Announcement Traffic Indication Message

Announcement Traffic Indication Message (ATIM, sometimes called Ad-hoc Traffic Indication Message) is a message type used to notify peers of pending data transfers in an ad-hoc wireless 802.11 network.

Rationale

While power management is fairly simple in infrastructure networks, as all the traffic goes through the access point which can coordinate sleep times of stations (therefore conserving power by only waking them up when there is data to be transferred), IBSS networks lack such a centralized traffic information repository, thus necessitating peers to negotiate data transfers individually.

Operation

ATIM is a type of message which peers use to notify other peers AbOUT pending traffic. ATIM, along with some other types of control messages, is sent by the peer wanting to transfer data in the so-called ATIM window at the beginning of each beacon interval. During the ATIM window all stations need to be listening; in case of unicast, Recipient shall reply with an acknowledgement of the ATIM, indicating that it will stay on during the rest of the beacon interval and is thus ready to receive. With multicast or broadcast no acknowledgements are issued and all stations should stay awake for the rest of the beacon period unconditionally.

When ATIM window ends, stations which must not send or receive data are allowed to power off until the next beacon interval.