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I understand that there are 14 channels in WiFi band ,each channel designated by its center frequency,if a WiFi device is transmitting in one channel does it makes use of all frequencies in the channel?For example in channel 1,starting frequency is 2.401Ghz and end frequency is 2.423(center freq-2.412).if it is transmitting only at center frequency,what is the use of other frequencies in channel 1.

Can anyone help me on this?

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Have you read this page on Wikipedia: List_of_WLAN_channels ?

In particular this image:

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shows that the type of Wifi (802.11b, 802.11b etc) determines how wide (how much space) a channel takes. Especially the N variant needs a wide channel with a lot of sub channels. Then in total only 3 non-overlapping channels result.

And when using the N variant with 40 MHz wide channels, only 2 remain!

The numbering of the (sub) channels is mainly historic, in the early days of Wifi the channels were much less wide, which also means lower data rates.

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Any communication system that operates by modulating a carrier (center frequency of a band) must occupy a certain bandwidth around that carrier if it is to transmit information. If you only transmit a steady carrier then there is no information being transmitted and no bandwidth is being used (the spectrum of a steady carrier frequency is just an impulse at that frequency). Once you modulate that carrier (either amplitude, frequency, or phase), the bandwidth increases. Even just turning the carrier on and off will cause the transmission to occupy a certain bandwidth (the faster the carrier is turned on and off more information can by sent but more bandwidth is used). So each channel of the WIFI system is assigned a carrier frequency but in operation its transmission will occupy some portion of the bandwidth around that frequency.

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