[Soekris] 4511 question?
jim at netgate.com
Wed Aug 18 20:57:03 UTC 2004
On Aug 18, 2004, at 9:58 AM, Wes wrote:
> Can you use the mini-pci and pcmcia slots at the same time. Like if I
> two wireless cards, one pcmcia and one mini-pci would this work?
Depends on your definition of 'work', I suppose.
Two radios in close proximity could be a problem for several reasons.
One is injection locking. If the LO on the two boards
are close enough in frequency, they might co-couple, with disastrous
results. Doesn't seem to be a problem for two cards, but let me tell
you about 14...
Also, with medium to high gain antennas operating in the same band is a
really bad idea, and was previously discussed on the bawug list:
As an example, lets take two 200mW (23dBm) radios with an ACR of 41dB
and antennas with a back or side lobe @ 6dBi, separated by 1m.
23dBm + 6dBi (gain of transmitting antenna) + 6dBi (gain of receiving
antenna) - 40dB (path loss in 1m) = -5dBm
This (-5dBm) is the signal level at the adjacent receiver. If it is
operated on an adjacent channel to the transmitter (i.e. the tx is on 6
and the receiver is on 1 or 11)
then we get an additional 41dB of adjacent channel rejection (this is
the spec on Prism2/2.5, the IEEE 802.11b minimum is 35dB).
Thus, the "signal level" presented at a receiver while the other unit
is transmitting will be -46dBm. if you think of it another way, the
noise floor is now at -46dBm,
instead of the normally-assumed -101dBm. This 55dBm delta presents real
issues. Those of you with an RF background will understand the
understatement inherent in what I just said.
If the unit were to be restricted to operation on channels 1 and 11,
then there would be perhaps another 15dB of ACR, but this only provides
an artificial noise floor of -61dBm.
Both -61dbm and -46dBm are high enough to 'set' CCA in the adjacent
receiver. That is, while unit "A" is transmitting, unit "B" will not,
because it will determine
that someone else "quite close by" is already transmitting. Get
enough isolation, and you will eventually run into the worse situation
where CCA is not set,
and all you get is a very high noise floor.
Also badness ensues when Unit "B" is receiving (from a distant client),
and the signal level at the off-channel unit isn't high enough to set
CCA. In this situation, the other unit *could* start to transmit, and
would destroy the packet that was being received at the first unit.
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