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I've read everywhere that wireless access points interfere with each other and should be put on distant channels. This even applies to access points on the same wired network with the same SSID from what I've read.

Doesn't that imply that wireless clients, especially higher-powered ones, will also interfere with each other on nearby channels - or the same channel? Even when they are connecting to the same access point?

If this client-to-client interference exists, are there any rules of thumb for when it becomes a problem?

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2 Answers 2

Two clients connected to the same AP... aren't supposed to interfere -- it's the AP's job to coordinate things (RTS/CTS handshake.) The collision avoidance system checks for an active transmitter (which might not be associated with the same AP) before transmitting. But as with all things in the Real World(TM), there are cases where it can still happen because the radios are spaced far enough apart to not be able to hear each other.

Co-channel interference from the inherent overlap of 2.4G channels does cause problems. The stronger the signal the worse it will be. 802.11N in the 2.4G band will almost never be able to use a wide channel, as it's required to drop to a narrow channel if it detects anything in the side channels -- bluetooth, cordless phone, baby monitor, any thing.

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You may be somewhat misinterpreting what you've read. Most (recent) wifi radios are good at rejecting adjacent channel interference, so your concern shouldn't be with radios on different channels. An AP on channel 1 will not cause much interference with another on ch 6, for example. The same goes for clients.

Edit:

When two workstations are associated with the same AP, the AP coordinates their transmissions using CSMA/CA. So they don't interfere so much as they compete for available time slots. The more clients you have, the more competition there is for airtime and throughput decreases.

Co-channel interference can occur when there are two nearby APs on the same channel. In this case, the APs are not in sync, so clients associated with AP1 are not in sync with clients associated to AP2. The clients will interfere with each other and the other AP, causing deferred transmissions, retransmissions, etc, all taking a big bite out of your throughput.

Using nonstandard frequencies (like channel 7) is just wrong and will cause everyone problems.

Also, +1 to everything @RickyBeam said.

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I am talking about about co-channel interference. Could my phone and my laptop, connecting to the same AP, potentially interfere with each other? –  The Phil Lee Mar 19 at 20:26
    
As well as nearby channels, since channel 7 for example in a 2.4 ghz N network seems to occupy channels 5-9. –  The Phil Lee Mar 19 at 20:30
    
Who says ch.7 is "nonstandard". There are 14 channels in the standard. In the US, we're allowed to use 1 thru 11. 1, 6, and 11 don't overlap, so that's what most people use, but there are, in fact, 11 channels one can use. –  Ricky Beam Mar 19 at 23:18
    
@RickyBeam The channel designations (with 5 MHz spacing) developed well before wifi was invented. It really is incorrect to say we use 1, 6 and 11, since wifi bandwidth is 20MHz (b/g). More accurate to say the wifi frequencies are centered on 1,6,11 -- they span several channels. I mean nonstandard in the sense that anything other than 1,6,11 will interfere with everybody and subject yourself to interference from other wifi xmiters. –  Ron Mar 20 at 1:29
    
Using channel 7 in HT40+ mode is the same as using channel 11 in HT40- mode. If there is no one using channel 5-11, using channel 7 isn't a bad idea. –  BatchyX Mar 20 at 8:44
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