802.11g @ 6M or 802.11b @ 5.5

Which would perform better in poor signal levels. say -85dBm.

I can find plenty of sites comparing the 2 technologies, an OFDM is a better
and more robust modulation. Anyone got any real life benefits?

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sat, 27 Sep 2008 14:25:28 +0300, "TheDragon"
> <(E-Mail Removed)> wrote:
>
>>Which would perform better in poor signal levels. say -85dBm.
>
> Which what? Oh, it's in the subject line. Do me a favor and NOT put
> half your question in the subject line, and the other half in the
> body. I get easily confused.
>
>>I can find plenty of sites comparing the 2 technologies, an OFDM is a
>>better
>>and more robust modulation. Anyone got any real life benefits?
>
> OFDM is more resistant to multipath (frequency selective fading) than
> CCK. OFDM is also slightly more resistant to interference effects
> than CCK. You'll also get more range out of 6Mbit/sec ODFM because
> the theoretical receiver sensitivity is about 3dB better than
> 5.5Mbits/sec CCK. I can supply numbers (tomorrow) if you need them.
>
> I like to lock the speeds of my point to point links to some minimum
> speed. I usually use 12Mbits/sec OFDM. The idea is so that the link
> doesn't constantly change speeds up or down.
>
> If you want real-life examples, it would be helpful if you would
> explain what you're trying to accomplish.
>
>
> --
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

Thanks for your response. Yes a few real life examples would be great.

I have built a cyber cafe and recently added WiFi access both inside and
outside (Higher Power, up a pole)

The outside users range from close by laptops, to distant fixed connections
using external antennas.
I noticed the speed was always changing, often down as low as 1mbs, so I
fixed it thinking 802.11b was better as I figured the speeds of g wasn't
really needed. so I fixed it at 5.5mbs. Then reading about modulation for
wifi discovered OFDM used in 802.11g at 6 mbs+ maybe would be better for the
weaker users.
Fixed so the weaker users doesn't drag the entire system down to 1M just so
they can connect reliably, I would rather a user not be able to connect and
force them to move location.

I chose 6mbs as a trade between speed and SNR requirements.

The backhaul ADSL link is only 1Mbs so high speed WiFi isnt needed.

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sun, 5 Oct 2008 08:48:23 +0300, "TheDragon"
> <(E-Mail Removed)> wrote:
>
>>The outside users range from close by laptops, to distant fixed
>>connections
>>using external antennas.
>
> Do you really want long distance users?
>
>>I noticed the speed was always changing, often down as low as 1mbs,
>
> In the presence of interference, the bit error rate climbs, which
> causes the access point to reduce speed.
>
>>so I
>>fixed it thinking 802.11b was better as I figured the speeds of g wasn't
>>really needed.
>
> Wrong. The advantage of using higher speeds is that they use less air
> time. That leaves more air time for the slower connections.
>
>>so I fixed it at 5.5mbs. Then reading about modulation for
>>wifi discovered OFDM used in 802.11g at 6 mbs+ maybe would be better for
>>the
>>weaker users.
>
> I use 12Mbits/sec. 6 and 9 are just too slow.
>
>>Fixed so the weaker users doesn't drag the entire system down to 1M just
>>so
>>they can connect reliably, I would rather a user not be able to connect
>>and
>>force them to move location.
>
> Well, one option that you should consider is to simply disable 802.11b
> speeds. That sets the minimum speed to 6Mbits/sec. Beacon and
> management frames are also sent at 6Mbits/sec instead of 1Mbit/sec,
> yielding more air time (and thruput).
>
>>I chose 6mbs as a trade between speed and SNR requirements.
>
> If you can see collisions on your AP, you'll probably find that higher
> speeds drastically reduce those.
>
>>The backhaul ADSL link is only 1Mbs so high speed WiFi isnt needed.
>
> Have you checked your traffic for abusers? Things like massive
> downloads, running servers, Bitorrent, and such tend to bring the
> system to a crawl. Good luck.
>
>
> --
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

I have already disabled 802.11b to deny access to these slow devices.

I don't have an option to set a min speed, only a fixed speed. I have
options of Tx Rate and Default rate, which is default, 1-2 or all. I have
left the default rate at 1-2, and the Tx Rate is on 6Mbs forcing everyone to
connect at 6 or nothing. I may look at increasing this to 12, at the expense
of blocking access to some long range users.