Nonstandard Devices

Although the previous section described how 802.11-standards–based devices access the wireless medium, this section discusses devices that fall outside of the 802.11 standard. These devices use the 802.11 technology in a way that violates or extends an area of the standard and that might prove useful in your network. The specific devices under consideration are:

• Repeater APs

• Universal clients (workgroup bridges)

• Wireless bridges

Although each of these devices provides useful networking tools, you should remember that they are not currently defined in the 802.11 standard, and there are no interoperability guarantees because different vendors may define different mechanisms for implementing these tools. For the reliability of your network, should you choose to use these, you should ensure that they are only interfacing to devices from the same vendor or devices for which the vendor ensures interoperability.


Repeater APs

You might find yourself in situations where it is not easy or convenient to connect an AP to the wired infrastructure or where an obstruction makes it difficult for an AP on your wired network to directly associate clients in an area of your deployment. In such a scenario, you can employ a repeater AP. Figure 2-18 shows this scenario, where Elaine is not directly visible to AP2 but she can see AP3, which is not connected to the wired network but can see AP2.

Much like a wired repeater, what a wireless repeater does is merely retransmit all the packets that it receives on its wireless interface. This retransmission happens on the same channel upon which the packet was received. The repeater AP has the effect of extending the BSS and also the collision domain. Although it can be an effective tool, you must take care when employing it; the overlapping of the broadcast domains can effectively cut your throughput in half because the originating AP also hears the retransmit.


Wireless Bridges

If you extend the concept of a workgroup bridge even further to the point where you are connecting two or more wired networks, you arrive at the concept of wireless bridges. Similar to wired bridges, wireless bridges connect networks. You might bridge wirelessly because you need to connect networks that are inherently mobile. Alternatively, the networks to be connected might not be co-located, in which case wireless bridging provides a method for connecting these networks. The main distinction between bridges and workgroup bridges is that the latter are only wirelessly enabling a small network in an office environment, whereas the former can connect larger networks often separated by distances much greater than what is found in the WLAN environment. In fact, many vendors offer products that provide ranges which far exceed the definitions and limitations of 802.11. Figure 2-20 shows a wireless bridging example.

As shown in the figure, one of the bridges assumes the role of the AP in a WLAN network, and the other bridges act as clients. Although the basic 802.11 MAC and PHY sublayer technologies are utilized in wireless bridging, individual vendors have their own proprietary methods for the encapsulation of wired network traffic and for extending the range from a MAC and PHY sublayer perspective. For this reason, once again you should ensure that your wireless bridges are certified to interoperate.