multiple WAPs - physical proximity

I was at a friends small office, and noticed their WAPs in the back room.

For some reason, they had acquired a couple of Linksys b/g WAPs and had "stacked" them on top of each other..... they fit so nice :) So the antennas were almost touching each other.

I was wondering, if you were going to run 2 WAPs in very close proximity, how "should" you physically arrange them ?

My RF knowledgebase only has mental entries for 2m & 450 repeaters.

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Bad idea.

802.11g and all spread spectrum transmissions do not stay within a nice neat spectral mask. In theory, spread spectrum transmissions spray RF well outside the official occupied bandwidth. For example, if you have an access point running on Ch 6, you'll have the bulk of the RF energy between Ch 4 thru Ch 8, but here will still be some RF all the way out to Ch 1 and Ch 11. The amounts involved are not huge, but at the very close proximity of the boxes (and presumably the antennas), self-interference between radios is going to be a problem.

Even if the system uses a mess of external antennas and coax cables, there will be case leakage problems. The WAP54G and WRT54G boxes (the only ones that stack neatly) are not internally shielded. They will radiate through the case to each other quite nicely.

The band pass filter at the antenna is useless because it only prevents out of band radiation. It merrily passes everything that's in band.

The effects of such interference is not huge. It mostly shows up as a high packet loss or a loss in thruput. Having the connection speed slow down is also an indication of problems. The easiest test is to use ping:

Start -> run -> cmd ping -t ip_address_of_access_point

You should see a *consistent* 2-3msec latency. If it varies all over the place, and/or shows timeouts, you've got a problem. However, that's for just one access point. Generate some traffic to one of the other access points, and see how it affects the ping results. If they were fine with just the initial setup, but where adding traffic on the

2nd access point cause the latency to climb, then congratulations, you have self-interference.

I spent yesterday getting IRLP running to our club repeaters. Some bright person (me) suggest that this would be a great time to move racks around. My back and wallet both hurt. Tomorrow we do EchoIRLP.

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