Can we extend range with repeaters?

Q: Can you extend the range of 802.11 networks using wireless repeaters? I have an application where I need mobile coverage in a large parking lot of trucks. Rather than hardwired access points, can you use multiple wireless repeaters to extend the range back to a central base station? And what version of 802.11 would be most suitable for outdoor, long-range usage? Range is more important than bandwidth and speed in this application. - Bob U.

The Wizards gaze deeply into their crystal ball and respond:

Bob O'Hara, Airespace

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Q: Can you extend the range of 802.11 networks using wireless repeaters? I have an application where I need mobile coverage in a large parking lot of trucks. Rather than hardwired access points, can you use multiple wireless repeaters to extend the range back to a central base station? And what version of 802.11 would be most suitable for outdoor, long-range usage? Range is more important than bandwidth and speed in this application. - Bob U.

The Wizards gaze deeply into their crystal ball and respond:

Bob O'Hara, Airespace

Several vendors offer wireless routing systems that extend outdoor 802.11 coverage. Available systems range from less expensive consumer-grade access point bridging products that can extend wireless coverage by only one access point per repeater, to more expensive carrier-class wireless repeating systems that can extend outdoor coverage by multiple hops.

For U.S. deployment, you will want to use 802.11a (5 GHz) for the backhaul transport, and 802.11b (2.4 GHz) for the client interface. Features to look for when choosing an outdoor repeating system include:

· Bi-directional outdoor amplifiers - These products typically consist of a low noise pre-amplifier receiver and a power amplifier for transmitting. A bi-directional amplifier will significantly increase the operating range and performance of the outdoor links.

· Internal directional antennae - By deploying systems with an internal directional antenna, you can better target the desired coverage areas while limiting unwanted RF overflow into adjoining areas.

· Optimized 802.11 MAC - A system with support for 802.11 MAC layer services optimized for outdoor use will provide better overall performance. The original 802.11 standard was optimized for indoor systems. You should look for systems that have optimized the 802.11 MAC for outdoor links. Optimizations would include adjustments to MAC services, such as an extended ACK timeout to support long RF links and RTS/CTS to help solve hidden node problems. Systems with these modifications will increase overall performance and provide better reliability for outdoor use.

· Outdoor rated enclosure units - The system unit casing needs to be rated such that it can sustain extended temperature ranges so that it will not fail due to harsh climate conditions. The system should also be able to support a variety of outdoor enclosures such as mounting brackets and power adapters for streetlight installation.

WLAN systems that can provide all of the outdoor-related features mentioned above, plus offer dynamic RF intelligence and centralized policy management, represent the best-of-class available on the market today.

Scott Haugdahl, WildPackets

Repeaters can extend the coverage of an 802.11 network, with some caveats. For one, a repeater must receive a frame and repeat it on the same channel, meaning your bandwidth is effectively cut in half. Even though you indicated that range is more important than bandwidth, at longer ranges clients will be transmitting as low as 1M bit/sec. About half of that is usable for data, because of 802.11 overhead. Cut that in half again because of a repeater, and even more so with multiple users sharing the same channel, and you could run into a bandwidth issue.