Reverse SR-VCC

One of the biggest assets mobile network operators have is the ability to handover voice calls seamlessly between different radio access technologies. If the calls starts in a UMTS network and the user leaves the coverage area of the 3G network, the call is handed over to the GSM network without the user noticing it. The reverse direction from 2G to 3G is also standardized but I have yet to observe any network actually doing it during an ongoing call. It could actually be quite useful so background data transfers such as email reception, facebook updates, etc. can continue during a voice call, something that is usually not possible unless the network supports GSM DTM (Dual Transfer Mode).

Anyway, things get a bit complicated in the LTE domain. First, there is no single Voice over LTE solution clearly dominating today, probably due to the lack of networks and devices. But let's say, for arguments sake, that we'll get there one day. How do you hand over a Voice over IP call to GSM once the user runs out of the LTE coverage area. The solution is Single Radio Voice Call Continuity, or SR-VCC for short, which has been specified some time ago in 3GPP TS 23.216. The procedure is voice technology agnostic so it can be used by IMS, VoLTE (an IMS implementation) and also VoLGA.

But once you are there, how do you get back to LTE or UTMS during the call? Well, not at all so far. However, there's a study item currently worked on in 3GPP Release 10 in TR 23.885 that gives a number of options of how reverse SR-VCC could be implemented. How interesting! I'm tempted to read it but I guess I'll just hold off until I see the first (forward) SR-VCC implementation used in a real the network. Have fun putting it into a TS until then 🙂

4 thoughts on “Reverse SR-VCC”

  1. Nice however two things which are not clear.
    In case of VCC does the data transfer continues (I suppose it is since both are IP based).
    Second what happens in the case of circuit switched fal-back? Does the LTE Data transfer continues on LTE network? Is it handed over to 3G or 2G? If so do you need GSM DTM to continue the data transfer?

  2. Hi dbudbu,

    yes, the data transfer can continue in case the fallback is done to a 3G network. In case of GSM the data transfer is interrupted, unless of course DTM is available in the network.


  3. Hi Martin,

    Thanks for your great blog!
    I have read about UTRAN(HSPA) which supports voice call over PS domain in the 3G network. And the SRVCC for UTRAN(HSPA) to UTRAN is also described in 23.216. But I don’t know how to start a voice call over PS domain by 3G RRC signalling, do you have any ideas about this feature?

  4. Hi ShuWen,

    Well, if you use Skype for making a voice call over 3G, RRC signaling will not be different from the one you see today for any other kind of data service. But I guess that was not the intent of your question.

    For VoLGA and VoLTE: There are some means for both systems to interact with the core and radio network to set the quality of service for the voice packets. In LTE that would be the establishment of a default bearer and in UMTS it would be a secondary PDP context. The same thing, just different words. That in turn then triggers a number of radio bearer reconfigurations, e.g. having one radio bearer in RLC unack. mode for the voice packets. Youll find some details here:


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