A wireless bridge connects two or more wired LAN segments, in a point-to-point or a point-to-multipoint configuration. Wireless bridges operate in half-duplex mode and provide Layer 2 connectivity, as do Ethernet bridges.

Wireless bridges can operate in different modes, just as APs do. There are four modes that a wireless bridge can use to communicate:

1. Root mode – Only one bridge in a group will assume the role of root bridge. It will communicate with non-root bridges and other client devices, connecting them to the main wired network. A root bridge cannot associate with another root bridge.
2. Non-root mode – Non-root-mode bridges communicate with root-mode bridges. Some wireless bridges will also support clients. When supported, this is a special bridge mode in which the bridge is acting as both an AP and a bridge. The diagram below illustrates wireless bridges in both root and non-root modes. The bridges can be in different buildings located as far apart as 25 miles.

Wireless Bridges in Root and Non-Root Modes

3. Access point mode – A wireless bridge can operate as an AP just like an AP can operate as a bridge. In some cases, the bridge can still perform bridging functions. Other times, the bridge becomes an AP and no longer bridges. A wireless bridge operating as both a bridge and an AP is pictured below.

Wireless Bridge in Access Point (AP) Mode

4. Repeater mode – A wireless bridge in repeater mode is situated between two other bridges. It serves to increase the length of the wireless bridged segment, as pictured below. The disadvantage of this configuration is reduced throughput. All frames need to be repeated using the same half-duplex radio. Repeater bridges are non-root bridges. The wired port will likely be disabled in this mode.

Wireless Bridge in Repeater Mode

Installing a Wireless Router

Wireless Bridge Options

Wireless bridges support the same options as access points, including the following:

• Antennas – Fixed and detachable antennas (some with diversity) are available, just as with APs. However, diversity antennas are less likely to be used here since the bridges at each end of the link are static. The environment does not tend to change much. Multipath reception is much less likely to be a problem. Detachable antennas are especially useful for bridging. This is because it allows placement of the antenna outside, perhaps on the roof, while placing the bridge indoors. Most often, directional or semi-directional antennas are used with wireless bridges.
• Filtering – The same filtering options, MAC and protocol-based, are available for wireless bridges.
• Modular radio cards – As with APs, a single slot allows upgrading the radio and two card slots, let the bridge to perform both the access point and bridge functions, potentially eliminating the need for two devices at each end of the link. Of course, when these two functions are both performing on a single radio card, throughput is significantly reduced.
• Variable output power – Useful without amplifiers, attenuators, and cable length variation, variable power can facilitate an outdoor site survey. Used with the additional devices, this can make it easier to fine-tune a bridge link without exceeding FCC power limitations.
• Wired connection types – Typically, various Ethernet/802.3 options are available. A full-duplex wired connection is important in order to maximize the throughput of the wireless bridge.

Workgroup Bridges

Workgroup bridges (WGBs) are often confused with wireless bridges. They are similar, but not the same. A WGB aggregates or groups multiple wired LAN client devices and bridges them to an access point. On the AP, the clients all appear as a single WLAN client device.

A WGB is especially useful in an environment such as a mobile classroom or office, in which a small group of users need to have access to the main network. The individual MAC addresses of the actual client devices will not be seen on the access point attached to the main network. A wireless bridge could be used, but that would require a second wireless bridge at the main site. That is because wireless bridges only communicate with other wireless bridges. Examine the following example configuration using a wireless workgroup bridge.

Example Wireless Workgroup Bridge Environment

Workgroup Bridge Options

Because the WGB is a type of bridge, the options available are similar to those on wireless bridges. There will be a limit to the number of stations that the WGB can connect to the main network. The number ranges from 8 to 128, depending on the manufacturer and the model. More than 30 clients is likely to cause a significant drop in throughput.