Beacon frame is one of the management frames in IEEE 802.11

IEEE 802.11

IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...

 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

Service set (802.11 network)

A service set is all the devices associated with a local or enterprise IEEE 802.11 wireless local area network .-Service set identifier :...

. 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)

TU (Time Unit)

In the IEEE 802.11 standard, a unit of time equal to 1024 microseconds. A whole number of such units are used to describe several intervals in that standard. The use of the term is to avoid using the term "millisecond", which is slightly different...

. It is a configurable parameter in the AP and typically configured as 100 TU .

• 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
  Traffic indication map
  The Traffic indication map information element is covered under section 7.3.2.6 of 802.11-1999.pdf. The IEEE 802.11 standards chose to use a bitmap to indicate to any sleeping listening stations if the Access Point has any buffered frames present for it...
  
  (TIM)