N (SGSN) and gateway GSN (GGSN). There may be more than one of these in a single network. The SGSN interfaces with the GSM network using the Gb or Iu-PS interface. A mobile station (MS) first attaches with the SGSN supporting its region of location. The SGSN gets subscriber data from the HLR over the Gr interface. It manages information related to mobility, location, security and authentication of the subscriber. The SGSN is further connected to the GGSN over the Gn interface through the packet Core Network Planning and Optimisation Packet Data Network Other PLMN Figure 4.42 GPRS core interfaces domain, which is IP based. The SGSN is connected to the GGSN of other PLMNs (in the case of roamers) using the Gp interface through the BG (border gateway). An SGSN helps in establishing the PDP context between the MS and GGSN. It may be optionally connected to the MSC/VLR using the Gs interface to send location information. The SGSN capacity is measured in terms of the total number of subscribers that can simultaneously attach to it, simultaneous PDP contexts and data throughput. The GGSN provides access to packet data networks (Internet/Intranet) based on the PDP address. Packet data units (PDUs) coming from a PDN are routed by the GGSN to the SGSN to which the subscriber is attached. The SGSN and GGSN may reside in the same hardware although they are mainly separate. The GGSN is essentially a router while the SGSN may be a switching platform. The GGSN connects to the SGSN over the Gn interface while it uses the Gi interface to connect to the Internet via the firewall. In a layer 2 core switch separate VLANS are created for Gn and Gi interfaces to segregate traffic. For an external packet data network, the GGSN is the first point of contact. The capacity of the GGSN is measured in terms of the total number of simultaneous PDP contexts and data throughput. Access Point Name (APN) The access point name (APN) is the reference to an external packet data network to which the mobile user wants to connect. In the GPRS backbone