igitalengineeringlibrary.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 Figure 10-2 The GPRS Tunneling Protocol (GTP). IP Backbone GTP Protocol Stack GMM/SM SNDCP Gn, Gp UDP/TCP UDP/TCP Other BSSGP L1Bis Location management messages Mobility management (MM) messages Transmission plane Tunnels are used to carry encapsulated tunneled PDUs (T-PDUs) between a given GSN pair (GGSN-SGSN or SGSN-SGSN) for individual mobile stations. The key tunnel ID, present in the GTP header, indicates to which tunnel a particular T-PDU belongs. GTP Messages The GTP header is a fixed-format, 20-octet header used for all GTP mes- sages, as shown in Figure 10-3. Version Is set to 0 to indicate that this is the first version of GTP. Spare 1111 Are unused bits, set to 1 by the sending side, and not evaluated by the receiving side. Message type Indicates the type of message, is set to the decimal value 255 for T-PDU, and takes a value from 1 to 52 for signaling messages. Length Gives the size of the GTP message excluding the header. 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 SGSN-to-GGSN (Gn) and GGSN-to-PDN (G) Interface Octets Figure 10-3 Version Spare "1111" The GTP message header. Message Type Length Sequence Number Flow Label LLC Frame Number Spare "11111111" Spare "11111111" 13-20 TID Number (MCC, MNC, MSIN - + NSAPI) Sequence number Is a transaction identity for signaling messages and an increasing sequence number for tunneled T-PDUs. Flow label Identifies unambiguously a GTP flow. LLC frame number Is used as the inter-SGSN routing update procedure to coordinate the data transmission on the link between the mobile station and SGSN. Spare bits X Indicates the unuse