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Appendix C:

Channels Explained

The ins and outs of channels used in each band have wound up in this

appendix, as you may need to know the details only when something

goes wrong—or you may be among the more technically inclined

readers who want to know more about the technical minutiae of Wi-Fi.

In this appendix, you can learn about why the 2.4 and 5 GHz channels

are organized the way they are, and what happened to 15 missing

5 GHz channels.

Channels in 2.4 and 5 GHz are 20 MHz wide; an optional 40 MHz

wide, or double-channel, option was added in 802.11n, although Apple

allows 40 MHz wide channels only in 5 GHz. 802.11ac expanded this to

80 MHz and 160 MHz wide channels; Apple included the mandatory

80 MHz width, but not the optional 160 MHz addition. The two bands

have different ways of defining and making those channels available.

Channel availability varies from country to country. Apple lists which

channels it supports in the technical specs for its base stations. You can

also see

a table of 5 GHz channels worldwide in Wikipedia.

MHz and Mbps

Megahertz does, in fact, correlate to megabits per second. Shannon’s

Law (or the Shannon-Hartley Theorem), a bit of information theory,

says that there’s a direct relationship that ties the width of a channel

and the ratio of signal to noise to the achievable data rate. Twice the

channel width means up to twice the raw data (Figure 102).

Figure 102: Shannon’s Law (image via Wikipedia).

In case you were wondering, the formula is: maximum bit rate equals

channel width in hertz multiplied by log2 of the sum of 1 + signal

divided by noise.

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Apple AirPort Networks Anleitung zur Fehlerbehebung Seite 53