I’m confused by the concept of channels of different bandwidths and data “flowing” through them.
I think of a wave as having a fixed frequency. Let’s say 2400 MHz, that’s the frequency of the wave. And then you can modulate the wave so it carries data. For digital modulation, that means you vary the amplitude or power level of the transmission.
Okay, so when you have a channel, say a 20 MHz channel, then what exactly do you have? That’s a frequency range. How do you “fill” it with data? Do you have many waves or still one wave, how do you take advantage of the range exactly?
Thanks!
In: Technology
There are many ways to modulate the carrier with data. Let’s look at your 2400Mhz carrier AM radio. If I sent 10 cycles per bit, then I can send 240Mb/sec – so that’s my bandwidth. Lots of radio circuitry can filter out pretty high noise levels if there are 10 cycles in which to figure it out. If I only send 2 cycles for each bit, I have more bandwidth in the channel, 1.2Gb/sec but a tiny bit of noise can cause my filters to make a bit error. While it might seem like 5 times more data, if I have to wait for a packet retransmission, my effective bandwidth in actual use might be much, much lower. The actual bandwidth peak is a complex combination of factors like the shielding of conductors and the sources of background noise.
> And then you can modulate the wave so it carries data. For digital modulation, that means you vary the amplitude or power level of the transmission.
And/or you can vary the frequency or phase. In FM transmission, you would change your 2400MHz signal up or down by up to 10MHz, and your ‘bandwidth’ would be just that – your band width, the difference between the lowest and highest frequency you’re sending.
Other modulation schemes which use phase and amplitude together are in use too. One example is [QAM](https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation).
If you’re transmitting a digital AM signal by pulsing a single carrier frequency on and off, then your bandwidth is the result of a tradeoff based on how fast you’re pulsing on and off relative to the frequency of the carrier wave. If you’re pulsing a carrier wave on and off very fast, approaching the frequency of the carrier itself, then the spectrum of what you’re sending gets quite wide. If you transmit a pure 2400MHz signal at constant level forever, never stopping or starting, then that would be a 0 bandwidth signal, a pure frequency. But when the signal is stopping and starting, the band gets wider than 0. This is just a mathematical consequence of how frequencies work. A great video to get intuitions about this is here: https://www.youtube.com/watch?v=MBnnXbOM5S4
A wave does not necessarily have a fixed frequency. If you want to send information on it, you need to modulate it, as you stated. Unmodulated carriers do not carry information. Regardless of what kind of data you have, you can act on the amplitude, on the frequency, on the phase or on a combination of those. You could even have multiple waves or subchannels.
What matters is, whatever modulation you choose, if you want to modulate data onto the carrier, you end up modifying its frequency. The basic wave (carrier) could still have its original frequency (e.g. AM), but you modulate it with data that has its own frequency band (for example speech with 300 Hz to 3.4 kHz). What results is a modulated wave that needs at least one side band 300 Hz to 3.4 kHz away from the carrier to be able to carry information.
So the bandwidth is the frequency range reserved for a given transmission, where the information is packed.
The best analogy I’ve ever heard is to think of a river. The speed the river flows would be your data speed. The size of the river is your bandwidth. With one boat, you can take full advantage of the rivers speed. When you start adding more boats, they each have to slow down a tiny bit to make room for the others. At a certain point you have too many boats which slows them all down to a crawl because there simply isn’t enough room.
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