[MLB-WIRELESS] Multiple APs

[Andrew Dean]

Yes, yes i did find out

 /beats wireless gear...

Not fair, its always gotta be hard doesn't it.... heh

----- Original Message -----
From: "Roger Venning" <r.venning at telstra.com>
To: "Donovan Baarda" <abo at minkirri.apana.org.au>
Cc: "Jamie Lovick" <jalovick at doof.org>; <melbwireless at wireless.org.au>
Sent: Monday, August 19, 2002 2:06 PM
Subject: Re: [MLB-WIRELESS] Multiple APs


>
> Donovan, see the short paper I just found:
> http://cwc.ucsd.edu/~rgholmie/WirelessPaper/WirelessPaper.html, paying
> particular attention to the section under 802.11 :-). This explains the
> way the medium access control works. I've also commented inline below.
>
> Roger.
>
> Donovan Baarda wrote:
> <snip>
> > Yeah, that's what I thought. With all the AP's on the same channel, they
> > would be colliding with each other all the time. Wouldn't you ideally
want
> > any two APs on the same channel not only out of each others range, but
> > sufficiently out of range that no client in the middle could see both?
> >
> Not necessarily true. If on different ESSID I believe that they use
> different spreading code, and the result is dimished throughput not as a
> result of an ethernet like 'collision' but rather because of greater
> 'interference' ie. high detected noise level.
>
> > This has probably been beaten to death on this list, but I can't find
exactly
> > what the benefit of AP's is. There is a lot of waffly stuff about how
> > "infrustructure mode" relays all traffic through the AP to clients and
how
> > they can do bridging, but I still don't get it.
> >
> They implement the Point Control Function which allows for higher
> channel utilisation due to reducing and coordinating the period where
> contention occurs.
>
> > With all the clients in peer-to-peer mode, the whole system would be
just
> > like ethernet, with colliding clients backing off and retrying. This
gives
> > you effective bandwidth of 1/2 the total available bandwidth. This I
> > understand.
> Ahh... I don't believe it converges to 1/2, I thought the classic ALOHA
> result was something like 37% (with some relationship to e?) and that
> slotted ALOHA was twice as efficient or something.
>
> >
> > With an AP in the mix, all traffic must be relayed through the AP. This
> > means packets between clients must be transmitted twice, once to the AP,
and
> > again to the destination client. Now unless AP's negotiate some sort of
> > collision avoidance, you still have 1/2 total available bandwidth from
> > collisions, and then you halve it again because of the double transmits,
> > giving you only 1/4 total available bandwidth. The only use I can see
for an
> > AP is relaying between two clients that are in range of the AP but not
each
> > other.
> >
> I'm not certain that the A/P will *always* relay the frame. Also
> remember though, that in a typical enterprise situation that this isn't
> a really bad thing if it does becuase most traffic is destined somewhere
> else and not locally anyway.
>
> > Is there some other voodoo that AP's do that buys you something that
I've
> > missed? Is it something to do with the incomplete direct conectivity
between
> > all clients? Surely this could be better handled by smart dynamic
routing
> > between clients in peer-to-peer mode?
> >
> I would say yes (and am working on it as described at
> http://users.bigpond.com/r.venning/wireless.html), but there may be
> performance issues that need to be resolved as Ferni found out.
>
> Roger.
>
>
>
>
>
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