Friday, May 22, 2009

CS Fallback

The primary alternative to VoLGA for voice over LTE is an approach known as CS Fallback. Defined in 3GPP release 8, CS Fallback is widely accepted as an interim solution to solving the voice over LTE ‘problem’.

As the name implies, CS Fallback ‘falls back’ from the LTE network to use the Circuit Services inherent in the 2G and 3G network to make and receive voice calls.

If this sounds counter-intuitive, that’s because it is.

LTE is the fastest, lowest cost access mobile access network the world has ever developed. Why would an operator deploy LTE and then *not* put its primary revenue generating service (voice) on it? Even worse: devise a system that forces users back to the previous network. It doesn’t make any sense.

With CS Fallback there are implications on the user experience with LTE. First, there is undoubtedly an additional delay in making a phone call. Switching networks takes time, and that’s added time the user is waiting to make or receive a phone call.

Switching networks is also when call drops tend to happen. So when’s the best time to hand over to another network? The exact moment the user knows they have a call coming in? Probably not.

Next, the phone needs to stop what it is doing on LTE (streaming a video? Internet radio? …) and re-connect to the 2G or 3G network. As a user, you need to decide to cancel your data session to answer the phone. Not exactly ideal if you bought the phone for high speed data. The data session may be able to hand back to the 3G network, but certainly all is lost if the phone falls back to 2G.

Thinking of using LTE for combinatorial services (voice and video)? Good luck. No voice, no combinatorial services on LTE. Everything will have to be on 3G.

Then there is the cost. To support CS Fallback, a new interface must be developed on the MSC. If there’s one thing mobile operators don’t want to do, it’s change the MSC. Not only are MSC upgrades expensive, they require a fair bit of testing and validation. The MSC is the heart of the core voice service so any change is approached cautiously.

As an interim solution, it’s clear that CS Fallback falls short.

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