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I am very confused about when to use bandwidth as bit rate and where not. Are these terms one and the same?

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closed as unclear what you're asking by jojek, Laurent Duval, Peter K. Mar 21 '16 at 11:47

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  • $\begingroup$ Could you please provide more details? Bandwidth and bit rate can be used in different contexts, with slightly different meanings $\endgroup$ – Laurent Duval Mar 19 '16 at 12:32
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In communications engineering, bandwidth is the measure of the width of a range of frequencies, measured in Hertz.

Rate is the number of transmitted bits per time unit, usually seconds, so it's measured in bit/second. Equivalently, it can be given in symbols/time unit.

The rate is proportional to the system bandwidth. The Shannon Capacity is one theoretical way to see this relation, as it provides the maximum number of bits transmitted for a given system bandwidth in the presence of noise.

Also, in the specifications for each communication's standard, you can find the exact relation between system bandwidth and achievable rate (which is smaller than the Shannon Capacity), and these depends on other parameters such as type of channel and modulation order, but the two magnitudes are still proportional.

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As hinted by Mr Duval, the term "bandwidth" has been adopted by networking engineers to mean something similar to "transfer rate" (just as "wideband" in networking tends to mean "high speed"). You've tagged your question "digital communications", though, where the term is very precisely defined.

Bit is the unit of information in information theory. Intuitively, a bit is the information conveyed by the answer to a yes/no question, where each answer is equally likely.

In communications we want to transfer information from a transmitter (information source) to a receiver (information sink). The information transfer rate is measured in bits per second. Informally, you can think of this as the number of zeros and ones that a system can transmit per second.

However, it turns out that information is transmitted and received using physical, analog signals (voltages or electromagnetic (radio) radiation). These signals pass through a physical medium, called a "channel", that acts like a filter: only certain frequencies make it from the channel input to its output. So, the channel is said to have a bandwidth, just like any filter.

One of the main jobs of communications engineers is to transfer information as fast and reliably as possible over a channel of given bandwidth. Basically, the problem is how to design a physical signal that:

  • Fits over the channel bandwidth
  • Conveys bits at a high rate
  • Does so reliably (with few bit errors at the receiver)
  • Does so at feasible complexity at both transmitter and receiver.

The design of such signals is, in a broad sense, covered by the term "modulation".

I hope this is clear -- please comment if you need clarification.

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  • $\begingroup$ So, is maximum bit rate the bandwidth of the channel? And what effect does a sampling frequency has on deciding bit rate? $\endgroup$ – user20119 Mar 20 '16 at 2:16
  • $\begingroup$ I've tagged it "digital communications" :) because the original tag was "dsp core" (which is always misused, and which should probably be deleted altogether). $\endgroup$ – Matt L. Mar 20 '16 at 8:22
  • $\begingroup$ Thank you all for all your views...I am fresh to stack exchange and it's application. This is my very first question. I am not a native English speaker. Apologies for anything wrongly done..Thank you!! $\endgroup$ – user20119 Mar 20 '16 at 18:33
  • $\begingroup$ What bothers me actually is that I don't manage to relate them to each other in a formula. What formula relates both of them (bandwidth and bitrate)? I have read somewhere that increasing the bitrate decreases the bandwidth. But I don't see how. $\endgroup$ – LandonZeKepitelOfGreytBritn Jan 30 '17 at 17:14
  • $\begingroup$ @trilolil The bandwidth is related to the pulse (or symbol) rate: $R_s \leq 2B$. Then, the bit rate is related to the pulse rate: $R_b = k R_p$, where $k$ is the number of bits carried by a symbol (e.g. 4 bits per 16-QAM symbol). $\endgroup$ – MBaz Jan 30 '17 at 17:29
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Bit rate is the number of bits transferred/processed per second. Bandwidth in measured in bits per second. So, you confuse distance with meters. In fact, you should confuse the bandwidth with throughput.

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  • $\begingroup$ Yeah I get you..Could you plz extend your views so that we could get the deeper meaning of it, like the bits processed per sec and bandwidth dimension..how does this really work in communication? $\endgroup$ – user20119 Mar 19 '16 at 13:58
  • $\begingroup$ @user20119 Learn what is one bit. Learn what is a second. Then put two and two together. It is not different from speed concept. $\endgroup$ – Valentin Tihomirov Mar 19 '16 at 16:35
  • $\begingroup$ What about modulation and bandwidth in Hz? $\endgroup$ – Matthias W. Mar 19 '16 at 20:06
  • $\begingroup$ @MatthiasW. I do not know. I think that bandwidth is the infromation you cant transfer per second. I guess that you somehow can measure it in Herts (the wider is your channel, the more information you can pass). But bps is the simplest measure. I do not bother about hertz as long as wikipedia prefers bps. Wikipedia says that it is spectral bandwidth which is measured in Hz and it is related to Nyquist. $\endgroup$ – Valentin Tihomirov Mar 19 '16 at 20:20

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