Traffic Indication Map (TIM) is an Information element and is part of the Beacon frame that is sent out by the Access Point at regular intervals.
The TIM Information element provides information on the current DTIM count and DTIM period and also provides information on whether Buffered Multicast/Broadcast Data or unicast data for different 802.11 Stations are buffered at the Access Point.
The TIM information element is provided below for reference
FIG COURTESY – 802.11-2012 Standard
The Element ID is 5.
The Length parameter indicates the length of the Information element starting from the DTIM Count to the end of the Partial Virtual Bitmap Octets
DTIM Count – is a down counter indicating the count till a DTIM Beacon is to be transmitted. When the DTIM count is zero, the beacon is a DTIM beacon and the Access Point will transmit buffered multicast/broadcast packets immediately after the DTIM beacon
DTIM Period – The DTIM Period indicates the number of successive beacons before a DTIM beacon transmission
Bitmap Control – Bit 0 of the Bitmap control indicates whether there is buffered Multicast/Broadcast Data to be transmitted at the Access Point. Bits 1-7 provide the Bitmap offset for the Partial Virtual Bitmap.
Partial Virtual Bitmap – this is a bitmap of 1-251 octets (2008 bits) where-in each bit corresponds to an Association ID of a connected station.
It is termed as Partial virtual bitmap because only a section of the bitmap (the AIDs that have buffered unicast data at the AP) is transmitted in a beacon.
NOTE: The Access Point provides a Unique Identifier to a WLAN station on a successful association – that is termed as Association Identifier (AID) for the associated WLAN Station
The TIM information element wireless capture is shown below
The Math involving the generation of the Partial Virtual Bitmap to be transmitted and the bitmap offset is described below
The standard states this
“the Partial Virtual Bitmap field consists of octets numbered N1 to N2 of the traffic indication virtual bitmap, where N1 is the largest even number such that bits numbered 1 to (N1 * 8) – 1 in the bitmap are all 0 and N2 is the smallest number such that bits numbered (N2 + 1) * 8 to 2007 in the bitmap are all 0. In this case, the Bitmap Offset subfield value contains the number N1/2, and the Length field is set to (N2 – N1) + 4.”
From the above snippet – it is known that the
Bitmap offset field = N1/2 (value of bits 1-7 of bitmap control field)
N1 is always an even number. It is the largest even number * 8 below which all Association ID (AID) bits are zero
N2 is selected such that all bits (Association IDs) from (N2 +1) * 8 are zero