As described in a previous post one of the issues with LTE CSFB is what happens when the UE falls back to the target RAT but the the LA is not same as the one the UE is registered in during the combined attach procedure. This typically occurs on the borders of Location Areas (LA) and Tracking Areas (TA) or when the target layer is 3G and the UE can only acquire 2G (e.g. indoors).
In order to avoid MT call setup failures, operators must implement either MTRR (Mobile Terminating Roaming Retry) described here or MTRF (Mobile Terminating Roaming Forwarding) which will be described in this post.
The signalling flow for MTRF is shown above (click to enlarge) and essentially consists of 3 phases.
Phase 1 and phase 2 are identical to the MTRR signalling flow, with the exception that the network elements involved must support MTRF and signal as such with the applicable IE (MTRF Supported) defined in the specs.
In phase 3 however rather than cancelling the call setup procedure and re-initiating it towards the "new" MSC like MTRR, with MTRF the old MSC is used as a relay and the call setup procedure continues with an additional PRN & IAM procedure.
So essentially MTRF manages to cut down the signalling flow and the whole MT procedure completes in 3 phases as opposed to 4. This results in a reduced call setup delay and as such better customer experience.
In the final post on the subject I will describe the IWF solution which overcomes the problem of misaligned LA & TA (and a number of other issues) with the inclusion of an additional core network element.
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