WiFi performance and quirks
hmurray at megapathdsl.net
Thu Mar 26 06:43:13 EDT 2009
wmb at laptop.org said:
> I can easily explain the 1.4 MBytes/sec figure - that's exactly what
> you'll get if you send maximum-size Ethernet frames at a rate of one
> frame per millisecond. The 1/ms rate limitation is easily explained
> by the USB macro-frame time.
Thanks. That matches exactly what I'm seeing.
1.4 megabytes/sec is 11.2 megabits/sec. If the packet format is ethernet
(and I counted right) the per-packet overhead would drop the throughput down
to 10.9 megabits. My tests get up to 10.8. That's close enough for me.
> I did a lot of work to make the NANDblaster send as fast as possible,
> using the mesh in a restricted mode to avoid retransmissions. I got
> to 2.8 Mbytes/sec. Ricardo Carrano told me that the fastest rate
> that has ever been measured using Linux is 1.4 MBytes/sec.
2.8 megabytes/sec is 22.4 megabits/sec. I found one web blurb that counts
all the overhead. It said that the best 802.11g can get is 22. I didn't
check anything, but it seemed reasonable and gave enough info that I probably
could check things.
The killer is a long preamble to be backward compatible with 802.11b.
Thinking that if one XO is limited to 11 megabits, I did the obvious thing of
running 2 TCP streams in parallel, one to each XO.
My Linksys AP seems to crap out at slightly over 7 megabits/sec. That's what
I get with one TCP stream from PC->AP->XO. If I run 2 tests to separate XOs,
I get roughly half the bandwidth on each.
Sending in the XO->AP->PC direction gets "interesting". One TCP stream gets
10 megabits. Two streams get under 1 megabit/sec each. If I chop the socket
buffers down to 4K, things work much better. They get about 5 megabits each.
What a rathole. I think I'll stop digging myself in deeper.
Does anybody know of a handy database of throughput for USB WiFi dongles
and/or APs? I poked around a bit but didn't find anything.
These are my opinions, not necessarily my employer's. I hate spam.
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