GSN) performs the following key tasks: authentication and mobility management; protocol conversion between the IP backbone and the protocols used in the BSS and MS; collection of charging data and traffic statistics; routing data to the relevant GGSN when connection to an external network is re- quired (all intra-network MS to MS connections must also be made via a GGSN). The Gateway GPRS Support Node (GGSN) acts as the interface between the GPRS network and external networks; it is simply a router to a sub-network. When the GGSN receives data addressed to a specific user, it checks if the address is active. If it is, the GGSN forwards the data to the SGSN serving the mobile: if the address is inactive the data is discarded. The GGSN also routes mobile originated packets to the correct exter- nal network. 3.2.1.1.1 Terminal Attachment to the GPRS Network The connection between a GPRS terminal and the network has two parts: The UMTS Development Platform 1. Connection to the GSM network (GPRS Attach) - When the GPRS terminal is switched on, it sends an 'attach' message to the network. The SGSN collects the user data from the HLR and authenticates the user before attaching the terminal. 2. Connection to the IP network (PDP context) - Once the GPRS terminal is attached, it can request an IP address (e.g. 172.19.52.91) from the network. This address is used to route data to the terminal. It can be static (the user always has the same IP address), or dynamic (the network allocates the user a different IP address for each connection). Dedicated standard (ETSI specified) interfaces assuring the interconnection between the key network elements and enabling multi-vendor configurations include: Gb-interface (SGSN-BSS); Gn-interface (GSN-GSN); Gp-interface (inter-PLMN interface); Gi (GGSN-external IP networks); Gr (SGSN-HLR); Gs (SGSN-MSC/VLR); Gd (SGSN to SMS-GMSC/SMS-IWMSC). Other GPRS elements illustrated in Figure 3.3 are: Domain Name Servers - These are standard IP devices that convert IP names into IP addresses e.g. vm