[MLB-WIRELESS] 4 (almost) non-overlapping channels

[Ratbaggy]

A 1/5/9/13 scenario may well be a good compromise ending up with with 4
channel separation instead of 5 and an "overlap" of only 2 MHz, BUT........

Ultimately at a site with co-located AP's it would be nice to have enough
isolation so as one AP could be receiving and  another transmitting at the
same time.  The isolation could be obtained in a number of ways:-
*Physical separation, Vertical and/or Horizontal
*Antenna pattern & orientation
*Frequency separation
*Different polarization
*Physical RF barriers eg. opposite sides if a tin roof
*and some others

To make things even more difficult to calculate, you need to consider the
wide band transmitter noise form your particular AP, the signal strength of
the AP you are trying to receive and interference from other sources outside
of your control.

In a bad scenario, say with two omnidirectional antannas mounted side by
side, even with one unit on channel 1 and the other on 13 you may not have
enough isolation to transmit and receive simultaneously.

Assume TX power of +20 dBm
Wideband TX noise 60 dB below your transmit power.
Adjacent omni antennas.  15 dB isolation.
Some feeder loss. 2 dB

=> noise level at RX = +20 -60 -15 -2 = -57 dBm
Even with a good RX signal, say around -65 dBm, we're in big trouble.

If you are forwarding traffic from one interface to the other, expect more
than 50% drop in bandwidth, despite the use of channels 1 & 13.

Of course, if an amplifier is in use the problem will be MUCH worse.
Conversely, if the interfaces are at opposite ends of the house with high
gain antennas pointing in different directions and one vertical and one
horizontally polarised you "might" get away with 4 channels separation
without a drop in bandwidth.

Dave.

----- Original Message ----- 
From: "Dan Flett" <conhoolio at hotmail.com>
To: <melbwireless at wireless.org.au>
Sent: Monday, June 27, 2005 11:39 AM
Subject: [MLB-WIRELESS] 4 (almost) non-overlapping channels


> I've found an interesting article about non-overlapping channels in
> 802.11b/g
>
> http://www.extremetech.com/article2/0,3973,708876,00.asp
>
> Conventional wisdom says that channels 1, 6 and 11 are the only
> non-overlapping channels in 802.11b/g.  This article claims that channels
1,
> 4, 8 and 11 (and 13 or 14 where allowed) can be used with minimal or no
> interference issues.   The article mainly deals with APs, evenly spaced in
> an indoor environment.  It doesn't take into account a situation where you
> might have four high-powered APs on a single mast.
>
> Also, using channels 4 and 8 in an environment where everyone else is
using
> channel 6 might be a problem.
>
> Dan
>
>
>
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