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When scanning a channel for wireless signals it is of course possible to pick up signals from surrounding channels. What I'm curious about (and forgive my n00b-ish-ness regarding the physics behind how signals are received/interpreted) is does picking up a signal from an adjacent channel affect any parts of the signal or data, such as the received signal strength measurement?

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Signals from other channels are picked up as noise, lowering your signal-to-noise ratio (SNR). In principle, it could be picked up as data, but will almost always fail some very basic checks (correct preamble, destination MAC address, ...).

The channel mapping is built so 802.11b channels have sufficient distance to not influence each other. With 802.11g, the transmission bandwidth is wider, but the channel mapping was retained for compatibility purposes.

Interfering transmissions cause information to be lost, which is generally corrected for by a retransmission, decreasing efficiency of the system.

If any access points using the g protocol are around, it is generally best if everyone uses channels 1, 6 and 11 exclusively, as this avoids interference between adjacent channels, leaving only conflicts on the same channel, which the protocol is built to avoid even between different networks.

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  • \$\begingroup\$ So what then happens to the data that fails the basic checks you mentioned? Is it dismissed? To give some context I'm specifically looking at monitoring beacon/probe packets, and trying to figure out the best way to sweep channels for their presence. I was hoping to find some correlation between RSS and channel difference to indicate which channel a transmission really came from without having to completely switch the receiver to that channel. \$\endgroup\$ – thanby Jun 12 '14 at 12:16
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    \$\begingroup\$ Discarded or ignored. The packet starts with a certain bit sequence to give the receiver an idea of the transmitter's clock rate, if that is distorted, data recording is never started. If the preamble looks valid, but the recipient address is neither our station address nor a multicast address, reception is terminated to conserve power, and the partial packet discarded. If the packet is received in full, but the checksum at the end does not match, the packet is discarded. \$\endgroup\$ – Simon Richter Jun 13 '14 at 11:34
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    \$\begingroup\$ In order to find out which channel the packet was sent on, you'd have to look at the OFDM subchannels (WiFi uses many narrowband transmissions, digitally mixed to a single broadband channel), however I'm not aware of any implementation that will actually provide that information to the higher layers. The RSS is just a summary over multiple packet lengths, while the interferer you are looking for is most likely also using short bursts. \$\endgroup\$ – Simon Richter Jun 13 '14 at 11:38

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