dress is assigned for a subscriber on demand, that is, when she attaches GPRS. Dynamic address is released in GPRS detach, when the MS is disconnected from the GPRS network. The major upgrades in the BS subsystem are new channel coders in BTSs and packet control units (PCUs) in BSCs. PCUs take care of the packet transmission between MSs and SGSN. 5.7.2 GPRS Network Elements GPRS is designed to leave BSSs almost unchanged. A PCU is added to the BSS and it routes packet-switched data to a separate GPRS core network. The roles of the new network elements are introduced below. 5.7.2.1 SGSN The SGSN support node is responsible for the delivery of packets to all MSs within its service area. SGSN plays the same role in GPRS as MSC/VLR in the circuit-switched GSM network. It detects new MSs in its area, performs authentication, ciphering, and IMEI check, and it sends and receives packets to and from the MS. It also collects charging data records (CDRs), performs session and mobility management, and supports SMS. Mobility manage- ment contains GPRS attach/detach, routing area update, location area update, cell change (in ready mode), and paging. Cell change corresponds to handover and for that SGSN takes care that unacknowledged packets are routed to a new cell and possibly to the new SGSN if the new cell is under different SGSN. Session management contains PDP context activation, deactivation, and modification. PDP context activation means establishment of a "virtual circuit" between the MS and GGSN for IP packet transfer. The SGSN handles protocol conversion between the IP protocol and the LLC protocol that is used between SGSN and MS. The SGSN performs TCP/IP compression according to RFC 1144 to save radio capacity [3]. 5.7.2.2 GGSN The GGSN support node acts as a logical interface to external packet data networks. GGSN acts as a router and hides the GPRS network infrastructure from the external networks. GGSN remains an anchor point when SGSN is Mobile Communications changed due to a cell change. When the GGSN receive