Operators and vendors have agreed a standard which could speed up the arrival of femtocells - the indoor base stations which are designed to improve coverage of 3G networks indoors by piggy-backing on home and office broadband.

The 3GPP group of mobile operators and equipment vendors has agreed an architecture including a standard femtocell protocol. The Iu-h standard - developed with help from the Femto Forum group - will allow operators and users to buy femtocells from different vendors, and allow competition.

It will be completed and published by by 3GPP in December 2008; despite recent optimism, this is much quicker than expected, and puts the femto standard on track to become part of the 3GPP 8 group of standards, due in December.

"At our meeting in March, we catalogued all the possible ways to do this interface," said Simon Saunders, chair of the Femto Forum. "We did a bit of naming and shaming. They were all good viable candidates, but there were just too many of them. The operators were making it clear they were not going to move forward till they got this sorted out."

This operator involvement put the hardware vendors under the cosh, said Saunders: "The operators specifically put forward absolute requirements." This produced a surprising result - the vendors agreed that none of their solutions completely met the operators' needs, and put together a compromise. "Three weeks ago, everybody gathered in Reston Virginia, knocked the rough edges off the compromise, put it to a 3GPP meeting."

The Iu-h standard could boost operators' confidence to roll out large numbers of femtocells since standard devices will be available quickly. "All the femtos we have out there will be remotely upgradeable," said Will Franks, chief technology officer of femto maker Ubiquisys. "It will require some work on the network side, but as far as operator is concerned, the most important thing is to show there is an upgrade path. It's one thing to replace a gateway, but thousands of femtocells would be more difficult."

The standard defines how femtocells (known as "Home NodeBs" or HNBs) communicate across the Internet to gateways on the core of the phone network. To some extent it is a stepping stone as it works with 3G networks, but will need modification for the forthcoming LTE standard, said Franks. "This applies to W-CDMA (3G) femtocells, but the protocols for LTE are different."

This doesn't matter because LTE handsets will be able to fall back to 3G when the user goes indoors, and will still get better performance through proximity to the cell.

The standard is a compromise between UMA - originally defined for roaming between cellular and Wi-Fi networks - and other proposals. It owes a lot of its existence to behind-the-scenes diplomacy by the major backer of UMA, Kineto, and includes some - such as the use of a separate SIM card in the femtocell, as required by the UMA approach.

"It will be harder for non-UMA vendors to meet the standard," said Franks - Ubiquisys uses UMA. "We see most operators wanting SIM cards, but some don't. It's sensible to make it optional."