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« Cheap Residential Gateways | Main | Article Archives »

April 7, 2001

Billions of Conflicts a Second: 2.4 GHz coexistence

Filed 4/5/01 by Glenn Fleishman

Note: update on 4/12/01 at bottom of article.

An invisible battle with billions of conflicts a second may rage all around us this summer as three competing short-range wireless networking standards begin wide deployment. The battle will be fought not just in the electromagnetic spectrum, but also in the field, as manufacturers ship equipment without clear expectations for their customers quality of service.

The three standards are Bluetooth, IEEE 802.11b, and HomeRF, all of which use the unregulated 2.4 GHz (gigahertz or billions of cycles per second) band. This band, which spans 2400 to 2483.5 MHz (megahertz) in the U.S., and similar frequencies worldwide, can carry megabits of data per second.

The FCC and its counterparts in most other countries have agreed to allow any individual or organization to use all or part of this band without a license via certified low-power, short-range “radios.”

These radios must use spread spectrum transmission, a technique in which no single frequency is used exclusively for more than a short period of time. The three networking standards employ two different spread spectrum methodologies, which put them at loggerheads for use of the band when they’re used within a few feet to a few dozen feet of each other.

All three networking styles divide the available bandwidth into channels. But Bluetooth and HomeRF use frequency hopping (FH), in which the transmission hops in predefined patterns from channel to channel across the entire 83.5 MHz spectrum. IEEE 802.11b, on the other hand, divides the spectrum into overlapping 22 MHz channels, and sends all its information through those swaths.

Bluetooth is designed to exchange data among portable peripherals (digital cameras, cell phones, PDAs, etc.) using a laptop or desktop as a controller. IEEE 802.11b is an extension of Ethernet to wireless networking; the “b” marks the latest, high-speed version of up to 11 Mbps. HomeRF also networks computers (up to 10 Mbps in the 2.0 version), but allows voice and data to intermingle with scheduled priority assigned to voice packets. The range of the networks runs from about 10m radius for Bluetooth to 30m or further for HomeRF and 802.11b. (One manufacturer claims 1,000 feet for line-of-sight with their 802.11b access point, and future chipsets may increase the maximum distance.)

The collision between these technologies may intensify this summer as equipment starts to be widely deployed. HomeRF’s 10 Mbps protocol was approved by the FCC in August 2000, and manufacturers like Proxim, Siemens, and Motorola are just now gearing up. (The HomeRF Working Group has said that the same approval applies to their future 22 Mbps HomeRF 3.0 specification, not yet released.)

Bluetooth, according to many companies, is about to burst forth as the cost lessens and the availability of chipsets rapidly tools up. Recent setbacks include a failure of the largest test to date at a trade show, and Microsoft’s apparent decision to not include native Bluetooth support built into Windows XP, its next-generation consumer operating system due out this fall.

IEEE 802.11b has found wide adoption in the home, small office, and corporation, as well as in public places, where service providers are using it in airport terminals, coffee shops, and hotels to connect business travelers to the Internet. Enabling central hubs cost as little as $300 (more for enterprise versions), and PC and PCI cards run from $100 to $200.

The conflict between these standards is apparent if you visualize an 84-lane highway on which cars of all shapes and sizes tool along with one thing in common: no windows. Some cars are Yugo size in height and width (Bluetooth), occupying a little more than line, but veering wildly from side of the highway to another. Other cars are super road yachts (802.11b), towering 50 feet in the sky and filling 22 lanes, but they never move out of their position. And yet other mid-size vehicles fill only five lanes (HomeRF), but they’re still careening around.

You can imagine the carnage this would cause. During rush hour, with lots of all three models zooming around, the juggernaut might crush the Yugos, but enough collisions cause it to explode Pinto-style. Some cars get through – but at what cost?

The groups representing each standard are talking, but the results of conversations won’t affect currently deployed equipment; it may be until 2002 until new standards that allow better co-existence find their way into shipping equipment.

The IEEE formed an 802.15 group focusing on Personal Area Networks (PAN), with a task group devoted to developing agreement around turning the Bluetooth spec into an IEEE standard.

Another 802.15 task group is working on co-existence between 802.11 and Bluetooth that might necessitate changes in both specifications, possibly using a technique known as adaptive FH, in which frequencies known to be in use or full of interference are “learned” and ignored. (Imagine a car with radar that helps it avoid the crash-prone lanes.)

Although Bluetooth and HomeRF involve potentially fewer collisions, the Bluetooth Special Interest Group (SIG) – the consortium of companies promoting the specification – has regular technical contact with the HomeRF Working Group, Inc., HomeRF’s group of backers, according to Tod Sizer, a Lucent (Bell Labs) researcher and chair of Bluetooth’s co-existence group. Ken Haase, a marketing director at Proxim and general chairman of HomeRF, said, “We may occasionally bump into each other, but it’s a nominal impact.”

HomeRF and 802.11b pose a bigger conflict, because both technologies burn through lots of spectrum in a competing fashion. Steve Shellhammer, chair of the IEEE 802.15 co-existence task group said, “There’s an interference issue and it needs to be addressed” between 802.11b and HomeRF, but “our initial focus is on 802 standards.” Meanwhile, Haase said, “There are have been informal conversations but there haven’t been any formal meetings” between the IEEE and HomeRF.

The best the personal or business consumer of wireless technology can hope for is a quick resolution of the current conflicts, and the ability to update equipment that’s shipped via firmware updates or trade-ins. And perhaps a purely approach might favor a brighter future. Sizer said, “There’s quite a bit of self-interest for us to minimize impact not only on Bluetooth but on other users in the band.”

Update: 4/12/01: The IEEE 802.15 co-existence task group approved a proposal by Mobiliant and Symbol that will put Bluetooth and 802.11b on non-competing tracks. More details to follow.

1 Comment

I hope for a quick resolution of the current conflicts. A friendly approach might favor a brighter future.