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<TITLE>RE: [e2e] How many transmission attempts should be done on wireless networks?</TITLE>
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<P><FONT SIZE=2>David P. Reed writes:<BR>
<BR>
> Let me clarify that in making my comment about 802.11 systems that won't<BR>
> stop retransmitting until 255 tries have been made, I was citing Larry<BR>
> Roberts. I have not done such measurements. If Larry is right and<BR>
> common chipsets retry up to 255 tries, I support his point. Not<BR>
> everything Larry says is something I agree with, and I have not<BR>
> independently done a test of current chipsets myself.<BR>
<BR>
In 802.11 DCF, failure to receive an expected link-layer ack causes the sender<BR>
to wait a random number of (50us) slot times between 0 to to J. J is calculated<BR>
based on previous # of access attempts plus a bias factor. The first<BR>
retransmission may go out after up to 15 slot times (750us); subsequent<BR>
retransmissions may go out after up to 255 slot times (12.75ms) - the (channel)<BR>
contention window size. This increasing DCF backoff delay before sending is<BR>
intended to work around interference.<BR>
<BR>
Number of slots waited before attempting a resend is not number of resends.<BR>
<BR>
L.<BR>
<BR>
<<A HREF="http://www.ee.surrey.ac.uk/Personal/L.Wood/">http://www.ee.surrey.ac.uk/Personal/L.Wood/</A>><L.Wood@surrey.ac.uk></FONT>
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