How, Physically, Does More Bandwidth Translate Into More Bits Per Second?

14 Aug 2009 7:27:35 AM

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Charlie Uniman___

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How, physically, does more bandwidth translate into more bits per second?

From what I've read, transmitting data over the Internet for example, requires the transmitting device to distinguish 0's from 1's, if you will. I gather that having more bandwidth, i.e., having more frequencies with which to work, means being able to build a wave form, coded as a "1" for example, more quickly than would be the case with fewer frequencies available. Is this the _phyical reason_ why greater bandwidth translates into being able to transmit more bits per second.

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14 Aug 2009 8:27:35 AM

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falcanary___

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Re: How, physically, does more bandwidth translate into more bits per second?

I think the previous answer is geared towards channel allocation and not bit rate vs bandwidth. I will try to clarify the issue.

First, you must understand how FM works and how a digital signal converts to FSK (frequency shift keying). Therefore I will use a simple example.

Each bit takes up some frequency. For example, the bit '1' could take up 5 Hz carrier frequency on the bandwidth spectrum and '0' could take up 6 Hz on the bandwidth spectrum. Therfore, there is a seperation of 1 Hz between bits.

If I send a '1' followed by a '0' then on a meter I should see 5 Hz freqency and then a 6 Hz frequency back to back. We could speed this up to infinity and therefore have these 2 frequencies transmitting all data at infinite speeds.

In the real world however, a '1' may fall around the 5 Hz frequency but not exactly on it. It may be about 5 Hz. Why around 5 Hz and not exactly on it everytime? Because in the real world we have noise in transmission lines. Noise can distort the frequency causing it to be a little less than 5 Hz or a little more than 5 Hz. Imagine if the '1' bit is transmitted but on the receiving end it ends up receieved as 5.5 Hz. What happens now? It's not quite 5 Hz and it's not quite 6 Hz. Noise caused it to move 0.5 Hz away from 5 Hz. It is in the middle of 5 Hz and 6 Hz.

Now the receiving end has to decide if this is '1' or '0'. What are the odds the receiving end gets it right? By slowing down the speed the 1s and 0s that are transmitted , you can hold steady the 5Hz or 6 Hz for that bit. The faster you go, the less accurate the bit will fall exactly where it should on the frequency.

So, what are some possible solutions? Well, you can slow the frequency down or make the frequency seperation larger. For example, make a '1' be 5 Hz and make '0' be 10 Hz. Now you have 5 Hz seperation between bits instead of 1 Hz seperation. Now if the frequency of 5.5 Hz is received, the receiving end can determine that this should have been 5 Hz. It's highly unlikely it was originally transmitted at 10 Hz. Therefore, the receiving end detects a '1'.

But what have you lost by doing this increase in frequency seperation? You have taken up more bandwidth. Every cable has only so much bandwidth and if you have several channels on a cable all transmitting at the same time, you need to limit the amount of bandwidth each channel takes up or you will run out.

So, therefore, since most companies don't want to backdown on speed to the consumer (you), then they upgrade their technology (i.e. cables) to higher bandwidth cables such as fiber optic cables.

I hope this helped. Others, please feel free to expand on this topic. I've only discussed FSK modulation. Other techniques such as PSK, ASK, QPSK, bit stuffing, NRZI, CRC, and FEC codes can improve bit rate over channels.

You can read more about bandwidth limits. Shannon and Hartley came up with a theorem which still holds today and is used quite extensively. I even know someone who worked with Shannon at Bell Labs back in the day. Shannon's theorem is C = BW*log2(1+S/N) where C is the channel capacity in bits per second, BW is bandwidth, S is the signal power and N is the noise power.

WF
Telecom Engineer

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14 Aug 2009 9:10:07 AM

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tomchi___

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Re: How, physically, does more bandwidth translate into more bits per second?

Well bandwidth is used to refer to the number of bits per second, so I think the terminology is part of what is confusing you. The way that things work physically is it's own matter, and depends on the physical characteristics of the communication channel.

Oh, I guess the other reason that this is confusing is that bandwidth has a different, but somewhat related meaning when it comes to EM spectrum.

So let's look at some channels:

Wires: Data going over a wire can be coded in many ways, but in more recent days it has all about hi/low signals at every increasing bitrates.

Fiber Optic: Basically on/off signals with some ability to get more data through (i.e. get more bandwidth) by using different wavelengths of light.

Radio/TV: These are electromagnetic waves in the air, and the FCC regulates the allocation of 'bandwidth'. In this can bandwidth means particular frequency ranges assigned to a company or for a particular application. Depending on how you do your encoded you can wring a whole lot of bits/sec out of your slice of the EM Spectrum thereby getting more "bandwidth" out of your "bandwidth".

Um yeah... hopefully that makes things a little more clear?

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14 Aug 2009 9:52:21 AM

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Roderick T___

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Re: How, physically, does more bandwidth translate into more bits per second?

You've partly answered the question in your question. Let me explain.

In order to transmit data, the transmitting device needs to turn on and off. For example, if you want to send a coded message to your friend across the way, you can use a flashlight that can be turned on and off. However, in order for you send more data in the same amount of time, you'll have to turn the flashlight on and off really fast. The faster you can do this, the higher your frequency, the more data you can send, and thus the higher your bandwidth. That is, the higher your capacity to transmit data. Sure, you can have two flashlights, thus doubling your bandwidth; but your friend would need to double his/her method for getting your message.

All forms of electromagnetic waves have an associated frequency. The higher the frequency, the higher the bandwidth. This is the reason for moving toward using light because it's very high frequency (THz).

It's not necessarily having more frequencies that allow for greater bandwidth. It's also how high the frequencies are. Furthermore, since data has to travel along a medium such as wires or optical fibers, it's usually the properties of the medium which limits the utilization of the full bandwidth.

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How, Physically, Does More Bandwidth Translate Into More Bits Per Second?

Charlie Uniman___

How, physically, does more bandwidth translate into more bits per second?

falcanary___

Re: How, physically, does more bandwidth translate into more bits per second?

tomchi___

Re: How, physically, does more bandwidth translate into more bits per second?

Roderick T___

Re: How, physically, does more bandwidth translate into more bits per second?

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