Before we discuss how a WLAN station associates to a network, it is important to understand how a beacon is transmitted and the important parameters that are a part of a beacon frame. This article provides an overview of the beacon mechanism in the 802.11 WLAN standard.
802.11 Beacons are broadcast transmissions. In WLAN Beacons are a means of broadcasting capabilities and features that a particular Basic Service Set (BSS) supports. It is usually sent by the master of the network – it could be an Access Point sending a beacon in an Infrastructure BSS or an Ad-hoc/P2P master (Group Owner device) Station sending a beacon.
In addition to providing information on features and capabilities, beacons also send out information on current packet traffic queued for different stations connected to the BSS (via TIM – Traffic Indication Map). It can also be used to notify Stations about different capabilities (e.g. VHT/HT capabilities etc), events (e.g. channel switch) and to time synchronize the network via a TSF time stamp – i.e. all stations adjust their network time stamp to the TSF time that is broadcast in the beacon and adjust their internal clocks to be on the same network clock as of the BSS.
The Beacon also via different Information elements provides details of the features supported by the Access Point.
The below beacon capture indicates the above points
FIG : A Beacon capture example
A beacon transmission is periodic and is normally set in multiples of Transmission Units (TU). Each Transmission unit is equivalent to 1024 micro-seconds. 100 TUs (~100 milliseconds) is normally the default beacon transmission time that is set in most routers. Most stations refer to the time at which a beacon is expected to be broadcast as Target Beacon Transmission Time (TBTT). Time interval between two TBTTs is referred to as beacon interval
In addition to default beacon intervals – another important parameter that is used in normal Beacon transmission terminology is DTIM (Delivery Traffic Indication Map) interval. This interval is a multiple of the basic beacon transmission interval.
For e.g. – if the basic beacon transmission interval is 100 milliseconds and the DTIM interval is set as 3 – then every third beacon that is sent out is a DTIM beacon. Each beacon will have a DTIM counter ( a down counter) present in each beacon transmission. Hence, if the DTIM is set to 3, then every third beacon with a DTIM counter value of 0 is a DTIM beacon. The DTIM counter in each beacon transmission counts down as 2, 1, 0, 2, 1, 0 and so on.
DTIM transmissions are specific beacon transmissions by the Master of the BSS to indicate to all connected Stations that immediately after the transmission of the beacon – all multicast/broadcast data queued at the BSS Master Station (e.g. Access point/P2P GO) will be sent to all stations connected to the network in a burst transmission.
The below figures indicates TIM/DTIM operation
Fig Courtesy: 802.11 Standard
Fig Courtesy: TIM Information Element showing DTIM count as 2 and DTIM period as 3
Most Connected stations utilize the DTIM interval as a sleep interval and awake at Target DTIM transmission time to receive all the Multicast/Broadcast data queued at the AP/ P2P GO device. The TIM map in the DTIM beacon also indicates which station currently has buffered unicast data traffic buffered at the Access Point or the BSS Master station.
Access Point Contending for Beacon Transmission