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[Ham-80211] RE: ham-80211 Digest, Vol 29, Issue 1

David Young dyoung at pobox.com
Thu Jun 7 06:01:04 UTC 2007


On Wed, Jun 06, 2007 at 08:28:17PM -0400, ussailis at shaysnet.com wrote:
> There are two other things that can be important:
> 
> 1. DECREASING bandwidth will provide as much gain as increasing power.
> Reducing bandwidth by one-half provides 3 dB gain, the same as doubling
> power. Go down to 1 mbps if you are not there.

Vendors produce tidy-looking charts that show the relationship between
S/N ratio and delivery probability at each bitrate.  1 Mb/s usually has
a higher delivery probability than 11 Mb/s at the same S/N.  The charts
do not show the full picture, though.

I believe you will usually do best by selecting the bitrate dynamically.
Some of the research in the field implies that.  For example,
<http://pdos.csail.mit.edu/papers/roofnet:sigcomm04/index.html> examines
data from a real-world testbed and concludes, in part,

        "A practical conclusion from the data in this section is that
        although S/N does affect delivery probability, one cannot expect
        to use S/N as a predictive tool."

The bitrate selection algorithm that the WRT54 uses probably does *not*
follow these suggested guidelines, from the same research paper:

        "First, an algorithm should wait until a high bit-rate is
        performing very badly (i.e. delivering only half the packets)
        before it reduces the bit-rate.  Second, 11 Mbit/s often provides
        higher throughput than 5.5 Mbit/s even when the loss rate at 11
        Mbit/s is higher than 50%. Third, performance at a low bit-rate is
        not a good predictor of performance at higher rates: for example,
        there are many links with high loss rates at 1Mbit/s that would
        have a higher throughput at 11Mbit/s. These observations imply
        that bit-rate selection must be based on explicit measurements
        of throughput at the different rates, rather than on indirect
        prediction."

In the face of bursts of interference, a WLAN system may deliver
more packets at a high bitrate than at a low one, holding the packet
length in bits the same, because more packets may fit between bursts of
interference when their transmission time is shorter.  That is true for
microwave oven interference, for example.  You can read more about that,
<http://www.hpl.hp.com/personal/Jean_Tourrilhes/Papers/Adaptive.Frag.html>.

Dave

-- 
David Young             OJC Technologies
dyoung at ojctech.com      Urbana, IL * (217) 278-3933 ext 24




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