It seems counter-intuitive. Deploy a wireless network in a frequency band where there is little or no regulation - meaning that other operators can use that same chunk of spectrum without having to check on any interference implications - and then offer service level agreements to demanding business customers.

What may seem even more against the grain of common sense is that those business customers - rather than having to wave angrily the SLAs in front of their service providers' noses after being badly let down - appear generally satisfied with their service.

US-based TowerStream, which offers T1/DSL replacement services to business customers using WiMAX in the unlicensed 5.8GHz frequency band, reported an impressively low average monthly customer churn rate of 1.35 percent during 1Q 2008. Offering downlink speeds ranging between 512Kbps to 1Gbps, TowerStream says it can guarantee 99.99 percent reliability.

Urban WiMAX, which has rolled out a WiMAX network across London at 5.8GHz and 5.4GHz (another unlicensed frequency band), offers a 99.5 percent availability SLA. Its service portfolio includes symmetrical data products ranging from 2Mbps to 10Mbps (in 2Mbps increments) and is primarily targeting SME customers (10 - 250 employees).

While SMEs, arguably, are less likely than larger enterprises to push for a stringent SLA - particularly in the case of Urban WiMAX, which typically charges its VoIP and symmetrical data products 30 percent cheaper than comparable products from its rivals - price alone cannot explain why the unlicensed spectrum operator has accrued what it claims to be several hundred SME customers. Some larger customers, such as GE Money, Bovis, Swisscom, Dennis Publishing and Faber & Faber, have also signed up to Urban WiMAX. It indicates that Urban WiMAX, like TowerStream, is delivering on QoS.

One reason why WiMAX can cope better with interference in licence-exempt bands compared with proprietary broadband wireless access systems is because its uses OFDM (orthogonal frequency-division multiplexing), which splits data traffic into multiple sub-carriers. This makes it less likely for the message to be adversely affected by other traffic being sent in the same part of the spectrum.

Unlicensed frequency bands have other advantages. They don't require any hefty upfront investment on spectrum, which lowers the barrier to entry for smaller players. It can also provide a quicker route to market if the regulator is dragging its heels on WiMAX licence awards. And, according to suppliers, you don't need much unlicensed spectrum on which to build an attractive business case. "Typically the operator customers we work with they are happy if they can have up to 7MHz per channel, but sometimes as little as 3MHz channel will suffice [if there is minimal interference]." says Elvis Tucker, director of solutions and alliances at Aperto Networks. "At the top end [around 7MHz] you can provide operators' customers between 8Mbps and 10Mbps. And, by prioritising traffic of particular customers, as well as by prioritising the different types of traffic used by individual business customers, operators can offer SLAs on mission-critical applications and charge premium prices."

Aperto, which is TowerStream's main supplier, provides support on its fixed WiMAX portfolio for a number of licence-exempt and 'regulatory light' frequency bands (in addition to supporting licensed spectrum). These bands are 5.8GHz, 5.4GHz, 5.1GHz, 4.9GHz and, most recently, 3.65GHz.

But it is in the 5GHz band (ranging from 5.15GHz to 5.925GHz) that Sky Light Research identifies as enjoying the most unlicensed wireless activity in recent years. According to the research firm, 5GHz accounted for around 75 percent of all unlicensed wireless broadband shipments in 2007. It is less congested than 2.4GHz and has a stronger global presence than 900Hz, says Sky Light Research, as well as being available in most regions in the world.

However, following the FCC's decision last year to allocate 3.65GHz for wireless broadband, a frequency band that is lightly regulated in the US - a nominal registration fee is all that is required - a growing number of WiMAX vendors have had their WiMAX kit FCC-certified at this frequency. These include Redline, Alvarion, Aperto and Airspan (all of which offer WiMAX equipment at 5GHz as well). Towerstream has already expressed interest in using 802.16e at 3.65GHz, which, if successful, would open up another unlicensed opportunity for suppliers.

Yet there is no tier one vendor vying strongly for WiMAX market share in unlicensed frequency bands. "The volumes aren't there to make this attractive for us and I'm sceptical that it's possible to offer meaningful SLAs," says a spokesperson for a Tier One supplier.

And sceptics will further point out that of all the unlicensed frequency bands, only 802.16d at 5.8GHz has a profile developed by the WiMAX Forum. Outside of that frequency there is little chance of operators being able to take advantage of vendor interoperability.

The onus is on suppliers to convince operators that their WiMAX kit in unlicensed spectrum, as it is effectively proprietary, does not derail the business case.

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