Electronic Design

Get 16 Radios In One Wi-Fi Access Point

Wi-Fi access points are ubiquitous nowadays. Yet the XS-3900 Wireless LAN Array from Xirrus is unique—and the best in its field. It solves all of the major problems that current access points share, making you wonder if you really need the forthcoming 802.11n MIMO-based (multiple input, multiple output) access points.

The XS-3900's special physical design combines 16 IEEE 802.11a/ b/ g radios with high gain multisector directional antennas in a unique circular enclosure packaged with an integrated wireless local-area network (WLAN) switch and controller (Fig. 1). Each one of the 12 802.11a transceivers and four 802.11b/g transceivers has a directional gain antenna covering specific sectors of its 360° view (Fig. 2).

One of those 802.11a/b/g radios may be designated as an RF monitor. The 802.11a antennas provide 60° overlapping coverage, while the 802.11a/b/g radios have 180° overlapping coverage, all for redundancy and fallover. A 2-Gbit/s switch fabric and 16-port media-access controller (MAC) manage the wireless traffic.

The XS-3900 can blanket a 100- by 300-ft area. The gain antennas greatly extend the range of each radio. The extra antenna gain usually means a faster data rate. Also, the greater signal strength means the radio can ratchet up to the higher data speeds reliably. And with only one unit to install, the amount of cable pulling is reduced to a minimum .

PUT TO THE TEST
The Tolly Group, an independent testing and consulting organization, recently performed indoor tests in both line-of-sight and obstructed settings to evaluate the range and data rate of the XS-3900 with standard wireless notebook PCs using 802.11a/b/g cards. The XS-3900 provided 2.5 times the range, a throughput of three times the typical rate at 100 ft., and a Transmission Control Protocol (TCP) throughput of 13 times that of a leading enterprise access point.

The XS-3900 also makes it possible to support the treasured triple play of voice, video, and data over the LAN. Its Wi-QoS (802.11e) provides a switched Wi-Fi solution to tag each wireless packet with 802.11Q virtual LAN and 802.1p prioritization at the source of each packet's transition to the wired network. An enterprise can now extend its existing wired quality-of-service (QoS) policies to the WLAN to allow for discrete tagging and prioritization of mission-critical applications between the wired and wireless parts of the network.

Each Xirrus LAN array is capable of real-time, load balanced discrete classification and prioritization between the wired and wireless network. Also, each array can support 180 simultaneous voice calls, 45 simultaneous standard-definition TV (SDTV) video streams, wireless fast Ethernet back-haul links between the arrays, and fast roaming between radios with an array or between arrays. The total maximum bandwidth is 864 Mbits/s.

John DiGiovanni, Xirrus' VP, says that the XS-3900 was designed with enough headroom to support the future 802.11n standard when it is finalized. The built-in switching fabric is rated at 2 Gbits/s. The array's multichannel MAC is contained in three FPGAs so it also can be firmware upgraded to support the MAC level changes that will occur in 802.11n. It will be easy to replace the individual radio modules with 11n radios when the right time comes.

Xirrus also makes smaller units, like the XS-3700 with eight radios (four 802.11a, four 802.11b/g) and the XS-3500 with four radios (802.11a/b/g). The XM-3300 management platform can handle up to 500 WLAN arrays.

On a related note, Xirrus was recently selected to handle the networking chores for the Interop conference in New York City. The company just received a design patent for its WLAN array housing. Several other patents for this product are pending as well.

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