# Help! What is the relationship between bandwidth and bit rate?

#### m_p

Joined Nov 23, 2010
6
hello,

[first of all a big thanks to whoever has produced this website. i am doing a foundation degree in engineering and one of the modules i am studying is electrical and electronic principles. i am mechanical biased and have never studied electrical engineering. it is completely new to me, but this website has been very useful. i have read almost all of volume 1]

anyway, back to my question.
i am working on my maths assignment at the moment. it covers trigonometry and waveforms. one of the questions has left me completely puzzled. i need to explain the following statements:
1. "the bit rate for accurate digital communication is much less than the bandwidth of the channel"
2. "the shape of a digital signal is increasingly distorted as it travels down a transmission line"

i do not have a clue what a bandwidth is. we have not covered this in class and i think out tutor is expecting us to do some research (which i did, but i still cannot understand what the relationship between bandwidth and bit rate is. can anyone help?

#### thatoneguy

Joined Feb 19, 2009
6,349
Wiki on Signal Bandwidth.

To simplify it: How much information could you put on a signal that only changed state once per second? How much could you put on a signal that changed 1000 times per second, or 1 million (1Mhz)?

Example: 10Mbit Ethernet needs a line capable of 16Mhz. 16Mbit Token Ring runs needs a 20Mhz signal line.

As purely digital signals (square waves) move down a long transmission line, they get distorted (think ugly sorta sinewave looking thing). This distortion needs to be "cleaned up" by the receiving side, and the data recovered. This is the reason for distance limits on ethernet runs (200m).

#### m_p

Joined Nov 23, 2010
6
thanks, thatoneguy.
how did you work this out:
"10Mbit Ethernet needs a line capable of 16Mhz. 16Mbit Token Ring runs needs a 20Mhz signal line."
is there a formula for this?

#### m_p

Joined Nov 23, 2010
6
thatoneguy,
you owrote:
"As purely digital signals (square waves) move down a long transmission line, they get distorted"
what cause the distortion?

#### thatoneguy

Joined Feb 19, 2009
6,349
thatoneguy,
you owrote:
"As purely digital signals (square waves) move down a long transmission line, they get distorted"
what cause the distortion?
The inherent inductance and capacitance of the transmission line. It is better to think of it as a bunch of inductors in series with the signal, and a bunch of small capacitors across the signal line. This creates a low pass filter. If you have a square wave signal generator, a spool of Cat5, and an oscilloscope, try it out by injecting the signal on a pair and reading it on the scope (or network analyzer) at the other end.

As for frequencies, those are the stated required capabilities of Ethernet and Token Ring in their RFCs (Request For Comments). They have more on how much capacitance a line can have between pairs as well as a ton of other specifications. If really interested, you can read the RFCs for months.

RFCs define the standards, and everything is laid out in painful detail.