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What are beacon frames?
In computer networks, beacon frames management frames used in IEEE 802.11 based wireless local area networks (WLANs). A beacon frame contains network information needed by a station before it can transmit a frame. They are used for announcing the presence of devices in a WLAN as well as synchronisation of the devices and services.
Beacon frames are used as a part of BSS (basic service set). In infrastructure BSS, there is are access points (APs) that are wireless routers forming the base stations for access. All the devices communicate with each other through APs. The APs transmit beacon frames periodically so that the devices know the status of the network channel. In independent BSS, devices communicate in peer-to-peer basis and so beacon generation is distributed among the devices.
Frame Format of Beacon Frames
A beacon frame comprises of −
- IEEE 802.11 MAC header
- Body
- Frame check sequence (FCS)
The important fields in the body of a beacon frame are −
- Timestamp − It is the time associated with the beacon frame. It helps in synchronization of the devices in the WLAN. The devices updates their local clocks according to the timestamp of the beacon frame.
- Beacon interval − The time gap between two consecutive transmissions of beacon frame.
- Capability information − It contains information about the capability of the network. Sometimes referred as the type field, it determines whether the network is infrastructure based or ad hoc.
- SSID − Service Set Identifiers that serve as network names.
- Supported rates − It defines the transmission rates permitted by the channel.
- Frequency-hopping (FH) Parameter Set
- Direct-Sequence (DS) Parameter Set
- Contention-Free (CF) Parameter Set
- Traffic indication map (TIM) − It is a bitmap used in IEEE 802.11. The AP sends TIM periodically since a station needs to listen to at least one beacon during the beacon interval.
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