Re: Wireless Protocol Question

muppet <muppet.2xxo04@no-mx.wirelessforums.org> hath wroth:

>I have a question about the 802.11g protocol.
>
>If you have 10 users connected to an AP, 9 of them are close and can
>get speeds of up to 54Mb/s, 1 of them is right on the range point and
>can only connect at 1Mb/s, is it true everyone will be limited to
>1Mb/s?
>
>i.e. Is it true that all users are dragged down to same speed as the
>slowest connected user?
>
>I've been told today that this is the case, however I don't believe
>what I'm being told, but I've had a hard time trying to find something
>specific in plain english that answers this.
>
>If you know the answer to this, I'd also love a link to a technical
>document if you have one. If I go back and say to the person "It's not
>true, according to an Internet forum post" I'm not really going to have
>a leg to stand on, however if I say that -and- have some hard
>documentation, I will!

See:
<http://standards.ieee.org/getieee802/802.11.html>
You'll need:
IEEE-802.11-1999
IEEE-802.11b-1999
IEEE-802.11b/COR1-2001
IEEE-802.11g-2003
Warning: Reading these have been known to turn one's brain to mush.

The question is a bit muddled because two things are happening at the
same time. Most packets run at individual connection speeds, but not
in all modes or with all types of packets. The details:

Only one user can move traffic to/from an access point at a time. The
system is essentially simplex, where the AP and clients can only
transmit or receive, one at a time.

802.11g access points usually have at least two modes. The AP can be
in 802.11g mode, where the speed it 6-54Mbits/sec and the modulation
is OFDM, or it can be in the 802.11b compatibility mode, where the
speeds are 1-11Mbits/sec using various modulation schemes. There also
modes not specified which are used to obtain speeds greater than
54Mbits/sec. The problem is that when the AP is in one mode, it
cannot decode transmissions sent in the other modes.

Depending upon the algorithms used, switching between 802.11g,
802.11b, and Turbo-Whatever modes can be quite sluggish. This is what
I believe your sources were discussing. In the dim and disgusting
past, when a single 802.11b packet was heard, other traffic thruput
would slow down drastically because the AP was multiplexing between
802.11b mode and 802.11g mode. Typical was about 25-50% of the time
went to 802.11b. During this time, no 802.11g traffic could move.
802.11b also takes much more air time than the same amount of data
using 802.11g, so it can really impact the speed of the system.
There's a speed table, which uses this scheme, in the FAQ at:
<http://wireless.wikia.com/wiki/Wi-Fi#Performance_and_Speed>

More modern chipsets and algorithms have eliminated the fixed time
slicing and replaced it with a more efficient algorithm. I'm too lazy
to dig out the patents, but basically it prevents the 802.11b mode
from hogging too much AP time. The same is true for the faster
speeds, which can also do the same thing. To maintain high airtime
efficiency, it's fairly common for capacity limited systems (such as
WISP systems) to just disable 802.11b compatibility and to not support
turbo modes.

The other part of the question is whether each 802.11g client can
connect at its own favorite speed or whether the access point give
everyone the same speed as the slowest connection (assuming 802.11b
compatibility mode is disabled). It would be really dumb to run the
system at speed of the slowest client. Each connection speed is
independent. The proof is somewhere in the IEEE 802.11 specs. If you
can't find the reference, I'll dig for it when I have time.

Also, there's the not so minor problem of communicating 802.11
management and control packets (such as beacon, probe, auth, assoc,
flow control, etc). In 802.11b, they are all transmitted at the
slowest speed of 1Mbit/sec. I think it's the same with 802.11g at
6Mbits/sec, but I'm not sure and need to read the specs.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558