ement and registration and authentication. It also interacts with a mobile with packet data flow and functions related to it like compression and ciphering. These are handled by protocols such as the SNDCP (sub-network dependent convergence protocol) and LLC (logical link control). SGSN is also responsible for GTP (gate tunelling protocol) tunnelling to the other support nodes. Gateway GPRS Support Node (GGSN) The GGSN is connected to the SGSN on the network side and to the outside world external networks such as the Internet and X.25. As it is a gateway to the external networks, its main function is to act as a 'wall' for these external networks in order to protect the GPRS network. When data come from the external network, after verification of the address, the data are forwarded to the SGSN. If the address is found to be invalid, the data are discarded. On the other hand, the SGSN also routes the packets it receives from the mobile to the correct network. Thus, for the outside networks, the SGSN acts as a router. Border Gateway (BG) The border gateway interconnects different GPRS operators' backbones, thereby facilitating the roaming feature. It is based on the standard IP router technology. Legal Interception Gateway (LIG) The LIG performs 'legal' functions in the network. Subscriber data and signalling can be intercepted by using this gateway, thus enabling the authorities to track criminal activities. LIG is required when launching a GPRS service. Domain Name System (DNS) DNS does the translation of IP host names to IP addresses, thereby making IP network configuration easier. In the GPRS backbone, SGSN uses DNS to get GGSN and SGSN IP addresses. GPRS: NETWORK PLANNING AND OPTIMISATION Packet Control Unit (PCU) This is a new card that is implanted in the BSC to manage the GPRS traffic. The PCU has limitations in terms of the number of transceivers and base stations it can manage, thereby creating a bottleneck for the network design usually in terms of capacity. Increasing the capacity of the network leads t