Extending WLAN Range with Repeaters

Access points, which require interconnecting cabling, generally play a dominant in providing radio frequency (RF) coverage in most wireless LAN (WLAN) deployments. Wireless repeaters, though, are an alternative way to extend the range of an existing WLAN instead of adding more access points. There are very few stand-alone 802.11 wireless repeaters on the market, but some access points have a built-in repeater mode. Here's the basic information you need to know when using wireless repeaters.

In general, a repeater simply regenerates a network signal in order to extend the range of the existing network infrastructure. A WLAN repeater does not physically connect by wire to any part of the network. Instead, it receives radio signals (802.11 frames) from an access point, end user device, or another repeater and retransmits the frames. This makes it possible for a repeater located in between an access point and distant user to act as a relay for frames traveling back and forth between the user and the access point.

As a result, wireless repeaters are an effective solution to overcome signal impairments such as RF attenuation . For example, repeaters provide connectivity to remote areas that normally would not have wireless network access. You may have one access point in a home or small office that doesn't quite cover the entire area where users need connectivity, such as a basement or patio. The placement of a repeater between the covered and uncovered areas, however, will provide connectivity throughout the entire space. The wireless repeater fills holes in coverage, enabling seamless roaming.

A recent review of the repeating function of the D-Link DWL-900AP+ access point identifies a range increase of 160%. In the review, the original signal was lost at about 55feet from the router/access point. With the placement of a DWL-900AP+ set to repeater mode at the site of the original signal loss, the signal strength was still around 72% at 140 feet. The repeater provides the extra kick in situations where you need to reach remote users.

Nearly all WLAN repeaters currently available today are actually built-in functions of access points. For example, the Cisco 350 and 1200 allow you to configure the access point to behave as a repeater (and not as an access point). Buffalo Technology, however, does offer a stand alone repeater in their AirStation Pro Series WLA-AWCG. The advantage of the stand alone repeaters is that they are generally less expensive.

One downside of wireless repeaters, though, is that they reduce throughput on the WLAN. A repeater must receive and retransmit each frame on the same RF channel, which effectively doubles the number of frames that are sent. This problem compounds when using multiple repeaters because each repeater will duplicate the number of frames sent. Thus, be sure to plan the use of repeaters sparingly.

The configuration of a repeater is relatively straight forward. After switching the access point to repeater mode, you set the service set identifier (SSID) of the repeater to match the SSID of the specific (root) access points that the repeater will associate with. Most repeaters will, similar to wireless network cards, automatically associate with the access point with the strongest signal. However, you can designate specific MAC addresses of the preferred and secondary access points as an option. If the repeater cannot connect with the preferred access point, it will try to associate with the next one, and so on.

All in all, wireless repeaters are an excellent way to increase the radio range of an existing WLAN, especially if it's not practical to install an additional access point to fully cover the location. Just don't get carried away with installing too many repeaters to keep performance up and users smiling.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers computer-based training (CBT) courses on wireless LANs.

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