Wi-Fi System Operation
All 802.11 technologies operate in the same manner. The operational functions, such as connecting, scanning, and roaming between APs, are controlled by the MAC layer, which is specified in the 802.11 standard. This section covers how the main MAC operations of wireless networks work.
Beacons
Access points (APs) and wireless routers broadcast beacons via the airwaves, as shown in Figure A-1. The beacons provide a means of identifying APs and wireless routers to wireless clients. These beacons also contain pertinent information about the wireless network to inform nearby wireless clients of the network and to synchronize the wireless clients that are already part of the network.
Beacons are sent periodically, starting as soon as you plug in the AP or wireless router, at an interval of 1 to 65,535 milliseconds. The default interval that most vendors use is 100 milliseconds, which equates to 10 beacons being sent every second. For hotspot installations, leave the beacon interval set to 100 milliseconds.
Note
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Most APs and wireless routers allow you to disable service set identifier (SSID) broadcasting, which removes the SSID from beacons. This helps hide the network from others. Obviously, you would not want to disable SSID broadcasting on your hotspot network; however, this feature might be useful if you have a separate private network. |
The beacons contain information, such as the SSID (name) of the network, supported data rates, timestamp, and more about the wireless network. The wireless client uses this information to determine which AP or wireless router to connect to.
Wireless Client Scanning
Wireless clients (or radio cards) are constantly listening for activity in their specific frequency band. Wireless clients have two main methods to scan for wireless networks (APs and wireless routers):
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Active scanning
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Passive scanning
Active scanning is used when a wireless client wants to search for a specific wireless network. The wireless client is depicted in Figure A-2. It sends a packet, called a probe request, on each channel, asking if a network, or an AP/wireless router, is nearby. The wireless client can ask either for a specific network or for any wireless network.
If the wireless client is probing for a specific network, only networks that match the SSID in the request respond with a probe response. However, if the wireless client is probing for any wireless networks nearby, all the APs or wireless routers that "hear" the request respond with a probe response.
Passive scanning, by contrast, takes a hands-off approach. In this case, the wireless client just listens for beacons on each channel. Most wireless clients use a combination of active and passive scanning using proprietary mechanisms. Even when a wireless client is connected to a wireless network, it still periodically scans other channels for nearby wireless networks. This is necessary if the user is roaming through a facility to determine whether to connect to a different AP or wireless router. With both scanning methods, the wireless client stores the information from the wireless networks it discovers. Client software, such as Windows XP, displays a list of applicable wireless networks to the user.
Connecting
Wireless clients have a special sequence they go through each time they want to initiate a connection with a wireless network. The sequence is started when the wireless client is given instructions to initiate a connection with a particular wireless network, from software such as Windows XP or the vendor configuration utility within the computer. For example, the person who is using the computer can view a list of available networks and click a button to connect to a particular network. This initiates the connection process with the applicable network.
Sending and Receiving
In infrastructure wireless networks, such as hotspots, the APs or wireless routers are the coordinators that regulate the traffic from the wireless clients. A common misconception is that the wireless clients send and receive traffic directly to and from each other. However, as shown in Figure A-3, all the traffic goes through the AP, even if a wireless client is accessing or transferring shared files from another wireless client on the network. Figure A-4 depicts the traffic flow when a wireless client is accessing the Internet.
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