Prism2 Cards: Maximum Distance / Ack Timeout?

George Heeres george at msworldnet.com
Thu Mar 9 15:10:24 EST 2006


Please let me start off by apologizing if this message end's up in a new
thread. I'm subscribed in Digest Mode...and didn't change modes before
receiving your reply... :(

>Joe Parks wrote:
>I've gone over 9 miles without problems (hostap driver at both ends)
>using directional antennas.
>

>From my limited understanding of the PRISM2 chipset, I thought that most
of the functionality was based on the hardware/firmware. If that's the
case, one would assume there shouldn't be any difference between
performance of the linux-wlan driver vs the hostap drivers.

>That Signal to Noise Ratio isn't great. A directional antenna at one or
>both ends should help, if you can do that.
>
>Duplicate packets are indeed a sign that the ACK timeout was exceeded,
>but in your case that is likely because the sender failed to receive or
>decode the ACK, not that it wasn't fast enough.  And drops are
>definitely a sign that a packet wasn't received correctly.
>

Yes. I agree the SNR is not good. From the site survey, if I raise the
client antenna another 10' we get a 6 dB increase...so I will likely
need to do that.

>>1.) ACK timeout.
>My understanding of this problem is that it does exist, but manifests
>in the 12-15 mile range.  The fact that I've gone 9 miles without
>difficulty supports that notion.
>

What's really strange regarding this is that I have several test clients
which have a VERY, VERY poor signal strength (upper -88/-92). These
clients are about 1-2 miles away. Although the pings times occasionally
jump from 5-8ms to 100-200ms, I don't experience DUP packets and rarely
get a DROPPED packet. Perhaps it's a combination of the poor signal
strength along with the distance?

>>2.) Signal just not strong enough? What should I be shooting for?
>That would be my guess. There is info lurking on the web that
>details the Signal and Signal to Noise Ratios you should be trying for
>to achieve various throughput rates.  I don't have any links at hand
>but Google should be able to help you find it.
>

Well, I think I'm going to replace the Prism2 card at the tower first
with an Atheros based card (SR2, powered down to within FCC limits) and
see what happens then. I've recently read brief mentions / suspicions on
the MikroTik forum that the 2.9 version of their OS has poor Prism2
based performance? I can't find much more, nor a definitive answer on
this, than people's comparison of performance under 2.8 vs 2.9 when they
upgraded.

George



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