6.4 Management Operations

In a wired Ethernet, it is usually sufficient to connect the client device via cable to the nearest hub or switch to get access to the network. Physically connecting a wireless device to a WLAN network is of course not possible, as there is no cable. Also, a WLAN device has the ability to automatically roam between different access points of an ESS and is able to encrypt data packets on layer 2 of the protocol stack. As all of these WLAN operations have to be coordinated between the access points and the user devices, the 802.11 standard specifies a number of management operations and messages on layer 2, as well as additional Information Elements (IEs) in the Medium Access Control (MAC) header of data packets, which are not found in a wired Ethernet.

The access point has a central role in a BSS and is usually also used as a bridge to the wired Ethernet. Therefore, wireless clients always forward their packets to the access point, which then forwards them to the wireless or wired destination devices. To allow wireless clients to detect the presence of an access point, beacon frames are broadcast by the access point periodically. A typical value of the beacon frame interval is 100 milliseconds. As can be seen in Figure 6.7, beacon frames not only comprise the SSID of the access point but also inform the client devices about a number of other functionalities and options in a number of IEs. One of these IEs is the capability IE. Each bit of this 2-B IE informs ...

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