# Some More MAC Features

Use of RTS/CTS is controlled by an RTS threshold.

* RTS/CTS is only used for data packets longer than the RTS threshold.
* Pointless to use RTS/CTS for short data packets – high overhead!

Number of retries is limited by a Retry Counter.

* Short retry counter: for packets shorter than RTS threshold.
* Long retry counter: for packets longer than RTS threshold.

Packets can be fragmented.

* Each fragment is acknowledged.
* But all fragments are sent in one sequence.
* Sending shorter frames can reduce impact of bit errors.
* Lifetime timer: maximum time for all fragments of frame.

## Fragmentation

<figure><img src="/files/qT5aqttS1XrFxtOb54yi" alt=""><figcaption></figcaption></figure>

## Power management (infrastructure)

APs buffer packets to stations in power saving mode.

* APs announce in beacons which packets are waiting with the TIM (traffic indication Map).
* Broadcast/multicast frames are also buffered at AP.
  * Sent after beacons, same common timing period.
  * Uses Delivery Traffic Indication Map (DTIM).
  * AP controls DTIM interval.

STA in power save wake periodically to listen for beacons.

* If it has data pending, send a PS-Poll.
* AP sends buffered data to this PS-poll.

TSF (Timing Synchronization Function) assures AP and stations are synchronized.

* Synchronizes clocks of the nodes in the BSS.

<figure><img src="/files/6AZS1KhPVtfA3NWYN6Cv" alt=""><figcaption></figcaption></figure>


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