Start-up Xirrus offers high-capacity WLAN gear

By John Cox
Network World, 03/28/05

A start-up this week is unveiling a super wireless LAN access point that can offer greater capacity and more coverage than conventional access points.

The Xirrus XS-3900 Wireless LAN Array combines advanced antennas, up to 16 802.11a radios and a wireless-LANs switch in a package that looks like an oversized smoke detector. Xirrus software coordinates the 16 wireless-LAN radios to boost capacity. That's because users can connect to the array on 16 channels at the same time, each channel with a data rate of 54M bit/sec, compared with just one channel on a conventional, one-radio access point.

The effect is somewhat comparable to taping together 16 802.11a access points, but adding a special multi-sector antenna that directs the radio energy thereby extending its range, and adding some clever software that blocks interference between the radios and lets users use adjacent channels, again without interference.

One of the 3900 arrays is being used to cover a two-story, 7,800 square foot classroom building on the campus of the Viewpoint School, an independent K-12 school in Calabasas, Calif. The single array mounted centrally on the second floor ceiling handles coverage for the entire building and extends the wireless LAN roughly 150 feet outside the building, which covers part of the campus, says Paul Rosenbaum, the school's associate headmaster, COO and director of technology.

Previously, the building had four Cisco Aironet access points to cover the same area with adequate performance.

Rosenbaum already is weighing the use of the array in a new 40,000 square foot building under construction because the Xirrus products will reduce WLAN installation and maintenance costs. "Lots of client devices can associate with one [Xirrus] device. That's a huge plus," he says. Using the Xirrus DC power option will eliminate the need to install AC power lines to each array, he says.

"We'll be reducing cabling, the number of switch ports needed in the wiring closets and power cords," he says.

The array comes in four-, eight- and 16-radio models. Picture an 18-inch dinner plate with the radios mounted around the diameter. The array can have up to 12 802.11a radios and up to four dual-frequency radios, which can support either 802.11a or 11b/g clients.

Each radio has a sectorized antenna, which in effect, concentrates the radio's energy in a specific segment, or sector, instead of letting it radiate in all directions as in a conventional access point. By concentrating the energy, the array extends the radio's range, so that at any given distance, the available WLAN throughput is higher than a conventional device. Xirrus executives say that the typical range for an 802.11a access point is less than 100 feet, but the array can reach 175 to 200 feet. The company says the array has about twice the range, at any given data rate, of rival access points from Cisco, Aruba Wireless Networks and Trapeze Networks.

One radio can be designated as a radio monitor, constantly sweeping the airwaves to check signal strength and detect unauthorized WLAN signals.

The built-in WLAN switch, which Xirrus dubs the array controller, is where the magic occurs. The switch carries Xirrus software that creates a media access control layer that spans all the radios, instead of each radio having its own MAC address as in conventional access points. In effect, the Xirrus software creates one radio with 16 channels, all of which can operate at the same time.

The switch has two Gigabit Ethernet uplinks to connect upstream to the nearest wiring closet switch. To handle the traffic load, encryption processing and other tasks, it's powered by a 800-MHz PowerPC CPU, with 640M bytes of double data rate RAM.

A separate box, in various models, slots into a data center rack mount to manage 10 to 500 arrays via a GUI. This management system handles configuration, authentication, security policies and firmware upgrades. Xirrus supports the usual security standards: 802.1x for authentication, and 802.11i for security and encryption. The array works with back-end RADIUS servers.

Finally, the array supports 802.11e for QoS capabilities, and standard alphabet soup of security.

There is a DC power option: another rack-mounted box can power the array with DC power over a separate Category 5 cable or 16-gauge wire.

All products will ship in May. The list price for the array starts at $4,000 for the four-radio 3500 model. The eight-radio 3700 array costs $7,000; the 16-radio 3900 array costs $12,000. The management models range from $5,000 to $25,000, depending on the number of arrays being controlled. The remote power system starts at $2,000 for the chassis and one expansion module, which powers four arrays.

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