The significance of beacon frames and how to configure the beacon interval on Access Points

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Apr 30, 2012 11:55 PM
Jun 22nd, 2009


Introduction

The significance of beacon frames and how to configure the beacon interval on Access Points

Resolution

According to the IEEE 802.11 standard, every compliant Access Point (AP) periodically sends out management frames called beacon frames. The purpose of beacon frames is to advertise the presence of an AP in an area, its capabilities, and some configuration and security information to the client devices. The time interval between two consecutive beacon frames is called the beacon interval. The beacon interval is measured in Time Units (TUs), where each TU equals 1024 microseconds, so the default period between beacons is approximately 100 milliseconds. Beacon interval is a configurable parameter on Cisco APs, but changing this value is not recommended without careful consideration.

A misconfigured beacon can cause a client to not associate with the AP.


The beacon period is the amount of time between access point beacons in Kilomicroseconds. One Kµsec equals 1,024 microseconds. The Data Beacon Rate, always a multiple of the beacon period, determines how often the beacon contains a delivery traffic indication message (DTIM). The DTIM tells power-save client devices that a packet is waiting for them.

For example, if the beacon period is set at 100, its default setting, and the data beacon rate is set at 2, its default setting, then the access point sends a beacon containing a DTIM every 200 Kµsecs. One Kµsec equals 1,024 microseconds.

The default beacon period is 100, and the default DTIM is 2. Beginning in privileged EXEC mode, follow these steps to configure the beacon period and the DTIM:



Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

interface dot11radio { 0 | 1 }

Enter interface configuration mode for the radio interface. The 2.4-GHz radio is radio 0, and the 5-GHz radio is radio 1.

Step 3

beacon period value

Set the beacon period. Enter a value in Kilomicroseconds.

Step 4

beacon dtim-period value

Set the DTIM. Enter a value in Kilomicroseconds.

Step 5

end

Return to privileged EXEC mode.

Step 6

copy running-config startup-config

(Optional) Save your entries in the configuration file.

All WiFi networks are identified by a network name also known as Service Set Identifier or SSID. SSID is at most 32 characters string advertised in beacon frames which are periodically transmitted by APs. All clients in the proximity of an AP listen to these periodic advertisements and know the presence of a WiFi network. This is known as passive network scanning or discovery. Sometimes, wireless client devices send request frames to know the presence of a WiFi network. These frames are called “Probe Requests”. APs in the radio range of client listen to these request frames and respond thorugh “Probe Response” frames. These frames are very similar to Beacon frames and also contain the wireless network name. The process of discovering wireless network by sending probes is called active scanning or active discovery.

Probe Requests may or may not contain the wireless network name. When no network name is present in “Probe Request” frame, it is known as Null Probe. These types of frames are used by client to discover any WiFi network present in the proximity of the client. Sometimes “Probe Request” does contain the name of the WiFi network. These types of frames are sent by a wireless client device when it looks for the presence of pre-configured wireless networks.

1.jpg

Problem Type

Configure / Configuration issues

Device cannot associate

Products

Access point

Topology

Wireless client to AP

SW Features

Beacon interval

Reference

Configuring the Beacon Period section of Configuring Radio Settings.

Overall Rating: 5 (1 ratings)
rlcirillo Sun, 05/23/2010 - 13:40
"so the default period between beacons is approximately 100 
microseconds."

I believe you meant to say "approximately 100 milliseconds".   Default beacon period value on Cisco APs is 100 (TU, or 1024 usec), which translates into 0.1024 seconds - or 102.4 milliseconds.

May want to consider editing and up-versioning this article to prevent propagation of this typo.

Rajesh Premachandran Wed, 06/09/2010 - 00:00

Thanks. I have corrrected the typo.

Vinay Sharma Fri, 02/11/2011 - 08:32

thanks Rajesh for this useful information.

Regards,

Vinay

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Posted June 22, 2009 at 5:36 PM
Updated April 30, 2012 at 11:55 PM
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