How much binary data is transmitted across a physical wire in one clock cycle?

Question

How much voltage change, what I'm calling binary data, can be transmitted over a wire at one time in one clock cycle? I understand, I think, binary data is what we call it and the wire doesn't really send out a one or a zero. It is conceptual.

I've been reading the question and have a measure of understanding, but not great. I understand there is a frequency at which voltage is transmitted and represented by us visually/logically as binary data. But how much "data" can be transmitted over a wire in one cycle? Is one cycle one voltage transmission (a bit) across a physical wire? Like Morse code but with computers it ends up being really, really fast (Ghz).

Answer 1

Actually, the answer to your seemingly simple question is more complex than you'd readily believe!
The short answer is that one signal at a time can be passed through a single signal wire, in one cycle. The amount of data that symbol represents depends on the protocol used.

The long answer is that:

2-state protocols, like OOK (On-Off Keying), pass only one bit (on or off) per cycle;

1-dimensional multi-state protocols, like FSK(Frequency Shift Keying), PSK(Phase Shift Keying), FM(Frequency Modulation), or AM(Amplitude Modulation), can transfer a few bits of data at once;

Multi-dimensional multi-state protocols, like QAM(Quadrature Amplitude Modulation), can transmit fairly huge amounts of data in a single cycle (I've seen QAM 512 {9 bits per cycle} advertised).

Answer 2

Welcome to SE, Johnny. You have a few terms mixed up. I take it from the question you want a beginner's grasp of what's going on.

Hertz is the number of cycles per second.

A very simple explanation

A (simplistic) way to understand this would be to imagine a signalling system that transmits ones and zeros as pulses.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Transmission of code '1 1 0 1 0' using pulse-width modulation.

Encoding the data

In our simple transmission system a '1' is transmitted using a long pulse. A zero is transmitted using a short pulse. In Figure 1 we show how to encode the signal using pulse-width-modulation and transmit a binary signal.

Data speed

• If we transmit one pulse per second we will have a 1 Hz signal (one cycle/second) and we will transmit one 'bit' (binary digit) per second.
• If we increase the transmission speed we can transmit move bits per second.

For such a simple system you can think of the data speed as being the same as the pulse rate. So to transmit 9,600 bits per second you would have 9,600 Hz.

Limits on data speed

So why don't we transmit everything at high speed? There are several reasons but one of the easiest to appreciate is that cable has capacitance and inductance. The effect of these is to mess up the nice square wave we have generated and instead of a nice square-wave what comes out the other end is more of a triangular wave. As we keep increasing the frequency the signal gets worse and worse until we can't read it reliably.

There are various methods used to maximise transmission speed and reliability and, as the other answers have indicated, it gets highly technical pretty quickly. The simple general rule is that the longer the cable or transmission distance the more degraded the signal becomes and so lower data rates have to be used.

Please clarify your question if I've pitched this answer to high or too low.

Answer 3

As laptop2d and transistor say, NOISE! That's the key concept. Everything else is just signal (sorry, I couldn't resist that).

Back up a minute and simplify the question. Consider just an unchanging voltage on the wire, supplied at one point and measured at another. And let's say that the transmitting (supply) end is a theoretical textbook-perfect power supply. The transmitter drives a certain voltage on the wire. At the receiving (measuring) end, we ask the question, "What is the voltage?" If all we can say is, "Eh, about 1.2V.", that's not a lot of information. If we can truthfully say, "It is 1.23798570520664V, give or take a few femtovolts", then that is a lot more information. In fact, if there was no limit to the precision with which we could supply and measure a voltage (and nothing interfering with it along the way), there would be no limit to how much information we could stuff down a wire.

In the real world, our precision is limited because of noise. There is noise in any real voltage source, noise due to thermal excitation of the wire, noise in the measuring instrument, and so on. There are also other practical limitations because of real-world tolerances on component values, reference sources, and more, but we don't even need to think about those to see that we can't supply and measure a voltage with unlimited accuracy.

So, how do we get more bits down a wire? We trade off accuracy and time. For example, instead of trying to measure a voltage once at femtovolt accuracy, we split the data up and make multiple measurements at, say, millivolt accuracy, spread apart over some period in time. Now things start to get really interesting, because there are so many different ways you can do this, and the reason there are so many different schemes in use is because they each make a different set of tradeoffs with regard to speed, complexity, cost, power, robustness, and what have you. Also, as others have mentioned, once you throw the time element into it, you have a new set of problems in addition to pure noise, because there are distortions and reflections that limit how quickly you can make changes at the source and get a distinct result at the receiver.

So, to get back to the original question, "How much binary data is transmitted across a physical wire in one Hertz cycle?", it all depends on how you transmit and receive it. If you want to dig deeper, the field is called "Communications Theory", and Claude Shannon is its patron saint.

Answer 4

If the data is being transmitted synchronously with a clock shared by the transmitter and receiver, then the number of meaningful voltage changes (0 to 1 and 1 to zero. between clocks) will depend, to a first approximation, on the characteristics of the transmission medium and the impedance of the load.