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TIM(Traffic Indication Map)

The Traffic Indication Map (TIM) is a component of the beacon frame broadcast by Wi-Fi access points (APs) that plays a crucial role in power management for devices connected to the Wi-Fi network.

In daily use, the TIM mechanism operates transparently to users, allowing devices like smartphones and laptops to conserve power by remaining in sleep mode as much as possible, only waking when necessary to maintain network connectivity and communication.

  • Inclusion in Beacon Frames:
    • The TIM is included in the beacon frames sent by an AP at regular intervals (the beacon interval).
    • These intervals are typically set to 100ms but can vary depending on the network setup.
  • Structure:
    • The TIM contains a bitmap representing each associated client device by its Association ID (AID).
    • Each bit in the TIM corresponds to a different AID. If a bit is set to '1', it indicates that the AP has buffered data waiting for the device with that AID.
  • Power-Saving Devices:
    • Devices that are power-saving mode (PSM) will wake up at each beacon interval to listen for the beacon frame.
    • The device checks the TIM element within the beacon to determine if the AP has any data buffered for it.
    • If the TIM indicates that data is buffered, the device will send a PS-Poll frame to the AP to retrieve the data.
  • DTIM and Multicast/Broadcast Traffic:
    • The Delivery Traffic Indication Map (DTIM) is another element related to the TIM.
    • The DTIM period, a count of beacon intervals, indicates how often the AP will include buffered multicast or broadcast traffic in the TIM.
    • Devices in power-saving mode will wake up for every DTIM beacon to check for multicast or broadcast traffic.
  • Efficient Network Usage:
    • By indicating which devices have data waiting for them, the TIM allows devices to remain in a low-power state until they need to be active.
    • This conserves battery life on mobile devices by reducing the need for them to constantly check in with the AP.
  • Retrieving Buffered Data:
    • When a device finds its AID in the TIM, it sends a frame indicating it's ready to receive data.
    • The AP then sends the buffered data to the device.

 

NOTE : How an AP can figure out if a client support TIM or not ?

Here is the sequence of events for a client using power-saving mode:

  • Association: The client device associates with the AP and agrees on various capabilities, including power-saving modes.
  • Communication of Power-Saving Intention: When the client device wants to enter power-saving mode, it sends a frame to the AP indicating this intention. This is usually done through a control frame known as a Null function frame with the power management bit set.
  • Listening to Beacons: The client then listens to beacon frames at its specified listen interval to check the TIM.
  • Checking the TIM: If the TIM indicates that the AP has buffered frames for the client (by setting the bit corresponding to the client's AID), the client will send a PS-Poll frame to the AP to request the delivery of the buffered data.
  • Receiving Data: The AP sends the buffered data to the client, which may involve multiple exchanges of PS-Poll frames and data frames if there are multiple packets buffered.
  • Returning to Sleep: Once the data exchange is complete, the client may return to a low-power sleep state until the next beacon interval when it will wake up to check the TIM again.

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