Traffic indication map (TIM) is a structure used in 802.11 wireless network management frames.
The Traffic indication map information element is covered under section 7.3.2.6 of 802.11-1999 standard. The IEEE 802.11 standards chose to use a bitmap to indicate to any sleeping listening stations if the Access Point (AP) has any buffered frames present for it. Because stations should listen to at least one beacon before the listen interval, the AP periodically sends this bitmap on its beacons as an information element. The bit mask is called the Traffic Indication Map and consists of 2008 bits, each bit representing the Association Id (AID) of a station. For example, the TIM information element allows you to transfer 1 byte up to the entire 251 bytes (2008 bits) of the TIM, you are allowed to transmit a smaller TIM bitmap as it is expected that only a few number of stations will be asleep. Because of this the bitmap values passed in the TIM information element is called a partial virtual bitmap. To allow you to transmit only a partial bitmap you must make use of the bitmap control and length fields of the TIM information element.
The structure of the TIM is following:
Note: all fields are 1-byte long except the Partial Virtual Bitmap which can be 1–251 bytes long.