23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS
This clause describes one way to perform Selected IP Traffic Offload by the Traffic Offload Function at Iu-PS for UMTS network.
Figure B.1: Selected IP Traffic Offload from Traffic Offload Function (TOF) deployed at Iu-PS
TOF may include the following functions:
– NAS and RANAP message inspection to build/remove local UE context and local session context;
– Packet Inspection and Selected IP Traffic Offload enforcement;
– Uplink traffic offloaded by removing GTP-U header and performing IPv4-IPv4 Network Address Translation;
– Downlink traffic offloaded by reverse IPv4-IPv4 Network Address Translation and adding GTP-U header;
– Charging for offloaded traffic;
– LI for offloaded traffic;
– Offload traffic service continuity during intra-TOF mobility;
In this implementation example a deployment is needed that assures that all PS domain signalling from a UE goes through the same TOF instance. The TOF inspects the NAS and RANAP messages to build the local UE context and local session context. When a RAB is requested to be set up and should be offloaded, the SGSN includes the MSISDN, APN and the Charging Characteristics for the requested RAB to enable Iu-ps offload. The details of how the SGSN transfers these RANAP parameters to the TOF are described in the following clause B.2. The TOF records any necessary information e.g. the RAB ID/NSAPI, uplink TEID and downlink TEID, APN, etc. in its local context.
During the data transfer procedure, the TOF performs IPv4-IPv4 Network Address Translation for uplink offload traffic which matches the offload policies, and transparently transfers the non-offload traffic to the CN. The TOF adds the corresponding GTP-U header to the downlink offload traffic which has no GTP-U header, and then sends the downlink traffic to the RNC/HNB GW in the same way as would an SGSN.
When the TOF detects an Iu release message it could start an inactivity timer, which is longer than the RAU Timer. When this inactivity timer expires and the Iu connection is not re-established, the TOF deletes related UE context. The inactivity timer is stopped when the Iu connection is re-established for the UE.
When the TOF is configured to perform paging, the TOF pages a UE in idle mode for downlink offload traffic arriving at the TOF, and when the UE sends a Service Request message as the paging response, the TOF modifies the Service type IE in the Service Request message to indicate "Data" and sends it to the SGSN. When the TOF is configured not to perform paging, the TOF discards the received downlink offload packets for a UE in idle mode.