CSFB vs. SRVCC, or how to “sudo make me Voice over LTE” – take 1

Posted: September 2, 2013 in technical
Tags: , , , , , , ,

Finally, I’ve managed to get my favourite real-life geek (the non-real-life geek is Spencer Reed) to write an article (which shall become a series, I hope) for my blog. Here it goes, Alex.


The other day I realised that working in the 4G/EPC field has some advantages. One is that I get to see how things work and that is the coolest thing, but sometimes you realise things are so complicated it may be a good idea to wait for a while before upgrading.   Take for example the voice calls. In order to do voice calls in 4G you have 2 major options:

1.       VoLTE  which is VoIP using the 4G  (LTE and EPC devices) and IMS network – this is how things should be done in a real 4G network

2.       CSFB (Circuit Switched (CS) fallback TS 23.272) which means that when the UE(phone) needs to make or receive a call it is moved to an older 3GPP technology (2G/3G). This mechanism is used if the IMS network is not available or the UE is not able to do VoLTE for some reason (for example registration to the IMS network failed)

VoLTE is the elegant option but because VoLTE has some strict quality requirements it is still not very popular. The popular option is CSFB because it makes use of the already existing CS infrastructure.

In order to be able to do CSFB  one needs to first attach to the network in a special way. This special way is called Combined EPS/IMSI attach. This special attach basically means that while the user is attached in 4G and residing in 4G, it is also attached to the CS network.   To do this special attach the UE needs to set the attach type in Attach request message to Combined EPS/IMSI attach and indicate its capability for CSFC and/or SMS over SGs (old SMS). On receiving such a request, the MME will derive the VLR number and start an update procedure towards the MSC/VLR. The rest of the attach procedure is the same but the location of the UE will need to be updated in the CS network every time the UE moves to a new LA.

When the UE wants to initiate a call (let’s assume for now that the UE is in connected mode) it will send a special message to the MME. This special message is called Extended Service Request and it contains an indication that the UE is asking the MME to start the handover to the CS network. The MME will ask the eNB to start the procedure for handover. The eNB may ask the UE to do some measurements and then initiate a normal handover procedure. The funny thing here is that it is possible for the eNB to decide to move the UE to a different LA (location area) than the one to which the UE is already connected. In this case along with the handover comes a LA Update or a Combined RA/LA Update. Once the UE is moved to the MSC it can initiate the call (this can happen before the release of the resources on the source network).

That’s all for now. See you later!

  1. […] the CSFB vs. SRVCC, or how to “sudo make me Voice over LTE” – take 1 series, by […]

  2. […] continuing the CSFB vs. SRVCC, or how to “sudo make me Voice over LTE” – take 1 […]

  3. Florin says:

    Very insightful, as always. Good job Alex.

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