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Delivery traffic indication message
A delivery traffic indication map is a kind of traffic indication map (TIM) which informs the clients about the presence of buffered multicast/broadcast data on the access point. It is generated within the periodic beacon at a frequency specified by the DTIM Interval. Beacons are packets sent by an access point to synchronize a wireless network. Normal TIMs that are present in every beacon are for signaling the presence of buffered unicast data. After a DTIM, the access point will send the multicast/broadcast data on the channel following the normal channel access rules (CSMA/CA). This helps to have minimum collision and in effect, increased throughput. In cases where there is not much interference, or where the number of clients is limited, the DTIM interval has little or no significance. Usually a value of 1 or 2. Also see Wi-Fi as established by the Wi-Fi Alliance.
==802.11 standard==
According to the 802.11 standards, a Delivery Traffic Indication Map (DTIM) period value is a number that determines how often a beacon frame includes a Delivery Traffic Indication Message, and this number is included in each beacon frame. A DTIM is included in beacon frames, according to the DTIM period, to indicate to the client devices whether the access point has buffered broadcast and/or multicast data waiting for them. Following a beacon frame that includes a DTIM, the access point will release the buffered broadcast and/or multicast data, if any exists.
Since beacon frames are sent using the mandatory 802.11 carrier sense multiple access/collision avoidance (CSMA/CA) algorithm, the access point must wait if a client device is sending a frame when the beacon is to be sent. As a result, the actual time between beacons may be longer than the beacon interval. Client devices that awaken from power-save mode may find that they have to wait longer than expected to receive the next beacon frame. Client devices, however, compensate for this inaccuracy by utilizing the time-stamp found within the beacon frame.
The 802.11 standards define a power-save mode for client devices. In power-save mode, a client device may choose to sleep for one or more beacon intervals waking for beacon frames that include DTIMs. When the DTIM period is 2, a client device in power-save mode will awaken to receive every other beacon frame. Upon entering power-save mode, a client device will transmit a notification to the access point, so that the access point will know how to handle unicast traffic destined for the client device. The client device will begin to sleep according to the DTIM period.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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