Circuit Switched Fallback on AT&T LTE

I had the opportunity to try out AT&T LTE on the new iPad before I swapped for the Verizon model.  The network was very fast but ultimately the coverage and reliability wasn’t there (see my previous post on the subject here).

I have still been curious about how it would be on a phone, since my only experience on Verizon has been with a Thunderbolt and a Galaxy Nexus.  These two devices were pretty heavy on the battery.  AT&T claims that since they are using Circuit Switched Fallback technology in their phones, they will produce better battery life and have slimmer profiles.  However, at what cost? 

I’m an avid fan of The Verge especially the work of Chris Ziegler, one of their senior mobile editors.  Chris has written an article detailing CSFB with some information I found particularly interest:

What is CS fallback, exactly? CNET says it’s a technology that permits a device’s LTE radio to be disabled when you’re out of LTE coverage, but that’s not quite right - rather, it allows a phone’s legacy 3G radio to be “woken up” when needed (say, to receive a call) while using LTE alone the remainder of the time. To put it simply, when you’re in LTE coverage, the phone can usually leave the 3G radio turned off.

This sounds like a great way to save battery and seamlessly switch between 3G and LTE technologies — especially since they are both GSM based on AT&T. But here’s the catch that they don’t advertise:

CS fallback suspends LTE service as long as a call is underway. Newer releases of the 3GPP specifications - which cover the implementation of CS fallback - allow ongoing data transfers to be handed off from LTE to 3G when a call comes in, but you won’t continue to get LTE capability as long as the modem needs to be in 3G mode to handle voice services.

So while you will have simultaneous voice and data on AT&T’s legacy 3G network, as long as you’re on the phone, no LTE.

Chris recently reviewed the HTC One X on AT&T’s LTE network. This point illustrates why the CSFB implementation hurts:

If I had a complaint, though, it would be AT&T’s use of circuit-switched fallback, which shares circuitry between the LTE and HSPA+ radios. The benefit is reduced complexity and improved battery life, but the chief disadvantage is that LTE is automatically disabled whenever you’re on a call or the phone rings. In practice, what this meant for me is that my mobile hotspot tethering to my laptop would be unceremoniously interrupted whenever I received a call — and worst yet, the phone frequently wouldn’t reengage LTE after the call ended without toggling airplane mode on and back off again to “reboot” the radio.

I get the technical limitation behind this, but this is a HUGE issue that I think AT&T completely side steps. While Verizon keeps the CDMA and LTE radios active full time in their phones which can result in a battery hit, at least you can still use LTE while on the phone.

It seems to me Verizon scores additional points in the LTE category because of this. I’ve read the One X is very very good on battery life, but if you configure the Galaxy Nexus on Verizon correctly, in my experience, the battery life is very acceptable. I would much rather take the battery hit than be stuck with no LTE access when I want to use the phone, so Verizon will continue to be my network of choice.