ECN Forum Forums Technical Discussion Electrical Theory and Applications Bandwidth

What is a "simple" definition of bandwidth? Is this term only used with fiber optic cable?

http://www.auburn.edu/helpdesk/glossary/bandwidth.html

Bandwidth - Bandwidth (the width of a band of electromagnetic frequencies) is ordinarily means how fast data flows on a given transmission path. Specifically, it is the width of the range of frequencies that an electronic signal occupies on a given transmission medium. Any digital or analog signal has a bandwidth. Bandwidth is measured in Megabits per second.

[This message has been edited by Joe Tedesco (edited 11-28-2001).]

Thank you Joe.

That reference mentions a general definition that bandwidth is the range of spectrum taken by a signal, but otherwise it seems to be looking at it solely from the point of view of digital signal transmission.

I'm not too happy with the "bandwidth is measured in megabits per second" either. The latter is the rate at which data is transferred, and although everything else being equal a higher data rate requires a greater bandwidth, there are other variables in the equation.

In its purest form, bandwidth is the measure of the difference between the lowest and the highest frequencies employed to transmit the signal, be it analog or digital, and is measured in Hertz (or kHz, MHz, etc).

The term was in use long before fiber-optic cable was invented, and can refer to a signal sent down a twisted pair of wires, a coaxial cable, a microwave waveguide, or broadcast through the air.

For example, the signal sent down a regular phone line is normally limited to a range of about 300 to 3400Hz, a bandwidth of some 3.1kHz, while a good hi-fi system might have a bandwidth of 20kHz or more.

For comparison, an AM broadcast radio signal has a bandwith of 9 or 10kHz, an FM radio signal 200kHz, and a TV signal (U.S. system) about 6MHz.

Paul,

Very good explanations!!!

I also felt the reference was making more of an emphasis on Digital packets, rather than explaining an Analog envelope [which as you know is rather complex as compared to a packet].

The reference from Joe is just what Frank needs to use for FDDI LAN tech, so it's probably better on that angle to keep within the Digital / Binary spectrum as opposed to an explanation of complex Analog envelopes.

Once again - just my $0.02

Scott SET

Scott,

I guess almost everything these days is now angled toward digital signals, as this definition clearly illustrates.

I won't even start the argument about whether to measure bandwidth at -3dB, -10dB, etc.

For anyone still pondering this, just consider that today's 56.6kbps dial-up modems operate over the same 3kHz telephone bandwidth as did the 300 bps modems of yester-year.

Which brings to mind another related point to remember: Despite what some advertisement may say, bits-per-second and baud rate are NOT the same thing.

Scott,

The information Joe provided was exactly what I needed. Beyond that, it's well over my head. I do have one more question. In general, doesn't fiber optic cable provide the greatest bandwidth? That is to say compared to Category 5 UTP cable etc.

Frank,

You're right. In the most basic sense, light on a fiber optic strand has a greater (much greater) bandwidth than voltage and current on a unshielded twisted pair (UTP) of CAT5e or "CAT6" copper conductors.

How much greater is roughly indicated the cycles per second of the light verses the maximum cycles per second of the wire. On Wire, electrical frequency can go up to 200 to 300 million cycles per second before being distorted beyond recognition by the wire's total impedance. Light's frequency is roughly a MILLION times greater than the frequency of the electricity in common UTP.

The way that information is changed into a signal in the electricity or light is in different stages of development, UTP vs. fiber, as is the hardware that modulates the electricity or light, so the million-fold difference above may be less or greater depending on the individual schemes compared. But basically, fiber is a fat pipe for information compared with UTP.

Al

Good question Frank!

I'm a bit 'stuck' on fathoming this word "Bandwidth" as it is used. As Paul described it I can understand how it is a measurement of the range of frequencies employed in the transmission of a signal.

What I am having a problem with though is how it relates to the transmission speed. It seems to me that the word "Bandwidth" should not be measured in Mps. Or that they are not necessarily that directly related.

Al,

I understand your analogy that "basically, fiber is a fat pipe for information compared with UTP." I find it interestingly ironic that the "fastest" Fiber is actually the thinnest.

Bill

Bill,

I, too, stand in awe at the information flow that a little hair of glass is capable of.

Speaking to transmission speed. . .when information is crunched by electronics into a binary stream of "on" and "off" and the binary stream is compressed by an exotic algorithm into a shorter, simpler binary stream, there is additional bandwidth gained over the basic analog bandwidth. The better the compression algorithm, the faster a fixed block of info will travel over a connection of fixed analog bandwidth. The actual signal on the wire or fiber is traveling at near light speed, but that is not the speed in consideration. Rather, it is how long it takes to disassemble a piece of info into a binary code, manipulate the code into a minimum of actual bits to be sent down the pipe, sending down the pipe, reversing the manipulation recreating the original binary code (with error detection and correction) and, finally, recreating the info.

So, there are at least two "bandwidths", the analog and the digital, that together work to develop a "transmission speed". Various noise will create errors that slow down the overall transmission by causing retransmission.

Sounds like a transporter. Beam me up Scotty.