First large-scale 802.11n wireless LAN now operational

Morrisville State College has 720 access points that create a pervasive, reliable, high-throughput wireless LAN

150Mbps per radio

In November, MSC and its systems integrator, IBM Global Technology Services, finished installing the 11n access points. During the summer, the college deployed Meru's new MC5000 high-end controller, which was designed for large 11n networks. Currently, all 11n gear is based on Draft 2 of the pending IEEE 802.11n high-throughput WLAN standard. Both radios in the Meru AP 320 can support as much as 150Mbps of shared throughput for student and faculty laptops that are fitted with embedded or plug-in 11n radios.

This year, freshman received new college-owned Lenovo ThinkPad T61s notebooks, with a built-in 802.11a/b/g/n chipset. Those PCs access the campus WLAN on the 5GHz band, via the radios in the Meru access point. The second radio, on the 2.4GHz band, will devote one 20MHz channel to 11b/g clients. The remaining two 20MHz channels will be combined into a wider 40MHz 11n channel to support laptops with an 11n plug-in card or USB dongle. Currently, nearly all such adapters work in the 2.4GHz band.

There is some early, anecdotal evidence from students that these legacy 11b/g clients are seeing slightly improved performance and more reliable connectivity. "The reliability of the [11a/b/g] connection has improved, by not dropping the connection like it did [in the past]," says MSC student Jason Witter, a junior in the Network Administrator program who also works on the campus laptop help desk. Witter has experimented with the wireless settings on his own laptop and found the changes did improve performance "a little bit" for the 11a/b/g radio connecting to an 11n access point.

Longer range, better signal quality

"The signal for each access point, for both radios, is definitely going further," Barber says. In effect, more access points can now "see each other," a situation that could cause interference. But Meru's architecture lets MSC put all the 2.4GHz radios on one channel and all the 5GHz radios on one different, non-overlapping channel. The result, Barber says, is a dense coverage with many access points in proximity that supports a lot of users and provides high throughput with no interference.

The quality of the 11n connection seems markedly better as well, though again this is based on early, anecdotal evidence. Barber says that during the summer he was testing an early Meru 11n access point, using his wireless laptop. "I was sitting at my desk and my laptop preferred to connect to the 11n access point rather than to an 11g access point that was closer to me," he says. "The laptop got higher throughput from the 11n connection and preferred that."

Another issue to be explored is whether the so-called legacy wireless clients will slow down performance for the 11n clients, as is the case today when 11Mbps 11b devices connect to an 54Mbps 11g access point: 11g clients end up behaving as if they were 11b clients. That shouldn't be a problem at MSC, according to Meru, because the Meru controller manages the client's access to the wireless medium, instead of leaving it to up the client, as is customary. Meru allocates each client the same amount of time to transmit or receive. During that time, the 11n client can transmit far more than an 11b or 11g client can, for example. Without this time-slicing, a slow client can hog the connection and degrade performance for faster clients.

One novel problem has been the disruptive interference in the 2.4GHz band caused by Microsoft Xbox game consoles. Barber and his team noticed a strange pattern of interference: a strong signal jumping around all the channels on that band. The team gradually narrowed the interference down to a few areas in some dorms, and by a process of elimination focused on game consoles.

To confirm it, Barber brought in his own Xbox from home, plugged it in, and found the same "very strong, crazy interference" pattern showing up on the radio-frequency analyzer. "It was even worse with multiple Xboxes in a given location," he says. So far MSC hasn't come up with a solution. But it was found that shielding the Xboxes with the antistatic bags used to protect electronic equipment from electrostatic discharge during assembly and shipping led to a noticeable drop in interference. It's probably not a long-term solution because "that's not good for the heat [level]," Barber says.

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John Cox

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