Access Points

Second only to the basic wireless PC card, the access point, or “AP”, is probably the most common wireless LAN device with which you will work as a wireless LAN administrator. As its name suggests, the access point provides clients with a point of access into a network. An access point is a half-duplex device with intelligence equivalent to that of a sophisticated Ethernet switch.

Access Point Modes
Access points communicate with their wireless clients, with the wired network, and with other access points. There are three modes in which an access point can be configured:

Root Mode
Repeater Mode
Bridge Mode

Root Mode
Root Mode is used when the access point is connected to a wired backbone through its wired (usually Ethernet) interface. Most access points that support modes other than root mode come configured in root mode by default. When an access point is connected to the wired segment through its Ethernet port, it will normally be configured for root mode. When in root mode, access points that are connected to the same wired distribution system can talk to each other over the wired segment. Access points talk to each other to coordinate roaming functionality such as re-association. Wireless clients can communicate with other wireless clients that are located in different cells through their respective access points across the wired segment, as shown in above.

Bridge Mode
In bridge mode, access points act exactly like wireless bridges, which will be discussed later in this chapter. In fact, they become wireless bridges while configured in this manner. Only a small number of access points on the market have bridge functionality, which typically adds significant cost to the equipment. We will explain shortly how wireless bridges function, but you can see from above figure that clients do not associate to bridges, but rather, bridges are used to link two or more wired segments together wirelessly.

Repeater Mode
In repeater mode, access points have the ability to provide a wireless upstream link into the wired network rather than the normal wired link. As you can see in the following figure, one access point serves as the root access point and the other serves as a wireless repeater. The access point in repeater mode connects to clients as an access point and connects to the upstream root access point as a client itself. Using an access point in repeater mode is not suggested unless absolutely necessary because cells around each access point in this scenario must overlap by a minimum of 50%. This configuration drastically reduces the range at which clients can connect to the repeater access point. Additionally, the repeater access point is communicating with the clients as well as the upstream access point over the wireless link, reducing throughput on the wireless segment. Users attached to the repeater access point will likely experience low throughput and high latencies in this scenario. It is typical for the wired Ethernet port to be disabled while in repeater mode.