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 US 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.
- Optimised 802.11 MAC - A system with support for 802.11 MAC layer services optimised for outdoor use will provide better overall performance. The original 802.11 standard was optimised for indoor systems. You should look for systems that have optimised the 802.11 MAC for outdoor links. Optimisations 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 centralised 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 1 Mbit/s. 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.
Another warning is that while some repeaters have single omni-directional antennas, some access points (which can be put into repeater mode) have two diversity antennas. If you wish to use higher gain antennas to extend the range, always use multiple omni-directional, not directional antennas. With a diversity antenna system, only one antenna is operational at a time, so it may not see transmissions from the opposite direction when actively transmitting, leading to potential interference and retransmissions. Use of multiple repeaters is generally discouraged. Simply put, the longer the wireless path, the more chance for failure.
As for which version of 802.11, even though the 802.11a frequency spectrum is less crowed then b/g, I would consider b/g for its farther reach, plus far more competitive market and devices to choose from.
Bob Myers, Chantry
It is possible to extend the range of an 802.11 network through the use of repeating technologies. In fact, the IEEE 802.11 protocol has been defined to support repeating through a couple of models. Wireless Distribution System (WDS), for example, allows an access point to not only connect to another access point over the wireless link, but it can also service 802.11 devices at the same time. If bandwidth isn’t a concern for the application, then 802.11b has the most flexibility when it comes to range today. However, remember that the available bandwidth is essentially halved if you are using the same radio for servicing users and for the backhaul.