Inter Working Function for CSFB

Traditionally support for CSFB requires an upgrade on the legacy MSCs to support the SGs interface. Additionally careful tracking area to location area mapping is required to avoid MT call failures when the UE falls back to a LA that is not the same as the one it registered on or alternatively call handling procedures such as MTRR or MTRF are required with associated upgrades on MSCs and HLRs.

Putting all this together can be an expensive and complicated process which is where the IWF for CSFB comes in. The IWF provides a number of advantages. At the most basic level it is an interface between the MME and the legacy CS network (as shown above). On one side it communicates using the SGsAP with the MME, while at the other side it communicates using MAP to the MSCs and HLRs. At the same time it provides a VLR function where all the LTE UEs are registered against. This is done by creating a "virtual" LA and updating the HLR accordingly. Any MT calls are thus routed to the IWF. At the same time it eliminates the need for TA - LA planning as all TAs can point towards this one "virtual" LA.

For any MO CS calls the UE fall backs towards the legacy network and discovers a "real" LA. This triggers a LA update and subsequently the call is set up.

For any MT CS calls the process is slightly more complicated and is illustrated in the diagram below (click to enlarge).

The call set up flow can be broken down into 4 phases. At phase 1 the incoming call is routed to the IWF and paging is initiated over the SGs interface. This triggers the CSFB procedure. At phase 2 the UE falls back towards a "real" LA and the LA update procedure is triggered. Phase 3 is the clever bit. Here the IWF sends the HLR an SRI to discover where the UE is located (note the the HLR is still waiting for a response to the PRN message from Phase 1). The HLR queries the MSC and responds. The IWF then uses this information to respond back to the original request from the HLR. The HLR then sends this back to the GMSC and the call is routed. Quite clever!

There is however one drawback with the IWF and that is that every call (MO & MT) requires a LAU before the call is set up. This adds approx. 1 to 2 seconds on the overall setup time. With the classic CSFB, the LAU is not performed and as such faster call setup times can be achieved.

To summarise then the IWF has the advantages of..
1. No upgrade to SGs for legacy network
2. No TA - LA mapping/planning
3. No requirement for MTRR/MTRF

..but the disadvantages of:
1. Longer call set up time due to mandatory LAU procedure