e 4-8 The GGSN. GPRS PLMN Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Function of GPRS Elements Function of GPRS Elements GGSN also performs a function similar to that of the Gateway MSC (GMSC) and is very close to that of a router in IP terms. Within the GPRS backbone, IP and X.25 data packets are encapsulated in the GPRS Tunneling Protocol (GTP): From a mobile station to the external data network, the GGSN strips off the GTP and lower-layer headers. Then, it delivers the IP and X.25 packet in its native form to the external network nodes. From the external data network to a mobile station, the GGSN performs the opposite operation: it adds GTP and lower-layer headers, followed by transporting the packet to the appropriate SGSN. Tunneling is the transfer of encapsulated data units within the Public Land Mobile Network (PLMN) from the point of encapsulation to the point of decapsulation. A tunnel is a two-way point-to-point path; only the end- points are identified. Home Location Register (HLR) The HLR is an existing GSM network element. It is responsible for keeping track of the mobile's location in the network (such as corresponding SGSN) as well as tracking the activity status (active or inactive) for mobiles in its domain. In order to support GPRS services, the HLR must be enhanced to include GPRS subscription data, including subscribed quality of service (QoS), statically allocated PDP addresses, and roaming permissions. The HLR is the main network database. Only one HLR is present (logi- cally) in any network, although it may be distributed. The information stored relates to all subscribers registered in the network. The presence of the information is independent of the location of the subscriber. Information contained in the HLR includes the Subscriber ID, MSISDN, IMSI, current location (switch area), and subscription services chos