802.11 Beacon Intervals – The Real Story

802.11 Beacon Intervals – The Real Story

By Tom Carpenter On 05/13/2014 - 16 Comments

A realistic look at 802.11 beacon intervals and their purposes. Great knowledge for those studying for CWAP!

You often hear that Beacon frames are sent, by default, every 100 milliseconds (ms); however, the reality is a bit more complex.

Beacons are sent by the AP at a regular interval defined as the Target Beacon Transmission Time (TBTT). The TBTT is a time interval measured in time units (TUs). A TU is equal to 1024 microseconds. The TU is often confused with 1 ms. The reality is seen in the definition of a time unit in the 802.11-2012 standard document, which reads, "A measurement of time equal to 1024 µs."

For example, if you set the Beacon interval to 100 (TUs), you are effectively setting it to 102,400 microseconds, or 102.4 ms.

The TBTT is called a target transmission time because it does not always occur at exactly 102.4 ms (if you set it to 100 TUs, the default). As you can see from the graphic in Figure 1, if the wireless medium is available at the TBTT, the Beacon will be transmitted then. However, if the medium is busy at the TBTT, the AP will contend for access to the medium like usual. However, if the medium is busy a second TBTT in a row, the Beacon is given high priority after the current transmission.

Figure 1: Beacon Frame Intervals

The Beacon serves several purposes, including:

• Passive discovery by seeking clients
• BSS time synchronization
• Communicate BSS features
• Update BSS features
• Power save management

This knowledge is valuable for network analysis and as knowledge for those preparing for the CWAP exam.

Editor's Note: This post was originally published in May 2014 and has been updated for accuracy and comprehensiveness.

Tagged with: Beacon, Wifi, Wireless, Network Analysis, BSS, CWAP, TBTT, Frames,

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