[CUWiN] good news everyone

David Young dyoung at pobox.com
Sat Aug 5 20:04:20 CDT 2006

On Sat, Aug 05, 2006 at 07:47:01PM +0200, Aaron Kaplan wrote:
> > hard disk, like a PVR.   How complicated is that? 
> > 
> (...) i did not mean this question :)
> > For the 1 thousandth time, we need a directional rooftop
> > device, of which, each house needs two or more, in 
> > order to construct our urban mesh WHOEVER we want. 
> > 
> oops!
> yes, i agree. But I think I have to clarify what I acutally meant:
> _given_ that many freifunk/funkfeuer/cuwin meshes are like they are now,
> and _given_ that our projects attract media attention and therefore - as
> it happend in VIE now - the attention of the (local) FCC
> ("funkueberwachung", "regulierungsbehoerde" in germany) and _given_ that
> - as you are all freaks - you installed high gain antennas. 
> THEN not everything is lost. You can still use these nice 24dBi high
> gain antennas on the manner i described before.
> In VIE we recently faced the problem that our main uplink was
> transporting bandwidth into the mesh via 24dBi antennas (which as TX
> antennas are to strong). We had to take them down and replaced them with
> "legal" less powerful antennas. 

Why not turn down the transmit power?  (I realize that isn't always
under one's control, but ISTR the WRT54 has a notorious "power hack.")

> Only to find out now that the Diversity dissociation loss on the linksys
> seems to be 26dB. Voila! We could have kept them there and just add a
> small antenna on the TX side. :)
> That was my point 
> Hm, and did I mention that you can switch RX and TX side in
> software ? ;-))

I am making a fine distinction, but calling these RX and TX antennas
may not be right.  Usually, if there are two antennas, both antennas are
used to receive.  An arbitrary antenna is the "default," where the NIC
listens while it is not transmitting.  When the NIC receives a signal
on the default antenna, it flips between the default antenna and the
alternate, measuring the energy level on each, and finally flips to
the antenna with the highest level to receive the packet.  In this way,
both antennas are RX antennas.

Both antennas may be TX antennas, too.  A NIC will usually send management
and data frames using the default antenna.  It will depend how the MAC
works, but if a NIC receives a data frame on the alternate antenna,
it may send the acknowledgement on the alternate antenna.  (You can
use a symmetry argument to argue that is the "right" way to do it.)
In this way, both the default antenna and the alternate antenna may be
TX antennas.

None of this is meant to diminish from your point, though, that having
two antennas on virtually every WiFi device presents opportunities!


David Young             OJC Technologies
dyoung at ojctech.com      Urbana, IL * (217) 278-3933

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