Prism2 Cards: Maximum Distance / Ack Timeout?

Joe Parks jphstap at
Thu Mar 9 14:02:06 EST 2006

On Thu, 09 Mar 2006 09:12:26 -0600
George Heeres <george at> wrote:

> The distances for these two clients from the access point is: 4.79 mi
> and 6.51 mi respectively.

I've gone over 9 miles without problems (hostap driver at both ends) using directional antennas.
> Each of these clients is seeing a signal strength of roughly -80/-84
> with a -92/-94 noise floor (roughly same signal strengths from both
> tower / client). I'm seeing ping times all over the place with dropped
> and duplicate packets:

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.
> Browsing around online, I found mention from someone regarding ACK
> timeouts using the PRISM2 based cards beyond 8 km (4.9 mi). Additional
> reading has confirmed that the ACK timeout on the PRISM2 based cards is
> fixed. The nature of the problem (duplicate / dropped packets, etc.)
> seems to confirm this as the problem.

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.

> 1.) ACK timeout. Is there anything that can be done or I just need to
> use different cards (ie. Atheros based with dynamic / adjustable ACK)

A recent wifi record distance setter claimed in an initial press release to have been using prism2 based cards to go some absurd distance.  That may have been an error in the original press release, but if not then they must have figured out a way around that problem.

If anyone knows anything more about this, I too would be interested in the details.

> 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.

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