Beacon frame

Beacon frame is one of the management frames in IEEE 802.11 based WLANs. It contains all the information about the network. Beacon frames are transmitted periodically to announce the presence of a Wireless LAN network. Beacon frames are transmitted by the Access Point (AP) in an infrastructure BSS. In IBSS network beacon generation is distributed among the stations.

Components of a Beacon frame

Beacon frame consist of MAC header, Frame body and FCS. Some of the fields are listed below.

• Timestamp

After receiving the beacon frame all the stations change their local clocks to this time. This helps with synchronization.

• Beacon interval

This is the time interval between beacon transmissions. The time at which a node (AP or station when in Ad-hoc) must send a beacon is known as Target Beacon Transmission Time (TBTT). Beacon interval expressed in Time Unit (TU). It is a configurable parameter in the AP and typically configured as 100 TU ^[1].

• Capability information

Capability information field spans to 16 bits and contain information about capability of the device/network. Type of network such as AdHoc or Infrastructure network is signaled in this field. Apart from this information, it announce the support for polling, encryption details also.

• SSID
• Supported rates
• Frequency-hopping (FH) Parameter Set
• Direct-Sequence (DS) Parameter Set
• Contention-Free (CF) Parameter Set
• IBSS Parameter Set
• Traffic indication map (TIM)

References