While trying to increase the speed of my internet, I just realized how Bandwidth is understood differently in the networking world. That makes me kind of confused with why increasing a Wifi Bandwidth from 20MHz to 40MHz increases speed, as people always say (provided no neighboring networks). It could be wrong but my first impression is that the signal CAN be sent two times faster, but not necessary. It just has the potential to use the upper half of the Bandwidth. Right? I don't get it. I know why FM and square wave needs more bandwidth in signals and systems though.

Can someone please explain to me how "physically" speed increases? Why more data is sent with wider bandwidth? Please no classic water pipe or highway analogy cause I am not even sure how the water is even behaving or how water is being pumped... Or should the analogy be more "high speed" lanes are available?

BTW is channel 11 theoretically faster than channel 1? Since the frequency is SLIGHTLY higher...?

  • The channel doesn't matter and it comes down to how WLAN works and which standard you're using. Bandwidth is also not understand differently in the networking world. There are just cases where bandwidth is context sensitive. See also List of WLAN channels which has quite a bit of information about how wlan channels work. – Seth Dec 13 '19 at 12:00
  • cause networking people tell me they miss use bandwidth vs throughput vs speed. but thanks. – Asians never die Dec 13 '19 at 12:38
  • "bandwidth for a single mode fiber can carry 10Gbps with a bandwidth of 20Ghz" they said. – Asians never die Dec 13 '19 at 12:44
  • In which case you're talking to people that actually (deeply) care about the physical layer. Most people that have to do with networks don't. Especially not in an optical medium as there is hardly anything you can do to influence. You probably should figure out what kind of "network people" you're talking to. After you've done that you need to take into account what level you're talking about. Talking about the actual physics of a connection is usually a special topic that's different e.g from talking about TCP/IP. – Seth Dec 13 '19 at 13:00

First of all, I'm not a physicist or a mathematician, so take this answer with a grain of (technical) salt. I'm sure people will correct me if I'm wrong ;)

Fourier theory tells us that any complex signal, like a square wave representing bits sent over the air, can be broken down into basic sinusoidal signals. Imagine having two sinusoidal waves with different frequencies. If you combine them together, you get a more complex shape:

Example of Fourier transform

(Author link)

How this translates to data transmission? If you send faster and faster pulses of data (as in higher throughput), your pulses will get shorter and shorter. To make these shorter pulses out of individual sinusoidal waves, will need more different waveforms that make up the individual components than with a low-throughput signal. These different waveforms will have more different frequency components (ie. more bandwidth in Hz).

Using a 40MHz channel will give you more different frequencies (and thus sinusoids) to make these combined pulse trains out of than 20MHz channels. This allows you to have shorter and shorter pulses, effectively increasing the throughput of your channel.

A real-life tradeoff here is that you would need an interference-free 40MHz window to do your communications in. If the interference on this wider window is higher than on a 20MHz window, using a 20MHz window might still result in more throughput (as there could be less corrupted data and retransmissions).

Which WiFi channel wouldn't really matter in a theoretical context. The only parameter that links the sinusoids and resulting waveforms together is the actual bandwidth in Hz. This bandwidth is the same for channel 11 as it is for channel 1, not resulting in a higher throughput on the link.

  • thanks for the reply, i thought the "pulses" you are referring to are actually modulated signals that WLAN standard offers. and i am confused how higher bandwidth can increase the throughput of those modulated signals. sending them faster? i am not sure if they are sent in pulses... i know wider bandwidth gives better quality of the signal. anyway, i am wondering what those "pulses" exactly are. – Asians never die Dec 13 '19 at 12:41
  • The OFDM modulation in WiFi still has to obey the same physical laws as any square wave signal, it's just a bit more complex (multiple vectors etc, to allow more bandwidth). The theory of "having more Hz equates in faster pulse trains, thus more throughput" still stands, there is just an added modulation step. – mtak Dec 13 '19 at 12:44
  • youtube.com/watch?v=rKy5dOl3Et4 got it solved! – Asians never die Dec 13 '19 at 12:56

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