When Wi-Fi equipment was originally deployed in Enterprise networks, the wireless interface to the network was merely a loose association of access points (APs), according to John DiGiovanni, director of marketing for WLAN products at Xirrus. However, this arrangement made managing and monitoring the RF environment difficult. It could also reduce a network’s security. To address these and other issues, the wireless LAN switch was developed.

While the wireless LAN switch enabled control and monitoring of all Wi-Fi access points, the physical interconnection of all APs to a centralized switch presented a new set of difficulties relating to cost and complexity. Xirrus developed the Wireless LAN (WLAN) Array to solve these problems. This ceiling-mounted device contains a WLAN switch directly connected to as many as 16 integrated access points (IAPs).

The WLAN switch is centrally located within the Wireless LAN Array. It contains a single multi-channel MAC that interfaces to the IAPs. The Xirrus MAC is implemented as a group of three Xilinx FGPAs, which collectively interconnect with a Power PC processor.

Within the WLAN Array housing, the IAPs are arranged in a circular configuration around the WLAN switch. Each IAP provides a separate RF channel of highly directional coverage that radiates away from the WLAN Array in a narrow radial sector. The channels are physically adjacent and form a seamless multi-channel region of coverage around the WLAN Array.

Because the IAPs provide highly directional coverage, the transmit power and receive sensitivity for each IAP is higher than would be possible for the same signal strengths in an omnidirectional AP. This also allows multiple Wi-Fi devices to operate in close proximity by providing a non-overlapping channel for each device. DiGiovanni stated this is important because the 802.11 specification requires that the RF medium be clear before wireless transmission is permitted.

Another unique functionality provided by the WLAN Array is backhaul RF mesh networking. Information exchange among the devices is packet-based, and up to three parallel mesh links can be established to increase throughput to 162 Mbps. In theory, there is no limit to the number of transfers, or “hops,” that a packet can make among the WLAN Arrays. However, each hop takes approximately 10 ms, and the practical limit to the number of hops depends on the application being run.

The devices support both the 2.4 GHz and 5 GHz frequency bands, according to DiGiovanni. The 2.4 GHz band as defined by 802.11bg provides three non-overlapping channels and is included to support legacy equipment. The 5 GHz band defined by 802.11a allows for 12 non-overlapping channels. This dual coverage is in keeping with industry trends to support both bands.

Both bands have multi-path issues, stated DiGiovanni. These can be addressed with the emerging 802.11n standard that will take advantage of multi-path. This standard will include changes in the PHY and MAC layers. It will also allow for channel bonding for a given wireless link to increase throughput.

In the meantime, the wireless access provided by the WLAN Arrays from Xirrus can improve RF coverage for a Wi-Fi Enterprise network. According to DiGiovanni, the highest average power consumption per device is 60 W. The sectored antennas mentioned previously nearly double the range, and thus quadruple the coverage area of a conventional wireless AP operating at the same power level, providing 54 Mbps throughput for 60,000 sq. ft verses 15,000 sq. ft for a conventional AP.

A central management system for the WLAN Arrays enables control of up to 500 units. Because of the wireless mesh networking capability of the WLAN arrays, no physical cabling is required except for a single remote Ethernet connection to the central management system.

At present, the WLAN arrays are powered by either 120 Vac or 48 Vdc, which can be configured for mutual failover. There is also a possibility for using PoE when new PoE standards define power level increases suitable for the levels needed for the WLAN Array. These developments would be in keeping with the benefit of the WLAN Arrays from Xirrus, which reduce the number of wires needed to support wireless capabilities.