Fixed Wireless Equipment

Many Many Radios in One Box

Touring the dismal floor of the Interop show in New York, we saw one thing that really intrigued us: a box with 16 radios in it. Here's the story.

by Alex Goldman ISP-Planet Managing Editor [October 10, 2006]

We talked to John DiGiovanni a couple of times during 2003 (see, for example, Talking About Hotspots at the 802.11 Planet Conference & Expo). Since then, he's moved from Nomadix (where he was director of marketing) to Westlake Village, Calif.-based Xirrus, where he is also director of marketing.

"I got started with the company before it was official," he says. "I was employee number five."

Xirrus, he says, has been in business for three years, and took a little over two years to develop its product, a quality access point that, the company says, makes Wi-Fi reliable for enterprise deployments. The company motto is "take the less out of wireless."

"What we see in wireless is similar to what happened in the wired Ethernet 10 or 12 years ago," DiGiovanni says. "The original architecture used shared bandwidth, but as you added more and more clients, they brought the networks to their knees. The addition of layer 2 switching allowed network architects to segment the medium, allocating bandwidth to users and applications."

With bandwidth for everyone, Ethernet took off. "That's when the hockey stick occurred."

Wireless, DiGiovanni says, is in the problem phase now. "There are more and more clients, more and more applications, and it's voice and video, not just data. Networks are not designed to handle it. We came up with this device we cal a Wi-Fi array. We took the core components, including a WLAN controller, a switch, and integrated up to 16 APs and a high gain antenna to maximize the available Wi-Fi bandwidth."

Three products
The company makes three arrays: the XS-3500 (4 radios), the XS-3700 (8 radios), and the XS-3900 (16 radios, shown at right).

Each product has 4 radios operating at 2.4 GHz (802.11 a/b/g). The 3700 and 3900 add radios operating in the 5 GHz band (802.11a). The product has an omni antenna for monitoring.

Total available bandwidth keeps doubling with each doubling of the number of radios.

The XS-3900 allocates its 12 802.11a radios a 60 degree sector each. Do the math, DiGiovanni explains, and you'll see that's 720 degrees of coverage, exactly two circles. "We wanted to ensure resiliency in the design, so if a radio went down, you'd still have coverage from adjacent radios."

In addition, if there's interference in one sector, each radio can be tuned individually, so the array can reduce power to radios receiving interference. "You can tune those radios down without affecting the rest of the solution," says DiGiovanni.

So is anything keeping the company from releasing a 32 radio array?

"The issue is spectrum," explains DiGiovanni. "There are 3 non-overlapping channels in 802.11b. The other channels are 5 GHz. There are 12 non-overlapping channels in 5 GHz, and the FCC recently opening up an additional 11 channels in the 5 GHz range, so there's enough spectrum for a 26 radio device."

However, DiGiovanni adds, the FCC rules on the new channels are a little vague, and equipment makers won't build for them until they can be sure that what they're making is legal.

The outdoors and the real bandwidth
Xirrus has some small outdoor deployments, generally, DiGiovanni says, on college campuses where there's already Wi-Fi within the buildings but not in the quad areas between them. For such deployments, the arrays can dedicate up to three radios to a single bonded backhaul.

In theory, that would offer 3 x 54 Mbps (802.11a) = 162 Mbps. "In our test, at a distance of half a mile, the yield was about 48 Mbps of TCP throughput, which is similar to the real throughput in a Fast Ethernet connection."

Xirrus claims the throughput for its XS-3900 array is 16 x 54 Mbps = 864 Mbps, but actual throughput should be less.

The efficiency of an array with many 5 GHz radios and a few 2.4 GHz radios depends on how many users have 5 GHz radios on their laptops. At the Interop show in New York, says DiGiovanni, about 60 percent of users connected to Xirrus arrays on the 2.4 GHz band and 40 percent on the 5 GHz band.

He expects the number of users in the 5 GHz band to rise over time, as there's less interference from non-data devices there. "There's more spectrum in the 5 GHz band and it's cleaner."

The company touts the fact that its APs lower the total number of devices on the network. At Interop, at New York City's Javits conference center, the company needed only 11 arrays, says DiGiovanni. The 11 arrays covered about 400,000 square feet on three levels.

"The vendor that supplied the solution last year deployed 150 devices," he adds.

Pricing and availability
The Xirrus arrays are available now. The list prices are:

• XS-3500 (4 radios): $3,000
• XS-3700 (8 radios): $6,000
• XS-3900 (16 radios): $12,000

—End