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Open Spectrum Faces Clogging

There’s no doubt that unlicensed spectrum and 802.11 equipment is a powerful combination. It allows anyone to setup a wireless broadband network when, and where, they want it.

But deploying mission-critical networks in shared, open spectrum is not without risk, and there are widespread concerns that a wireless “tragedy of the commons” could be just around the corner. The fear is that with too many users competing for too little spectrum, it’s only a matter of time before wireless LAN becomes so congested and unreliable as to be unusable (see WLAN: Future Imperfect?).

How valid these concerns are, and how they should be tackled, is the subject of this month’s Unstrung readers' poll: Unlicensed Spectrum: Is it broke? Can we fix it?.

Issues of interference in open spectrum were also addressed in last week’s Unstrung Webinar “Unlicensed Broadband for Metro 802.11 and Wireless ISPs.”

The event addressed how service providers, municipalities, and campus organizations could deploy outdoor and metro-area networks using 802.11 and 802.16 (WiMax) equipment. Speakers on the Webinar tackled head-on questions about how to manage collocation of equipment, mitigate interference, and generally keep the network up and running in the face of adversity.

Sony Kogin, product manager for Proxim Corp.’s (Nasdaq: PROX) MP.11 point-to-multipoint systems, acknowledged potential problems with open spectrum and said there is always a tradeoff between performance, collocation flexibility, and user density.

He said that although some installations attempt to overcome the problem by brute force (drowning everyone else with illegal amplifiers), there are more graceful solutions.

Proxim’s chosen route for its outdoor product is to use low-cost 802.11 OFDM radios, but strip out the 802.11 MAC and replace it with a more deterministic upper-layer protocol called WORP (Wireless Outdoor Routing Protocol) that is similar in concept to the TDM protocols used in WiMax. “In essence, it’s a complete departure from the collision-based protocol based on 802.11,” he said.

Kogin’s other tips were to use smaller channel bandwidths (5 MHz or 10 MHz instead of the usual 20 MHz) to increase the number of non-overlapping channels, and to appoint a local-area frequency coordinator.

Bert Williams, vice president of marketing at metro mesh vendor Tropos Networks, argued that built-in contention control in the 802.11 protocol does allow “some graceful sharing” of spectrum, and said that in over 150 Tropos installations, interference has not been nearly as bad as expected. He said there are three main reasons for this:

  • A lot of devices are installed indoors, with low power configurations that don’t leak much signal outdoors
  • A lot of the outdoor installations today are point-to-point solutions that run on rooftops that are much higher than the streetlight level that Tropos uses (“We’re not seeing them, we’re spatially in different areas.”)
  • Mesh networks can automatically route around interference to preserve the integrity of the network, even if some links suffer interference


Terry Boland, director of product marketing at the wireless mesh division within Nortel Networks Ltd. (NYSE/Toronto: NT), made the point that contention and interference could be reduced by using multi-radio mesh nodes, with separate 802.11g (54 Mbit/s over 2.4 GHz) radios for client access, and 802.11a (54 Mbit/s over 5 GHz) radios with focused beams for the transit link.

Boland also anticipates the introduction of WiMax radios into the mesh node, initially for backhaul and transit links. This will likely result in networks being deployed in a mixture of both licensed and unlicensed spectrum, he said.

The hour-long event can be reviewed, for free, by visiting our online Webinar archive: here.

To take the poll click here: Unlicensed Spectrum.

— Gabriel Brown, Chief Analyst, Unstrung Insider

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