full-duplex 802.11?

[Simon Byrnand]

At 12:20 16/07/02 +0930, Brett Lymn wrote:
>On Tue, Jul 16, 2002 at 12:32:42PM +1000, Bruce Janson wrote:
>>     ...
>>     That was my understanding but I did not (and still do not) know
>> whether the current wireless devices allow selection of two channels
>> sufficiently far apart to avoid that overlap.  Moreover, given that the
>> pairs of colocated cards would be only "bus-slots" apart within the PC
>> there might be other shielding-related problems.
>> 

The problem is not (just) frequency overlap, but frequency proximity. All
radio receivers suffer to a greater or lesser degree from a phenomenon
called "desensing". In laymans terms, a strong signal on a nearby (but not
overlapping) frequency has the effect of reducing the apparent receiver
sensitivity on the desired frequency. The stronger the nearby signal and/or
the closer in frequency, the more the effect. This is not just a small
effect, when the output of a transmitter is close to another receiver the
effect can be extremely severe.

>I have talked to someone that has put two wireless lan cards into a
>laptop, from what he was saying he had no problems with the set up.  I
>was surprised he could do it physically but he told me he had a
>wavelan card and a cisco card (the cisco does not have the lump at the
>end) so they physically fitted into the pcmcia slots.
>
>Given this, I would not totally discount the idea.  Perhaps instead of
>going full duplex you could just trunk the interfaces together and let
>the trunk driver load balance the packets for you?

The problem is in practice you wouldn't achieve the apparent benefit of the
two cards operating simultaneously. 

Unless the received signal from the other end of the link was very strong,
or there was a great deal of (signal) seperation between the two local
antennas then when one card was transmitting, it would be desensing the the
receiver of the other card to the point where it would fail to receive the
signal from the far end. Effectively you would have half duplex forced apon
you regardless of whether you liked it or not, except the overall
performance would probably be even slower than a normal half duplex
connection due to the lost packets sent during times of desensing.

Regards,
Simon