A while back I made a post about the performance of CSFB in a live network. Even though it was quite insightful it only captured a single CSFB procedure so from a statistical point of view the performance impact of the call setup delay was not that valid.
Recently Ericsson and Qualcomm produced a paper which summarises their findings from multiple tests and I have presented the most relevant table above. The multiple columns represent the performance impact of the different "flavours" of CSFB. As the implementation becomes more complex (from right to left) the call setup delay is reduced.
The basic impementation is what was initially conceived in the standards in 3GPP rel8. In order to save some time when the UE is redirected to UMTS, it can skip reading some SIBs. This can be either a proprietary implementation or some 3GPP rel7 fucntionality can be used which allows the UE to skip some SIBs and inform the network about it, which can then send the relevant information in connected mode.
SI Tunnel, specified in rel9, allows for the SIBs to be sent to the UE while still in LTE as part of the RRC Release with redirection message.
Finally a handover can also take place which will keep the UE in dedicated mode and allow for the fastest call setup.
In my opinion I think the SI Tunnel approach will be the mainstream solution as it offers a good balance of performance Vs complexity.
Interesting enough looking at the CSFB log I posted previously here, we can see that in this occasion the UE actually used some proprietary SIB skipping fuctionality as some SIBs such as SIB11 and SIB5 were not read by the UE. As SIB5 is mandatory for initiall access I can only assume that the UE had previously camped on that particular cell and had stored the SIB5 and SIB11 in memory.
The full paper that contains a lot more interesting information can be found here.
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