When describing 802.11b/g and the 2.4 GHz channels, or 802.11a/n with 5 GHz channels, textbooks (CWNA Official Study Guide) often show some figure with channels spaced such that certain channels cannot overlap and therefore ISI is avoided. The explanation is that ISI occurs in cases of multipath, where the signals of the same frequency interfere. This does make sense to me, i.e. that signals of the same frequency can interfere, and so non-overlapping channels would avoid ISI.
My understanding is that the 802.11 design assigns 1 signal per constant width of "frequency space" in a channel, for example
- 1 signal per 2 MHz in a DSSS channel
- 1 signal per subcarrier in OFDM channel
If we look at the figures in the URLs, the spacing of the channels and frequencies show a sequential order by Hz. What's not clear to me, is whether the same exact sequence must also hold for the timing of each signal's arrival. For example, consider 2.4 GHz channel 1: does the 2.410 GHz signal always arrive at the receiver before the 2.414 GHz signal? After the first 1 cycle, I would guess that the signals within a single channel arrive in order of lowest to highest Hz. But does this precise order hold after any length of time? What would prevent signals at different Hz from arriving at exactly the same instant? Or if simultaneous arrival of different-frequency signals is already expected to occur, then how does the radio discern which is the correct signal to receive?
Best Answer
These types of radio frequency transmissions are "frequency division multiplexed" (FDM). The different channels (ie, frequencies) transmit and receive at the same time. It's like a cocktail party where each conversation chooses a different pitch range -- the tenors have no problem hearing each other right through the basses at the next table.
In FDM, the receivers are tuned so that they only pickup transmissions in the channel (the frequency range) to which they are supposed to listen. So over a wider range (eg, the 2.4Ghz wifi) there are smaller divisions of frequency ranges into channels. The transmissions move at nearly the speed of light, so any transmitters on the same frequency -- the same channel in a give frequency band, eg 2.4Ghz's channel 11 -- will instantly be talking on top of each other. So any two 2.4Ghz channel 11 devices within range of each other will collide instantly.
Perhaps the part that isn't obvious is that electro-magnetic theory shows that superposition of E&M waves is not a problem. It's like waves in the open, ocean; You can have short spaced three foot waves (a highish frequency), moving over the surface at the same time you have long ocean swells (a lowish frequency). At the E&M receiver, you "simply" tune to the right frequency and you can pick the signals you want out of the noise. (And the E&M spectrums is very very noisy.)