 Signaling is used to create, modify, and delete tunnels. In the transmission plane, GTP uses a tunneling mechanism (GTP-U) to provide a service for carrying user data packets. The choice of path depends on whether or not the user data that will be tunneled requires a reliable link. The GTP-U protocol is implemented by SGSNs and Gateway GPRS Sup- port Nodes (GGSNs) in the UMTS/GPRS backbone and by Radio Network 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 (G) and GGSN-to-PDN (G) Interface Gr or Gc Figure 10-1 GTP-MAP protocol The GTP reference converting GSN model. UTRAN UTRAN Other PLMN Controllers (RNCs) in the UTRAN. SGSNs and GGSNs in the UMTS/GPRS backbone implement the GTP-C protocol. GTP is the protocol used between GSNs, as shown in Figure 10-2. As the GGSN may be linked to different kinds of PDNs, GTP enables multiproto- col packets to be tunneled through the GPRS backbone on the Gn interface (between GSNs within a PLMN) and on the Gp interface (between GSNs in different PLMNs). GTP tunnels utilize the Transmission Control Protocol/Internet Protocol (TCP/IP) for protocols that need a reliable data link (such as X.25) and the User Datagram Protocol/Internet Protocol (UDP/IP) for protocols that do not need a reliable data link (such as IP). GTP includes the signaling and data procedures. In the signaling plane, GTP specifies a tunnel control and management protocol that enables the SGSN to provide GPRS network access for a mobile station. Signaling is used to create, modify, and delete tunnels. In the transmission plane, GTP uses a tunneling mechanism to provide a service for carrying user data packets. Signaling plane Path management messages (Echo Request/Echo Response messages) Tunnel management messages Downloaded from Digital Engineering Library @ McGraw-Hill (www.d