Every call Drop Matters ? Here comes SRVCC





If you are on the road driving. Would you want a phone call to get interrupted in any of the following scenarios

  • Talking to a prospective client
  • a job interview where you are being asked how soon can you join ?
  • (more realistic)  let’s just say an important make-up call with your girl friend or spouse (when they are really upset with you ).


Personally, the prospective client lead call drop did happen to me twice (for the same client). Obviously, I didn’t get it closed.

Ideally, you would want none of the above to happen to you nor to your clients.


What about call drops and how to avoid them, even if you have fast speed LTE connection.

SRVCC is one way to make that possible

I have gotten few questions related to SRVCC  in the past couple of weeks. So let’s touch on  SRVCC .


Motivation for Single Radio Voice Call Continuity (SRVCC )

If voice is already supported in LTE using circuit switch (CS) Fallback and voice over LTE (VoLTE) approaches etc.

Then why do we need SRVCC in the first place ?

The concept of SRVCC is important with the perspective that every call drop matters.


Let me explain , how …





Look at the map below.  Assuming LTE + VoLTE coverage is shown below in the map.

Let’s say if you are driving out on a given day from the city of Louisville,KY and your destination is Frankfort, KY . While driving out of Louisville towards Frankfort, you get on a call. It started on LTE (VoLTE).  While you are driving towards Frankfort. The coverage for LTE is bound to get bad , and there is a possible need to do handover onto 2G /3G in this case. Ideally, during this handover voice quality should not be affected. The handover should be seamless , in a way that your voice connection should not be interrupted from QoS perspective .

If SRVCC is not in place, you experience an interruption




Disclaimer: LTE (VoLTE) and 3G or 2G Coverage are shown for example purposes only. It does not reflect any real or live network coverage at these specific locations




You say, come on Azar…I would simply stop over in Louisville area , finish my call and get back on the road again. That sounds awesome

But the question is as a consumer how would you know where the point comes in where there is good LTE coverage vs. good 3G/2G coverage

or more specifically, if you do not have the option to stop at your will (public transport etc. )


How would seamless handover takes place from LTE to 2G or 3G ?


This is where SRVCC comes into play making sure if you are on an VoLTE call , your call or session ( either voice or data )  should remain connected and seamlessly it should be handed over to 2G or 3G ( whatever technology you have in place ) ,

so that you can close deals with your clients, continue to impress your interviewers or win the heart of your spouse.


Q: Is CS Fall Back and SRVCC one and the same thing ?

CS fallback is used to provide voice services in LTE, by redirecting the user to 3G or 2G coverage. Whereas SRVCC is applicable when LTE does have voice service available( VoLTE ) .

When LTE coverage is bad, then SRVCC can help in seamless transfer of voice or packet session to 3G or 2G without interruption.


Q: Is SRVCC only applicable from LTE to UMTS ?

Ans: Not only from LTE to UMTS. SRVCC provides mobility between VoIP (VoLTE) and a circuit switch voice call. It is applicable for the following

  • LTE VoIP to UMTS circuit switched domain
  • LTE VoIP to UMTS packet switched domain
  • LTE VoIP to GERAN circuit switched domain
  • LTE VoIP to 3GPP2 1xCS is also supported


Even though it has been specified  in the standards, as SRVCC support for the above bullet points.

Please note, equipment manufacturers or vendors can offer each of the above versions of SRVCC to different operators (clients) as on need basis or based on their business models at various price points.


SRVCC in action from LTE to UMTS

In this article though, we are discussing only Mobility from LTE ( VoLTE ) to UMTS .

The players involved in the overall operation are.

  • LTE RAN and Core Network
  • UMTS RAN and Core Networks
  • IMS

Architecture for SRVCC between LTE and UTRAN


Procedure Support for SRVCC  

SRVCC procedure requires support from both the network and UE side.


Network Support

During S1AP : Initial context Setup Request message includes an SRVCC operation possible field to indicate whether or not the eNodeB can apply SRVCC procedure based upon both UE and MME capability . The SRVCC Operation Possible field is a flag ,which reflects the combination of UE and MME capabilities.


Network support for SRVCC

UE side Support

-UE signals its capability to the MME during attach procedure. The Attach Request includes a flag with the UE network capability section which indicates wheter or not the UE supports SRVCC.


-UE signals its capability to the eNodeB during the UE capability enquiry procedure . Bit 27 of the Feature Group Indicator (FGI) bit string indicates whether or not the UE supports SRVCC. If bit is 1 , it means it supports SRVCC




UE support for SRVCC

SRVCC Operation


SRVCC Operation can be triggered as

  • With (packet switched ) PS inter-system handover for the data bearer
  • Without PS inter-system handover for the data bearer


In other words what it means is that procedure can move only voice connection to the 3G/ 2G or voice and data connection both to 3G or 2G.


SRVCC Operation 

Now that we have enough of the background and motivation available for SRVCC.


In the video below, you will see the Operation of SRVCC domain transfer.





Now having gone through the video. If you have any remaining questions or comments. Write them in the comments below.


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