Traffic Indication Map (TIM) Element

Traffic Indication Map is present only in beacon. The TIM element contains information useful for clients in low-power mode.

Besides the classical element ID and length, the TIM contains two types of elements: The virtual bitmap and DTIM information. 

The DTIM is not present in all beacons and all TIMs. The beacon will contain a TIM that will also be a DTIM (Delivery Traffic Indication Message). AP uses the beacon frames DTIM information to inform the cell if it has broadcasts or multicasts frames buffered. Stations in low power mode should wake up at least for every beacon that is a DTIM. The DTIM does not have to be in every beacon but can, for example, occur every two to five beacons.

When a DTIM shows that there is broadcast or multicast traffic buffered at the AP level, all stations stay awake. Just after sending the beacon announcing that buffered broadcast or multicasts that AP contends for the medium and then forwards the buffered broadcast or multicast to the cell. All stations recevied it and then can go back to sleep if needed. 

The first bit of the bitmap control field is used to announce the presence of multicast or broadcast traffic buffered on the AP. 

When Bitmap Control field is set to 1, the AP has multicast or broadcast buffered. When this first bit is set 0, there is no buffered broadcast or multicast.