AT&T is contacting some of its customers asking if they'd like to test an in-home extension to its cellular networks powered by a subscriber's own broadband—a femtocell. Femtocells use frequencies licensed by the carrier for data and voice, while handling backhaul through a customer-provided service.

An Ars Technica reader forwarded a customer survey question he'd seen after being solicited by AT&T for his opinion: "AT&T's new product is a small, security-enabled cellular base station that easily connects to your home DSL or Cable Internet, providing a reliable wireless signal for any 3G phone in every room of your house. The device allows you to have unlimited, nationwide Anytime Minutes for incoming or outgoing calls."

Sprint Nextel has been offering femtocells since last year; the advantage to the carrier is providing fill-in service in the home without deploying more base stations in an area. (See "Sprint's new femtocells offer cell coverage, backhaul costs," July 30, 2008.) AT&T has apparently been testing femtocells with its own employees since last year as well.

Femtocells differ from T-Mobile's UMA (unlicensed mobile access) approach, which also puts a specialized device in the home. With UMA, specialized handsets must have both cell and WiFi radios, and the firmware to handle seamless handoffs between the two network types. With a femtocells, the radio side is effectively identical with only the backhaul varying. T-Mobile also offers WiFi routers that feature two increasingly common VoIP-oriented protocols (one for power conservation, the other for packet prioritization).

Carriers pay enormously less to transit and account for voice and data over a customer's own broadband, and thus can offer so-called unlimited voice plans (which have some very high monthly limits). T-Mobile's HotSpot@Home service costs $10 (1 or more lines) per month adding to a minimum $40-per-month voice plan; Sprint charges $15 to $25 per month for the same thing.

Femtocells have few disadvantages for home users because the dedicated frequencies means that any WiFi network they may already have in place isn't degraded by cellular use, and vice versa.