Doubts raised over 802.11n for HD video

Spectrum-sharing could limit new Wi-Fi technology.

Although 802.11n has yet to be formally declared as a standard, the wireless technology may not turn out be the solution to delivering streaming HD video that many consumers are hoping.

Often touted as more than five times as fast as the current 802.11g standard, limitations caused by older networks could leave 802.11n devices operating at speeds of 50Mbps, just twice the rate of 802.11g.

Winston Sun, a senior member of the technical staff at wireless firm Atheros Communications, told vnunet.com that an 802.11n device connecting to an access point less than 20ft away can attain rates of 150Mbps, with the rate decreasing as the user moves away from the access point. 

Other experts estimate that most 802.11n configurations yield speeds around 100Mbps under ideal conditions.

The real slowdown comes when a non-802.11n (or legacy) Wi-Fi network is introduced.

Sun explained that the decrease is intended as a way to protect the legacy devices from 802.11n interference.

The new 802.11n 1.10 specifications require devices to switch from their normal 40MHz operating mode to 20MHz when a legacy network is detected.

This prevents the 802.11n device from dominating the Wi-Fi spectrum and leaving the legacy network connection slowing to a crawl.

When the 802.11n device adjusts itself from 40MHz to 20MHz, performance is roughly cut in half, according to Sun.

The decrease would leave the 802.11n devices operating at speeds of only 50-75Mbps, far less than the speeds first promised by 802.11n, and only two to three times faster than the current 802.11g standards.

"In the end, it is a very different message from what the supporters of .11n were saying," Kurt Scherf, vice president and principal analyst at Parks Associates, told vnunet.com. 

While a twofold increase would still allow for devices like Apple's iTV to stream compressed video, it may not be adequate for streaming uncompressed HD videos. 

"There seems to be this movement afoot to address uncompressed video, particularly uncompressed HD," said Scherf. "In that sense, I am not sure that the solutions as they stand have the throughput."

Scherf suggested that if reliable support for streaming HD video is going to happen, researchers may need to go back to the drawing board.

"There is going to be yet another solution needed if they want to do it wirelessly," he said.

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