d bits, which are set to 0 by the sending side and are not evaluated by the receiving side. Tunnel identifier (TID) Points out MM and Packet Data Protocol (PDP) contexts. This identifier is composed of the Mobile Country Code (MCC), Mobile Network Code (MNC), and Mobile Station Identification Number (MSIN). These are parts of the International Mobile Subscriber Identity (IMSI) and the Network layer Service Access Point Identifier (NSAPI), which is an integer value in the range 0 to 15, identifying a PDP context belonging to a specific MM context ID. GPRS Tunneling Protocol (GTP) Layer The SGSN interfaces with the GSM network, whereas the GGSN interfaces with the external world. The GTP is used to transfer information between 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. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface Chapter 10 the SGSN and the GGSN. In essence, IP is the protocol between the SGSN and the GGSN. As mentioned earlier, tunneling refers to the encapsulation of a user's data packet within another packet. The packets that reach the SGSN or the GGSN (maybe of different formats such as IP, X.25, and SO on) are encap- sulated packets with the source and destination support node addresses in the outer packets' header. As a result, the actual information from the user is not modified. This is useful because it supports multiprotocol packets to be tunneled through the GPRS backbone. The tunnels are established when an SGSN activates a PDP context with the GGSN. TIDs identify the tunnels. These are shown in Figure 10-4. Every tunnel has a unique TID. SGSN and GGSN tables are mapped according to the TIDs. The tunnel is destroyed when the context is deacti- vated. Tunneling is supported for inter-PLMN and intra-PLMN communi- cations. GTP Identities A many-to-many relationship exists between the SGSNs and the GGSNs. Therefore, multiple t