[MLB-WIRELESS] Cisco tool

[Jason Hecker]

With regards to the Cisco spreadsheet posted on the site, it's pretty 
damned nifty.  The 10dB fade margin is a very good idea and there for a 
reason, namely effects from the atmosphere (rain and fog) and dynamic 
multipath (this will change as reflections come off moving cars, trees, 
atmosphere turbulence, microwave ducting in moist warm air, planes) which 
can degrade the performance by up to 10dB.  This sort of thing can be 
improved if you employ space diversity.  If you have a card with 2 antenna 
connectors, like the Senao EXT2 device, and you space two antennas spaced 
apart (I forget how many wavelengths it should be), in receive mode it will 
pick the best antenna for signal strength because it has a 'voter' in it 
which decides which antennas has the best signal.  The one and same antenna 
will always be used for transmitting as space diversity is only useful when 
receiving.  If you do this then your fade margin is reduced.

I would suggest that for backbone links, space diversity is used for 
maximum reliability.  If the signal is too weak the cards will drop to a 
lower bitrate to compensate.


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