November 08, 2013, 12:49 PM — 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."
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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.
Consulting with Cisco, Georgia Tech decided to try some newer access points, with external antenna mounts, and selected one of Cisco's certified partners, Tessco's Ventev Wireless Infrastructure division, to supply the directional antennas. The TerraWave products also are compatible with access points from Aruba, Juniper, Meru, Motorola and others.
Patch antennas focus the radio beam within a specific area. (A couple of vendors, Ruckus Wireless and Xirrus, have developed their own built-in "smart" antennas that adjust and focus Wi-Fi signals on clients.) Depending on the beamwidth, the effect can be that of a floodlight or a spotlight, says Jeff Lime, Ventev's vice president. Ventev's newest TerraWave High-Density products focus the radio beam within narrower ranges than some competing products, and offer higher gain (in effect putting more oomph into the signal to drive it further), he says.
One model, with a maximum power of 20 watts, can have beam widths of 18 or 28 inches vertically, and 24 or 40 inches horizontally, with a gain of 10 or 11 dBi, depending on the frequency range. The second model, with a 50-watt maximum power output, has a beamwidth in both dimension of 35 degrees, at a still higher gain of 14 dBi to drive the spotlighted signal further, in really big areas like a stadium.
At Georgia Tech, each antenna focused the Wi-Fi signal from a specific overhead access point to cover a section of seats below it. Fewer users associate with each access point. The result is a kind of virtuous circle. "It gives more capacity per user, so more bandwidth, so a better user experience," says Lime.
The antennas come with a quartet of 36-inch cables to connect to the access points. The idea is to give IT groups maximum flexibility. But the cables initially were awkward for the IT team installing the antennas. Lawrence says they experimented with different ways of neatly and quickly wrapping up the excess cable to keep it out of the way between the access point proper and the antenna panel [see photo, below]. They also had to modify mounting clips to get them to hold in the metal grid that forms the dropped ceiling in some of the rooms. "Little things like that can cause you some unexpected issues," Lawrence says.
The IT staff worked with Cisco engineers to reset a dedicated controller to handle the new "high density group" of access points; and the controller automatically handled configuration tasks like setting access point power levels and selecting channels.
Another issue is that when the patch antennas were ceiling mounted in second- or third-story rooms, their downward-shooting signal cone reached into the radio space of access points in the floor below. Lawrence says they tweaked the position of the antennas in some cases to send the spotlight signal beaming at an angle. "I look at each room and ask how am I going to deploy these antennas to minimize signal bleed-through into other areas," he says. "Adding a high-gain antenna can have unintended consequences outside the space it's intended for."
But based on improved throughput and consistent signals, Lawrence says it's likely the antennas will be used in a growing number of lecture halls and other spaces on the main and satellite campuses. "This is the best solution we've got for now," he says.
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