May 23, 2024

Beacon Frame

1
Beacon
  1. Beacon frame is transmitted periodically to all the stations by broadcasting the information about the network.​
  2. It is transmitted by AP.​
  3. Default Beacon Interval 100 time units​
  4. A time unit is 1.024ms ​
  5. For every 102.4ms a beacon is Broadcasted by AP.​

Beacon Frame consists of​

Frame controlDuration IDDASABSSIDSequence controlFrame Body
1.Time stamp
2.Beacon Interval
3. Capabilities information
4.SSID
5.Supported Rates
6.FH parameter set
7.DS parameter set
8.CF parameter set
9.IBSS parameter set
10.TIM
11.Country Info
12.ERP Information
13.Extended Supported Rates
FCS
BEACON FRAME

Time stamp : It is the time associated within the beacon frame of sent and Received signal​.

Beacon Interval : Time period of every beacon sent (Default value = 102.4 ms)​.

Capabilities information : Consists of information about AP​ like

  • ESS Capabilities
  • IBSS Status
  • CFP Participation Capabilities
  • Privacy
  • Short Preamble
  • PBCC
  • Channel Agility
  • Spectrum Management
  • Short Slot Time
  • Automatic Power Save Delivery
  • Radio Measurement
  • DSSS-OFDM
  • Delayed Block Ack
  • Immediate Block Ack

SSID : Name of WiFi Network​

Supported Rates : It indicates whether data rate is Basic Rates or mandatory rates or supported rates​.

Frequency HOP parameter set : Used by legacy Frequency Hopping stations. This Information element is present when it uses FH physical layer .

Direct Sequence parameter set : It is generated by stations using Clause 15, 18 or 19 PHY and present when DS physical layer is used. ​

Contention free parameter set : Used with PCF, unused in real networks. CF parameter set is embedded in the Beacon if the AP which supports PCF will have this information element when beacon frame is transmitted by them​.

IBSS parameter set : Present only within beacon frames generated by stations .​

Traffic Indication Map : TIM element contains information useful for stations in low-power mode.

The AP uses Delivery Traffic Indication Map (DTIM) to inform the client if it as any broadcast or multicast frames are buffered which is needed to be send to the client.

ERP Information : It is present only on 2.4GHz network supporting 802.11g​

Extended Supported Rates : It specifies the supported rates not carried in the Supported Rates Element.Extended Supported Rates are used if there are more than 8 supported rates.​

Wire Shark Analysis

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