GPRS Backbone Network Architecture

3.9 GPRS Backbone Network Architecture

Figure 3.30 shows the architecture of the GPRS backbone network (see gray boxes) made up of GSNs.

Click To expand Figure 3.30: Architecture of GPRS backbone network. (From— [1].)

All PDUs conveyed in the GPRS backbone network across the Gn/Gp interface are encapsulated by GTP. GTP allows IP PDUs to be tunneled through the GPRS backbone network and allows signaling exchange to be performed between GSNs. UDP/IP are backbone network protocols used for user data routing and control signaling.

3.9.1 Tunneling

A GTP tunnel is a two-way PTP path between two GSNs used to deliver packets between an external PDN and an MS. A GTP tunnel is created during a PDP context activation procedure. A GTP tunnel is identified in each GSN node by a tunnel endpoint identifier (TEID), a GSN IP address, and a UDP port number. These identifiers are contained in IP and GTP PDU headers. There are two types of GTP tunnels:

  • GTP-U tunnel (user plane), defined for each PDP context in the GSNs;

  • GTP-C tunnel (control plane), defined for all PDP contexts with the same PDP address and access point network (APN).

The IP datagram tunneled in a GTP tunnel is called a T-PDU. With the tunneling mechanism, T-PDUs are multiplexed and demultiplexed by GTP between two GSNs by using the TEID field present in the GTP headers, which indicates the tunnel of a particular T-PDU. A GTP header is added to the T-PDU to constitute a G-PDU (or GTP-U PDU), which is sent in an UDP/IP path, a connectionless path between two endpoints.

Figure 3.31 illustrates a tunneling mechanism for IP packet sending toward MS.

Click To expand
Figure 3.31: Tunneling mechanism for IP packet sending toward MS.

All signaling procedures (path management, tunnel management, location management, mobility management) between GSNs are tunneled in a GTP tunnel in the control plane. A GTP header is added to the GTP signaling message to constitute a GTP-C PDU, which is sent in a UDP/IP path.

3.9.2 Path Protocols

The UDP/IP path protocol is used to convey GTP signaling messages between GSNs or T-PDU in connectionless mode. Each UDP/IP path may multiplex several GTP tunnels. An endpoint of the UDP/IP path is defined by an IP address and a UDP port number. For UDP/IP path, the IP source address is the IP address of the source GSN, while the IP destination source is the IP address of the destination GSN. Note that the IP addresses of GSNs within the GPRS backbone network are private. This means that GSNs are not accessible from the public Internet.