Xirrus' XS-3900 Wireless LAN Array

Multiple Models, Radios

The XS-3900 we tested includes 16 integrated APs, but Xirrus also offers scaled-down versions with four or eight radios for smaller buildings or fewer users. Regardless of the size you choose, the XS-3900 draws much of its appeal from the fact that a single device can provide service across every nonoverlapping channel of both the 2.4-GHz and 5-GHz unlicensed radio bands. That translates to an aggregate data rate of more than 800 MB per second. Two Gigabit Ethernet ports provide backhaul to the wired network, and a 10/100 Ethernet port is available for management. We couldn't effectively evaluate overall system performance (we would have needed at least 15 clients), but we were able to evaluate range and get a general sense of performance and management capabilities.

Only four of the 16 integrated APs (IAPs) in our test unit included dual-mode (802.11a/b/g) radios. The remaining 12 were restricted to 802.11a. When the device boots up, it scans the airwaves and assigns non-overlapping channels to each IAP. You can override the automatic RF settings to optimize coverage.

The IAPs provide service using sectorized directional antennas. Lower frequency 2.4-GHz signals propagate better indoors than 5-GHz signals, but by varying the antenna gain across the two bands, Xirrus attempts to provide roughly comparable RF footprints while providing substantial overlap between adjacent cells to avoid RF coverage holes. For 802.11b/g coverage, there are three 180-degree 3-dBi antennas equally spaced inside the unit. For 802.11a service, the array uses 12 60-degree 7-dBi antennas. Each 3 dBi of antenna gain translates into a doubling of the RF signal. An additional 360-degree omnidirectional antenna is attached to one of the 802.11a/b/g radios for dedicated RF monitoring.

Comparison Shopping

We assessed how well the array provides comparable coverage footprints across both bands and the product's relative coverage footprint compared with Cisco's popular dual-band Aironet 1200 AP. We used AirMagnet Surveyor 2.5 configured with our building's floor plan to plot the signal-to-noise ratio while conducting a thorough walkabout of the facility. We found the array's 802.11a coverage significantly exceeded that of the Aironet, which we expected given the higher gain directional antennas. The system eliminated many of the 802.11a dead spots left by other products and provided almost complete coverage of our test environment.

On the other hand, the 802.11b/g coverage fell somewhat short of expectations. Many of the areas covered by the Cisco were not well-illuminated by the XS-3900. Xirrus says these problems should be resolved in the final release.We performed throughput testing for a single node to get a general feel for how each radio stacked up against competitors. We used Ixia Chariot to measure network throughput at 20 feet and 100 feet from the AP. At 20 feet, we saw single-station 802.11a throughput of 22 Mbps, compared with 26 Mbps for the Aironet. At 100 feet, however, Xirrus' device outperformed Cisco's, maintaining its 22 Mbps throughput while Cisco's fell to 19 Mbps. Xirrus' performance for 11g was approximately 18 Mbps, compared with 19 Mbps for Cisco, both at 20 and 100 feet. For 11b, Xirrus delivered 5.6 Mbps on 11b compared with 6.2 Mbps for Cisco. Although these performance differences are slight, remember that we tested with a single client. The array's appeal lies in extended coverage and total system capacity.

Beyond Performance

The XS-3900 works with management environments using SNMP but for most operations, admins will manage the system through a Web server integrated into each array's controller or through the XM-3300 management appliance, which addresses configuration, policy and firmware revisions. The management interface is well-designed. It let us easily configure parameters for all the IAPs, including WEP, WPA and WPA2 security. For monitoring, a simple event log is included. Notably absent is rogue AP detection, which Xirrus says the XM-3300 provides.

The array's design makes it impossible to use conventional Power over Ethernet technology because the system draws significantly more power than can be provided through 802.3af PoE equipment. Each array can be powered through a standard AC outlet or through Xirrus' XP-3100 remote power system, which provides DC power to the array over Category 5 cable at up to 300 meters.

Dave Molta is a Network Computing senior technology editor. He is also assistant dean for technology at the School of Information Studies and director of the Center for Emerging Network Technologies at Syracuse University. Jameson Blandford is a lab associate at the center. Write to them at [email protected].(Editor's Note: 802.11 Coverage Maps for Xirrus WLAN Array Produced using AirMagnet Surveyor[Higher numbers represent higher signal levels])

In a market dominated by conventional smart APs and WLAN switches, the Xirrus offering is a breath of fresh air. However, just being different doesn't always lead to success. Prospective customers should determine whether the Xirrus approach is cost-effective compared with more conventional products.

With a list price of almost $12,000, the XS-3900 is priced higher than competitive offerings. On a per-AP basis, the Xirrus WLAN Array rings in at about $750 per AP, but unlike other competitive offerings, the array includes an integrated controller, which costs extra when purchased from wireless switch vendors. On the other hand, most wireless switch vendors let you support significantly more APs per controller. When Xirrus provides competitive pricing comparisons, it points out that the cost of installing a single WLAN Array is much less than the costs of deploying 16 APs. When you combine all those factors, the Xirrus offering looks much more cost competitive. The availability of lower-priced units, with fewer IAPs, also can lower the array cost for some installations.

Overall, we like Xirrus' strategy, particularly for organizations that embrace 802.11a as a key element of delivering scalable WLAN performance. In many environments, delivering service to a large number of users from a single infrastructure device makes sense. We expect to see other vendors expand their product portfolios in the future to include multiradio APs, though we're less confident that vendors will choose to integrate the controller subsystem as well. In some high-density user environments, such as classrooms, the Xirrus approach could have significant appeal, particularly with 802.11a clients. But to meet the needs of mainstream enterprises, Xirrus must broaden its portfolio to include more conventional AP offerings that work with array controllers.0