Direct Tunnel Feature Overview
The direct tunnel architecture allows the establishment of a direct user plane (GTP-U) tunnel between the radio access network equipment (RNC) and a GGSN.
Once a direct tunnel is established, the SGSN continues to handle the control plane (RANAP/GTP-C) signaling and retains the responsibility of making the decision to establish direct tunnel at PDP context activation.
A direct tunnel improves the user experience (for example, expedites web page delivery, reduces round trip delay for conversational services) by eliminating switching latency from the user plane. An additional advantage, direct tunnel functionality implements optimization to improve the usage of user plane resources (and hardware) by removing the requirement from the SGSN to handle the user plane processing.
A direct tunnel is achieved upon PDP context activation in the following ways:
Gn/Gp Interface towards GGSN: The SGSN establishes a user plane (GTP-U) tunnel directly between the RNC and the GGSN, using an Updated PDP Context Request toward the GGSN or the GGSN service of a collocated GGSN/P-GW.
Gn/Gp Interface towards P-GW When Gn/Gp interworking with pre-release 8 (3GPP) SGSNs is enabled, the GGSN service on the P-GW supports direct tunnel functionality. The SGSN establishes a user plane (GTP-U) tunnel directly between the RNC and the collocated PGW, using an Update PDP Context Message toward the GGSN/P-GW.
A major consequence of deploying a direct tunnel is that it produces a significant increase in control plane load on both the SGSN and GGSN components of the packet core. Hence, deployment requires highly scalable GGSNs since the volume and frequency of Update PDP Context messages to the GGSN will increase substantially. The SGSN platform capabilities ensure control plane capacity will not be a limiting factor with direct tunnel deployment.
The following figure illustrates the logic used within the SGSN to determine if a direct tunnel will be setup.