To boost its Wi-Fi capacity in packed lecture halls, Georgia Institute of Technology gave up trying to cram in more access points, with conventional omni-directional antennas, and juggle power settings and channel plans. Instead, it turned to new high-gain directional antennas, from Tessco's Ventev division.
Ventev's new TerraWave High-Density Ceiling Mount Antenna, which looks almost exactly like the bottom half of a small pizza box, focuses the Wi-Fi signal from the ceiling mounted Cisco access point in a precise cone-shaped pattern, covering part of the lecture hall floor. Instead of the flakey, laggy connections, about which professors had been complaining, users now consistently get up to 144Mbps (if they have 802.11n client radios).
"Overall, the system performed much better" with the Ventev antennas, says William Lawrence, IT project manager principal with the university's academic and research technologies group. "And there was a much more even distribution of clients across the room's access points."
Initially, these 802.11n access points were running 40-MHz channels, but Lawrence's team eventually switched to the narrower 20 MHz. "We saw more consistent performance for clients in the 20-MHz channel, and I really don't know why," he says. "It seems like the clients were doing a lot of shifting between using 40 MHz and 20 MHz. With the narrower channel, it was very smooth and consistent: we got great video playback."
With the narrower channel, 11n clients can't achieve their maximum 11n throughput. But that doesn't seem to have been a problem in these select locations, Lawrence says. "We've not seen that to be an issue, but we're continuing to monitor it," he says.
The Atlanta main campus has a fully-deployed Cisco WLAN, with about 3,900 access points, nearly all supporting 11n, and 17 wireless controllers. Virtually all of the access points use a conventional, omni-directional antenna, which radiates energy in a globe-shaped configuration with the access point at the center. But in high density classrooms, faculty and students began complaining of flakey connections and slow speeds.
The problem, Lawrence says, was the surging number of Wi-Fi devices actively being used in big classrooms and lectures halls, coupled with Wi-Fi signals, especially in the 2.4-GHz band, stepping on each other over wide sections of the hall, creating co-channel interference.
One Georgia Tech network engineer spent a lot of time monitoring the problem areas and working with students and faculty. In a few cases, the problems could be traced to a client-side configuration problem. But "with 120 clients on one access point, performance really goes downhill," Lawrence says. "With the omni-directional antenna, you can only pack the access points so close."
Shifting users to the cleaner 5 GHz was an obvious step but in practice was rarely feasible: many mobile devices still support only 2.4-GHz connections; and client radios often showed a stubborn willfulness in sticking with a 2.4-GHz connection on a distant access point even when another was available much closer.
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