In order to reduce the load of passing the user plan data by the SGSN, the SGSN may decide to establish direct tunneling between the RNC and the GGSN.

This will make the user plan data pass directly between the RNC and the GGSN, while keeping the control plan passing by the SGSN.

This will reduce the load on the SGSN, while keeping it still able to control the session. 

'Direct tunnel' means that during the establishment of the 'PDP context' between the MS and the GGSN. The SGSN sends the IP address and TEID of the GGSN to the RNC. In addition, the SGSN will send  the IP address and TEID of the RNC to the GGSN.

This is in order to start sending the user plan data directly between the RNC and the GGSN without passing by the SGSN.

Th SGSN can decide to use this function or not according to the configuration on it. This configuration instructs the SGSN to take its decision based on which RNC, and which GGSN exist the current session.

Direct tunneling procedure

During the 'PDP context activation' procedure, if the SGSN decided to use “Direct Tunnel”, 

• The SGSN will send the IP address and TEID of the GGSN to the RNC in the 'RAB assignment' procedure.
• Then, the SGSN will send the IP address and TEID of the RNC to the GGSN in the “Update PDP context request”.

Currently, the RNC and the GGSN are having the necessary information, in order to establish a direct tunnel for user plan data between them.