If conferences are any indication of the eventual success of a concept, then femto cells are well on the way. A packed event at London's Heathrow last week heard plans for indoor 3G base stations from vendors and - most importantly - significant expressions of support form mobile operators.

Femtocells are small cellular base stations, positioned inside buildings, providing coverage for 3G networks and using broadband for backhaul to the operator's network (read our explanation). It's not a new concept - cellular vendors like Ericsson have attempted to sell indoor base stations to enterprises for ten years or so, but it has become more practical now broadband is cheap and plentiful, base stations can be made at a consumer price-point, and there is a shortage of indoor coverage for 3G networks that have already cost operators dearly in licences and equipment.

Several vendors are offering femtocells, including ip.access, Ubiquisys and Radioframe, but operators haven't come out, in favour of it, or even announced live trials, yet.

The Avren conference saw a couple of moves to provoke a change. The formation of the Femto Forum standards and promotion group was launched, and Nokia Siemens announcing a femto controller intended for operators.

Operators looking seriously

For their part, operators gave a detailed account of the technical and business problems with the femto concept - a better sign that they are taking it seriously than presentations listing the benefits. The biggest of those problems are likely to be ones around interference, according to Jiri Valek, RAN systems expert at T-Mobile International.

If the femtocell is on the same channel as surrounding regular "macro" cells outside the house, then there may be interference between the two. The operator will also have to decide whether passers-by outside the house can use the femtocell (the "open island" approach), or not (the "closed island"). "US femtos will be open, while Europeans are less likely to want to share," said Valek.

If it's the former, then the operator will be using one customer's femto and the broadband that customer has paid for, to serve a different customer. If not, then what happens to a customer near the house, where the femto signal may make it harder to get onto the macro network?

The broadband backhaul will raise several issues. If it comes from a different provider, will the femto break its terms and conditions, and will the operator be able to provide any quality of service on it? "The operator will have to partner with the ISP, or the service may not work," said Valek.

Handovers between femto and macro cells will be tricky. "Hand-in will be more complex than hand-out," said Valek. In most cases customers will accept a "hard" hand-over, but the issue will become even more complex if business-grade femtos start to appear in offices, and users expect to roam from one femto directly to another.

To work cheaply enough, femtos will have to be installed by the user - but what happens if the user moves or unplugs a femto, and then doesn't get the service they've been expecting - or causes issues for those outside the house?

What if a tech-savvy person takes a femto abroad and uses it to get free "home" calls on a hotel broadband connection? Valek wants to see a way to ensure a femto is used in one location.

Other questions include the protocol used to manage the femto (several are available) and whether they act as a full node, or (as in femtos from Ubiquisys) use the much-maligned UMA scheme to masquerade as a handset.

Bring on the RFPs

These kinds of questions suggest operators are thinking seriously about femtos - as does the news that France Telecom is preparing an RFP for femtocells in trial quantities, which should help it scope out the business case for renting them to 3G customers, in small, medium and large volumes.

Orange sees a more heavyweight model, where it ships and installs the products, as well as managing them. "These are operator products," said Bruno Dachary, of launch director of Orange France Telecom's 3G service, contrasting 3G-based femtos with the Wi-Fi access points used in the Unik converged phone service, which FT has launched in France, somewhat more successfully than BT's similar Fusion service.

Dachary dismisses the idea that customers can "own" operator equipment, and believes femtos could be difficult to sell, since UK 3G licences require operators to tell the regulator when they add a node to the network.

Vodafone also has an RFP out for femtos, according to wireless news site Unstrung.

But how real are they?

This sort of actitivity has led analysts ABI Research to predict a boom, with 102 million users on 35 million femto cells by 2011. That's too much for other analysts like Dean Bubley, who says: "I'm becoming increasingly skeptical of the near-term opportunity, especially in mature broadband markets."

Bubley warns that homes are going to be full of other kit including Wi-Fi access points and broadband gateways, and most households will have phones from multiple operators: "How do you manage a house with two or more femtos?"

The 900MHz spectrum may soon be available for cellphones - penetrating better into buildings, it may reduce the crisis in indoor coverage, says Bubley. He also agrees that quality of service will be an issue, especially when an operator is using a different ISP's service to backhaul phone services, possibly alongside another providers IP TV, and an enterprise VPN in use by a home-worker.

"For mobile-only operators without their own broadband (or close ISP partnerships), the femto model assumes Net Neutrality, which has the potential to raise accusations of hypocrisy," says Bubley.