ket when a packet comes from the MS, and the last element to see the IP packet in the downlink direction. The SGSN can also perform compression, such as IP header compression [18-21], and content compression (using V.42 bis). On the control plane, the SGSN handles mobility man- agement by performing mobility management signaling towards the radio network and the GGSN. In addition, the SGSN performs signaling to other SGSNs during handovers. An inter-SGSN handover is performed when an MS moves to an area where the radio network is controlled by a different SGSN. Therefore, the MS can pass by multiple SGSNs while having an active connection open. GGSN is the topological anchor point of the mobility management in the GPRS network. It is the gateway between the GPRS network and external networks, such as the Internet. Thus it terminates the GPRS protocols and mobility management functions. As the GGSN is the mobility management anchor point, unlike the SGSN, it does not change during the MS's connection. Like the SGSN, the GGSN also performs both control and user-plane duties. On the control plane, it receives and answers session initiation, handover, and session teardown signaling from the SGSN. On the user plane, it is responsible for the routing from and to the external IP-networks. In addition, it also gathers charging information, and is a central point in Legal Interception (LI). As pointed out above, the MS can go through many SGSN while moving in the network and having a connection open. Since the GGSN is the anchor point, one connection can be connected only to one GGSN. However, the MS can have multiple independent, concurrent connections to multiple GGSNs at the same time. We will describe these concepts later in this chapter. New interfaces are specified to connect the new network elements to the existing network elements and to each other. These are the most important: Gb-interface is the interface between the BSS and the core network - hence, between the BSC and the SGSN. This interface comprises both o