802.11g AP/Bridge Collocation

pcklutz · Oct 24, 2005

I'm working on linking four offshore oil platforms with bridged point-to-point 802.11g links. 802.11g says radios must have at least 4 dB adjacent channel rejection (ACJ) for not more than 10% packet error rate (@36 Mbps data rate). I have measured an ACJ of 11 dB. System gain is 99 dB so I figured I need 88 dB of antenna isolation. When explaining this to client, he said overlaying the three available channels with collocated APs is common practice. In a typical indoor installation with collocated APs, isolation between antennas is likely to be only 40-50 dB. How can this work without greatly compromising range?

zappedagain · Oct 24, 2005

It is common practice to not use adjacent channels. More channel spacing helps.

You can dig into the public domain 802.11 specs, or here is what wikipedia says "Although the statement that channels 1, 6, and 11 are "non-overlapping" is incomplete, the 1, 6, 11 guideline has merit. If transmitters are closer together than channels 1, 6, and 11 (e.g. 1, 4, 7, and 10), overlap between the channels will probably cause unacceptable degradation of signal quality and throughput."

pcklutz · Oct 25, 2005

With only 3 channels in the 802.11g standard and needing all three, I have to use adjacent channels.

The specs are not in the public domain. I do have access to draft copies of 802.11a, b, & g.

I've requested customer referrals from the vendor but have received nothing yet. I am really hoping to contact someone who has done this or at least tried.

pstuckey · Oct 26, 2005

The IEEE 802 specs are offered free to the public 6 months after they've been finalized. See

http://standards.ieee.org/getieee802/

(802.11 a, b , g are there)

zappedagain · Oct 26, 2005

802.11b/g has 11 channels in North America, 13 in Europe except for Spain (2) and France (4). Japan has one or 13 depending upon the system.

pcklutz · Oct 27, 2005

pstuckey
Thanks for the link to the standards. Will check it out.

zappedagain
re: 802.11 channelization
See table 105, table 111, & fig 141 in 802.11b.
802.11g references the channelization defined in 802.11b.

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