, the APN is a reference to the GGSN, to be used in case different GGSNs are connected to different external networks. The access point is the logical connection that the GGSN provides, allowing mobiles to connect to external networks. Typically the subscriber might use one access point to connect to the corporate Intranet, another access point to access ISP and yet another for WAP (Wireless Application Protocol) or MMS (multimedia messaging service). The access point is configured on the GGSN. A single GGSN can provide several access points, even on the same physical interface. A sample APN is my.isp.com.myoperator.com.gprs. Example of GPRS Roaming When a subscriber of operator A is roaming in another PLMN (operator B) and tries to establish a PDP context, the APN defined in the handset (e.g. wap.operatorA.gprs) is mapped to the complete APN Core Network Planning and Optimisation Operator B (Visited Network) Operator A (Home Network) Inter PLMN Roaming User Border Backbone Border Gateway Network Gateway Backbon Backbone IP Netwol IP Network Internet Server Figure 4.43 GPRS roaming name (wap.operatorA.mnc001.mcc002.gprs) in the visitor SGSN using the IMSI information. The visitor SGSN asks its DNS for the GGSN IP address using the APN as the key. The DNS asks a root DNS whether it knows a DNS that can resolve this APN. Once the DNS of operator A is found, the IP address of the GGSN serving this APN is received. After receiving the GGSN IP address, the visitor SGSN creates a PDP context with the home GGSN serving the APN and a GTP tunnel is created over Gp interface. If allowed in the HLR and both operators have the same APNs, a visitor can use the visitor GGSN, as shown in Figure 4.43. Tunnel End Point Identifier The GPRS uses the Tunnelling Protocol (GTP) to transfer data between the SGSN and GGSN. A tunnel end point identifier (TEID) is a unique identifier for the receiving end of a GTP tunnel. It also uniquely identifies a PDP context. The receiving end side of a GTP user tunnel locally assigns the TEID value 