[e2e] How many transmission attempts should be done on wireless networks?
L.Wood at surrey.ac.uk
Sat Sep 19 02:02:02 PDT 2009
David P. Reed writes:
> Let me clarify that in making my comment about 802.11 systems that won't
> stop retransmitting until 255 tries have been made, I was citing Larry
> Roberts. I have not done such measurements. If Larry is right and
> common chipsets retry up to 255 tries, I support his point. Not
> everything Larry says is something I agree with, and I have not
> independently done a test of current chipsets myself.
In 802.11 DCF, failure to receive an expected link-layer ack causes the sender
to wait a random number of (50us) slot times between 0 to to J. J is calculated
based on previous # of access attempts plus a bias factor. The first
retransmission may go out after up to 15 slot times (750us); subsequent
retransmissions may go out after up to 255 slot times (12.75ms) - the (channel)
contention window size. This increasing DCF backoff delay before sending is
intended to work around interference.
Number of slots waited before attempting a resend is not number of resends.
<http://www.ee.surrey.ac.uk/Personal/L.Wood/><L.Wood at surrey.ac.uk>
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