Friday, 2 January 2015

Random question on Wifi- Beacon Interval

Question :

 Suppose Beacon packets are sent at 0, 100, 200,300..In this case if AMPDU has started at 95th ms
and is extended beyond 100ms upto 105 ms.When the next beacon will be sent ? 100ms or 105ms or
will be dropped and wait for next beacon interval?


Answer:


As per IEEE Std802.11-2012, Sec: 10.2.1.4

The AP schedules Beacon frames for transmission every beacon interval, but the Beacon frames
may be delayed if there is traffic at the TBTT.


Beacons frames are scheduled to be transmitted every 1024us. If on the 1024us, the medium is busy,
it is transmitted at the next available transmit opportunity.So the beacon will be transmitted at
105 msec.The next beacon will continue at previous interval i.e at 200 msec, so if we calculate the
beacon interval between the last beacon frame and this beacon frame it will be 95 msec and not 100 msec.

Target Beacon Transmission Time  (TBTT) is the time at which a node (AP or station when in Ad-hoc) must send a beacon. The time difference between two TBTTs is known as the beacon interval. The beacon interval is given in Time Units (TU), each TU represents 1024 microseconds. The beacon interval is typically set to 100 TUs (102400 microseconds, or 102.4 ms) and its length is two bytes.

Ref:

http://wireless.kernel.org/en/developers/Documentation/ieee80211/power-savings

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