Page 415 - From GMS to LTE

Wireless Local Area Network (WLAN) 401

Figure 6.6 Client device configuration for a BSS or ESS.

to encrypt data packets on layer 2 of the protocol stack. As all of these WLAN opera-
tions have to be coordinated between the APs and the user devices, the 802.11 standard
specifies a number of management operations and messages on layer 2, as well as addi-
tional Information Elements (IEs) in the Medium Access Control (MAC) header of data
packets, which are not found in a wired Ethernet.
The AP 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 AP, which then
forwards them to the wireless or wired destination devices. To allow wireless clients to
detect the presence of an AP, beacon frames are broadcast by the AP periodically.
A typical value for the beacon frame interval is 100 milliseconds. As can be seen in
Figure 6.7, beacon frames not only comprise the SSID of the AP but also inform 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‐byte IE informs a client device about the
availability of a certain feature. As can be seen in Figure 6.7, the capability IE informs
the client device in the fifth bit, for example, that ciphering is not activated (privacy
disabled). Other IEs in the beacon frame are used for parameters that require more than
a single bit. Each type of IE has its own ID that indicates to the client devices how to
decode the data part of the IE. IE 0, for example, is used to carry the SSID, while IE 1 is
used to carry information about the supported datarates. As IEs have different lengths,
a length field is included in every IE header. Since there is an identifier and a length field
at the beginning of each IE, a client device is able to skip over optional IEs that it does
not recognize. Such IEs might be present in beacon frames of new APs that offer func-
tionality that older client devices might not have implemented. This ensures backward
compatibility with older devices.
During a network search, a client device has two ways to find available APs. One way
is to passively scan all possible frequencies and just wait for the reception of a beacon