment of the connection between the UE and cellular network equipments, all PS user data (from or to the UE) is first sent to the GGSN. This means the UE connects to a mobile network and uses the NodeB, the RNC and the SGSN for different session establishments. However, end to end IP level communication takes place between the UE and GGSN. Later, the GGSN forwards the UE's data towards its destination according to per host specific routing on the GGSN. The connection between the UE and the GGSN is referred to as an active PDP context. Each PDP context has its own IP address that is assigned by a GGSN. As needed, each UE can have up to 16 activated PDP contexts and each of them is uniquely identified by a combination of the subscriber's International Mobile Subscriber Identity (IMSI) and a Network Service Access Point Identifier (NSAPI) [18]. In order to route the UE data correctly, a PDP context should exist from the UE to the GGSN, as shown in Figure 3. A PDP context is necessary only for data communication and not voice. MM is referred to as PMM (Packet Mobility Management) on GPRS 3G networks. In order to track a UE's location easily, mobile networks use a hierarchy of areas, such as Location Area (LA) and Routing Area (RA). Cells form an RA and RAs form an LA, as shown in Figure 4.  
An LA is a set of base stations that are grouped together to optimize signaling. An RNC manages radio channel allocation and handover for all cells in the same LA. Many nodeBs in the Universal  
Figure 4: Mobile network hierarchy. Figure 3: End to End packet data protocol (PDP) Context. Mobile Telecommunications System (UMTS) share a single RNC in UMTS. The size of an LA is an issue since an LA too big or too small could be a problem from the optimization point of view. The size of an LA may also affect paging on the RNC [19]. An RA is used in the PS domain by UEs that are attached to GPRS and have active PDP contexts. A change from an RA to another RA is done in same way as it is done during a change from one LA to another LA. The only difference is that an LA is controlled by an RNC whereas an RA is controlled by an SGSN. In a PS network, a UE can be in one of the three different PMM states: PMM-Detached, PMM-Idle and PMM-Connected, as shown in Figure 5.  
_PMM-Idle State:_ Once a UE is attached to a GPRS network, in addition to IP assignment, end-to-end resources are dedicated for the virtual channel between the UE and the GGSN, and also a timeout timer is started to track the UE activity. If the UE is idle for longer than a certain amount of time, the related systems such the GGSN and the SGSN will release their dedicated resources.