The Gn interface is used to control the signal between Controller and GGSN (Gateway GPRS Support Node) as well as for tunneling end user data payload within the backbone network between both the nodes.

GPRS Tunneling Protocol (GTP) transmits user data packets and signaling between controller and GGSN. GTP encapsulates traffic and creates GTP tunnels, which act as virtual data channels for transmission of packet data between controller and GGSN. A GTP tunnel is established between controller and GGSN through a create PDP context procedure for a data session initiated from UE.

A GTP tunnel is identified by a pair of IP addresses and a pair of GTP Tunnel End Point Identifiers (TEIDs), where one IP address and TEID is for the SGSN and the other is for the GGSN. TEID is a session identifier used by GTP protocol entities in the SGSN and in the GGSN.

The two TEID are defined each for GTP-C and GTP-U. GTP-U is a tunneling mechanism that provides a service for carrying user data packets. On both planes, a GTP header encapsulates the data package, called GTP Packet Data Unit (G-PDU), and a path implemented by UDP/IP is used as bearer of the GTP traffic. GTP-C is a tunnel control and management protocol and is used to create, modify and delete tunnels.

The controller supports the following categories of GTP signaling traffic:

  • Path management messages—The main purpose of these messages is to supervise an UDP/IP path to ensure that connectivity failure can be detected on time. This is managed by frequently sending GTP echo or echo response packets between the controller and the GGSN.
  • Tunnel management messages—These contain messages that establish, modify and release GTP tunnels.

The following diagram shows the message flow between the controller and GGSN for establishing a GTP tunnel.

Figure 1. Message flow between controller and GGSN