In an interview with CNET this week, AT&T Mobility's longtime chief Ralph de la Vega noted that the company will be employing circuit-switched fallback - sometimes better known as "CS fallback" or CSFB - in its upcoming LTE handsets, which he says will offer improved battery life and thinner device design compared to Verizon's offerings.

In brief, de la Vega's claims center primarily around two points: one, AT&T's LTE phones won't need to have two radios drawing power simultaneously, which naturally leads to improved standby and talk times. Because Verizon currently needs to operate two radios at the same time, its LTE phones presently include two discrete modems (one of which is typically integrated into the SoC), and more hardware leads to more bulk.

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.

In speaking to a Verizon spokesperson today, the company wouldn't comment on whether it uses CS fallback, but issued this statement (emphasis ours):

We have a year's lead deploying 4G LTE, to more than half the country, already with a dozen great phones, tablets and other devices. As with any new technology, the capabilities and functionality of these devices is rapidly improving. We have a track record of 4G LTE performance excellence that speaks for itself.

To be clear, de la Vega flatly tells CNET that his company will be the first with CS fallback devices on the market. Verizon's verbiage that "capabilities and functionality" are "rapidly improving" seems to be a tacit acknowledgement that it isn't employing the technology, which basically means that it needs to keep the CDMA radio turned on at all times to be ready to receive calls - hence de la Vega's claims of a battery life advantage on AT&T's part. It's believed that Verizon opted against the protocol over concerns that call setups in a mixed CDMA / LTE environment would take unacceptably long, meaning you could be waiting for an additional second or two before incoming or outgoing calls would connect on LTE-capable devices.

For AT&T, though, there are a couple catches. The company's decision to use CS fallback is a product of the fact that it isn't yet in a position to roll out voice service over LTE, which is the first step in allowing carriers to repurpose spectrum allocated for legacy services - 2G and 3G - to LTE. Some day down the road, that'll eventually mean more high-speed bandwidth for everyone, which is good when we've got 3.5- to 5-inch screens in our pockets. Of course, that's not really AT&T's fault - there have been a lot of moving targets in the industry and standards bodies around deploying voice on LTE networks. Verizon isn't in a position to offer voice on its LTE network either, but says it's on track to deploy it in 2012 (the service won't be able to hand off to CDMA, which is a big part of the reason the company hasn't deployed it yet - it's waiting until LTE overlays most of its coverage area so handoffs aren't a concern). For AT&T, though, CS fallback may be a longer-term stopgap solution: it has committed to moving voice to LTE, but hasn't yet offered guidance on when that will happen.

Furthermore, de la Vega doesn't mention that 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.

What's the takeaway? For AT&T, its claimed battery life advantage may be short-lived - it has yet to release any LTE phones, while Verizon is looking to move to voice over LTE in 2012. At that point, concerns of simultaneous radio use should go away.