TIM, DTIM, ATIM
. . . .
TIM, DTIM, ATIM
Traffic Indication Map
Delivery Traffic Indication Message
Announcement Traffic Indication Message
Traffic Indication Map
- If a station is part of a basic service set, it will notify the AP that it is
enabling Power Save mode by changing the Power Management field to 1. - When the AP receives a frame from a station with this bit set to 1, the AP knows that the station is in Power Save mode.
- If the AP then receives any data that is destined for the station in Power Save mode, the AP will store the information in a buffer.
- Any time a station associates to an AP, the station receives an association identifier (AID).
- The AP uses this AID to keep track of the stations that are associated and the members of the BSS.
- If the AP is buffering data for a station in Power Save mode, when the AP transmits its next beacon, the AID of the station will be seen in a field of the beacon frame known as the traffic indication map (TIM).
- The TIM field is a list of all stations that have undelivered data buffered on the AP, waiting to be delivered.
- Every beacon will include the AID of the station until the data is delivered.
- After the station notifies the AP that it is in Power Save mode, the station shuts down part of its transceiver to conserve energy.
- A station can be in one of two states, either awake or doze:
- During the awake state, the client station can receive frames and transmit frames.
- During the doze state, the client station cannot receive or transmit any frames and operates in a very low power state to conserve power.
- Because beacons are transmitted at a consistent predetermined interval known as the target beacon transmission time (TBTT), all stations know when beacons will occur.
- The station will remain asleep for a short period of time and awaken in time to hear a beacon frame.
- The station does not have to awaken for every beacon.
- To conserve more power, the station can sleep for a longer period of time and then awaken in time to hear an upcoming beacon.
- How often the client station awakens is based on a client variable called the listen interval and is usually vendor specific.
- When the station receives the beacon, it checks to see whether its AID is set in the TIM, indicating that a buffered unicast frame waits.
- If so, the station will remain awake and will send a PS-Poll frame to the AP.
- When the AP receives the PS-Poll frame, it will send the buffered unicast frame to the station.
- The station will stay awake while the AP transmits the buffered unicast frame.
- When the AP sends the data to the station, the station needs to know when all of the buffered unicast data has been received so that it can go back to sleep.
- Each unicast frame contains a 1-bit field called the More Data field.
- When the station receives a buffered unicast frame with the More Data field set to 1, the station knows that it cannot go back to sleep yet because there is some more buffered data that it has not yet received.
- When the More Data field is set to 1, the station knows that it needs to send another PS-Poll frame and wait to receive the next buffered unicast frame.
- After all of the buffered unicast frames have been sent, the More Data field in the last buffered frame will be set to 0, indicating that there is currently no more buffered data, and the station will go back to sleep.
- The AP will set the value of the station’s AID bit to 0, and when the next TBTT arrives, the AP will send a beacon.
- The station will remain asleep for a short period of time and again awaken in time to hear a beacon frame.
- When the station receives the beacon, it will again check to see whether its AID is set in the TIM. Assuming that there are no buffered unicast frames awaiting this station, the station’s AID will not be set to 1 in the TIM and the station can simply go back to sleep until it is time to wake up and check again.
Delivery Traffic Indication Message
- In addition to unicast traffic, network traffic includes multicast and
broadcast traffic. - Because multicast and broadcast traffic is directed to all stations, the BSS needs to provide a way to make sure that all stations are awake to receive these frames.
- A delivery traffic indication map (DTIM) is used to ensure that all stations using
power management are awake when multicast or broadcast traffic is
sent. - DTIM is a special type of TIM. A TIM or DTIM is transmitted as part of every beacon.
- A configurable setting on the AP called the DTIM interval determines how often a DTIM beacon is transmitted.
- A DTIM interval of 3 means that every third beacon is a DTIM beacon, whereas a DTIM interval of 1 means that every beacon is a DTIM beacon.
- Every beacon contains DTIM information that informs the stations when the next DTIM will occur.
- A DTIM value of 0 indicates that the current TIM is a DTIM.
- All stations will wake up in time to receive the beacon with the DTIM.
- If the AP has multicast or broadcast traffic to be sent, it will transmit the beacon with the DTIM and then immediately send the multicast or broadcast data.
- After the multicast or broadcast data is transmitted, if a station’s AID
was in the DTIM, the station will remain awake and will send a PSPoll
frame and proceed with retrieving its buffered unicast traffic
from the AP. - If a station did not see its AID in the DTIM, or if its AID was set to 0, the station can go back to sleep.
- The DTIM interval is important for any application that uses multicasting. For example, many VoWiFi vendors support push-totalk capabilities that send VoIP traffic to a multicast address.
- A misconfigured DTIM interval would cause performance issues during a push-to-talk multicast.
Announcement Traffic Indication Message
- If a station is part of an IBSS, there is no central AP to buffer data
while the stations are in Power Save mode. - A station will notify the other stations that it is enabling Power Save mode by changing the Power Management field to 1.
- When the station transmits a frame with this field set to 1, the other stations know to buffer any data that they may have for this station because this station is now in Power Save mode.
- Periodically, all stations must wake up and notify each other if any station has buffered data that needs to be delivered to another station.
- This recurring period of time when all devices must be awake to exchange this information is known as the announcement traffic indication message (ATIM) window.
- During the ATIM window, if a station has buffered data for another station, it will send a unicast frame known as an ATIM frame to the other station.
- This unicast frame informs the station that it must stay awake until the next ATIM window so that it can receive the buffered data.
- Any station that either has buffered data for another station or has received an ATIM will stay awake so that the buffered data can be exchanged.
- All of the other stations can go to sleep and wait until the next ATIM window to go through this process again.
- When the ATIM window expires, the nodes that have stayed awake go
through the usual CSMA/CA process to exchange the unsent data. - If a station is unable to transmit the data during this time, it will simply send another ATIM frame during the next ATIM window and then attempt to send the data during the following CSMA/CA period.
- Do not confuse the ATIM frame with the TIM field.
- The ATIM is a frame used for power management by ad hoc clients not communicating through an AP. The TIM is a field in the beacon frame that tells client stations in Power Management mode that the AP has buffered unicast frames for the clients.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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