What’s new in wireless bridging?

Wireless LAN bridges have long been available as an attractive internetworking option.  For example, over-the-air, multimegabit-speed LAN-to-LAN connections are appealing for linking sites within a metropolitan area. You avoid digging up the environment to run cabling or paying monthly recurring service provider fees.

Bridges also are a strong option for connecting temporary work locations – such as construction sites – that must communicate to a data center. The devices can reach distances of 20 miles or more, depending on geography and type of antenna used.

There are wireless bridges that operate in licensed and unlicensed bands. Those in the licensed band are usually considered less prone to interference, because having to get and pay for a license weeds out many network operators.

However, inexpensive 802.11b-based (11M bit/sec, 2.4 GHz) bridges and, more recently, 802.11a (54M bit/sec, 5 GHz) bridges have become available in the unlicensed bands. Witness, for example, Cisco’s announcement of its $5K, 802.11a-based bridge, the Aironet 1400 Series, in June. At this juncture, the 5 GHz band is still fairly virgin territory and should be resilient to interference.

There is one primary differentiator between an 802.11a-based wireless bridge and an 802.11a wireless LAN access point: To be useful, wireless bridges must transmit much farther than 300-foot WLANs, a capability requiring an adjustment to the MAC layer of the 802.11 standard.

To date, each supplier has developed a unique way to extend the window of time before the transmitting bridge requires an acknowledgement receipt from the receiving bridge, thus enabling longer-distance transmissions. Now the industry is considering the emerging 802.11j standard as a common way to support the longer distances.

802.11j is the Japanese flavor of North America’s 802.11a, the 54M bit/sec, Orthogonal Frequency Division Multiplexing (OFDM)-based IEEE standard. The standard is being tweaked to work in the 4.9-5.0 GHz frequency assigned to high-speed WLANs in Japan.

The window adjustment to the MAC in 802.11j is likely to be standardized in Japan for metro-area bridging, say IEEE aficionados, and will probably also become an industry standard for worldwide 802.11-based bridges.