An Access Point (AP) is a vital component for a WLAN. It is needed so that wireless devices can communicate with the network. An AP can also communicate with another AP using the wired network. This saves the wireless bandwidth. A single AP can be referred to as a Basic Service Set (BSS). If more than one AP is connected to a wired segment, the topology can be called an Extended Service Set (ESS). The wired network is called the distribution system.

Access points operate in one of the following modes:

1. Root Mode – This is the default mode of operation. In root mode, the AP is connected to the wired foundation of a network (usually the Ethernet) as shown below. An AP typically operates in root mode. If it also supports one or both of the other modes, the default will likely be root mode.

A Wireless Access Point (AP) in Root Mode

2. Bridge Mode – An AP operating in bridge mode essentially becomes a wireless bridge. Bridges are used to link together two or more wired segments. They may be located in different parts of the same building or in separate buildings. The graphic below shows a network using the AP as a bridge and not actual wireless bridges.

A Wireless Access Point (AP) in Bridge Mode

3. Repeater Mode – In repeater mode, an AP provides a wireless upshot link into the wired network. The AP repeater connects to clients as an access point. It connects to a root mode AP, as a client. This is shown below. There must be at least a 50% overlap of the cells involved. The AP repeater exchanges with both clients and with the upstream AP. Throughput on this wireless section is reduced. For these reasons, this configuration is recommended only if absolutely necessary.

A Wireless Access Point (AP) in Repeater Mode

Common Access Point Options

An AP is a portal, from architecture specified in the IEEE 802.11 for LANs to the 802.3 specifications for Ethernet or other network type. Available hardware and software options vary by manufacturer and model. Following are the options most frequently available on an AP:

• Antennas – A detachable antenna provides flexibility to select the best antenna for the AP location and usage. For example, cable can be used to connect an AP mounted indoors to an antenna mounted outdoors. Some APs are shipped with diversity antennas. This means that input from multiple antennas is sampled and the input signal with the best reception is used. Illustrated below are some of the available detachable antenna types, as well as an AP with diversity antennas. Antennas will be discussed in more detail in a later chapter.

Antennas

• Filtering – MAC address or protocol filtering can provide a basic level of security by screening out devices that are not in a defined MAC or protocol filter list. An example of a protocol filter would be an HTTP filter, which would only allow web browsing from the wireless LAN.
• Modular Radio Cards – A single PCMCIA slot allows upgrading by swapping the radio card. With multiple slots, an access point could perform both the AP function and the bridging function. It could also serve as multiple, co-located APs. In both cases, non-overlapping channels would be used.
• Variable Output Power – This option allows the administrator to control the power. Increasing the power allows clients to move further away and still remain connected. Lowering the power can help security, by limiting the size of the cells. Without variable output power, amplifiers, attenuators, long cables, or high-gain antennas might need to be implemented. When adjusting power levels in any of these ways, it is important to always be aware of FCC guidelines.
• Wired Connection Types – Almost all APs support Ethernet/802.3 connections, most frequently 10/100 802.3. Some APs will support other choices, such as 802.3 over fiber or Token Ring.