Direct Tunnel – GPRS Core Network Streamlining

While work is ongoing on 3GPP LTE (Long Term Evolution) and SAE (System Architecture Evolution), current 3G networks continue to be enhanced as well. Since the 3G air interface is in the process continues to evolve with HSPA (High Speed Packet Access) it was felt in the standards groups that the 3G core network should be streamlined to handle the increasing network traffic more efficiently.

One part of the network in particular has been waiting for optimization for quite some time. In today’s 3G packet core architecture the SGSN (Serving GPRS Support Node) which is the gateway between the radio network and the core network handles both signaling traffic (e.g. to keep track of a users location) and the actual data packets exchanged between the user and the Internet. Since the users location can change at any time, data packets are tunneled (encapsulated) from the gateway to the Internet (The Gateway GPRS Support Node, GGSN) via the SGSN over the radio network to the mobile device. The current architecture uses a tunnel between the GGSN and the SGSN and another one between the SGSN and the Radio Network Controller (RNC). All data packets thus have to pass the SGSN which has to terminate one tunnel, extract the packet and put it into another tunnel. This requires both time and processing power.

Since both the RNC and the GGSN are IP routers this process is not really required in most circumstances. The one tunnel approach now standardized in 3GPP thus foresees that the SGSN can create a direct tunnel between the RNC and the GGSN and thus remove itself from the chain. Mobility Management remains on the SGSN, however, which means for example that it continues to be responsible to modify the tunnel in case the mobile device is moved to an area served by another RNC.

The approach does not work for international roaming since the SGSN has to be in the loop in order to count the traffic for inter-operator billing purposes. Another case where the one tunnel option can not be used is in case the SGSN is asked for example by a prepaid system to count the traffic flow. A small limitation since in practice it’s also possible to connect such a system to the GGSN (via Diameter).

For the details have a look at the following documents:

  • Direct Tunnel 3GPP Work Item Description SP-060142_S2-060545
  • The TR (Technical Recommendation) describing the overall design and impact on existing functionalities: TR 23.809
  • The Change Request (CR) for 3GPP TS 23.060
  • And the latest version of the ‘GPRS Service Description;  Stage 2’ which contains the enhancements. TS 23.060 7.4.0