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I would like to be be able to calculate the delay for communications 
data within TCP/IP networking through cat5e networking cable. 

That is a pretty vague request so I will do my best to clarify (it is very likely I am missing a piece of fundamental electrical know-how here so sorry if that question is moot by its very definition!)

Lets imagine a very basic scenario of two computers with 100Mbps Ethernet NICs connected directly together sending data from one to the other, over a 10 meter piece of cat5e cable. Also lets image the same scenario with a 100 meter cable.

Transferring data at a rate of 100Mbps means we are encoding 100,000,000 bits of data on to the wire every second (ignore fancy encoding techniques that reduce this). That is one bit every 10 nanoseconds.

In the first scenario with a 10m cable length, after 10ns of initiating the transfer, one data bit has been encoded onto the wire as an electrical signal. How much more time will pass before it has reached the other end of the cable? How much later would it be if it was a 100m cable?

I assume this is some sort of propagation formulae taking into account the resistance of the material or similar, but I have no idea where to being, and I am a complete novice at electronics so please go easy on me :)

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    \$\begingroup\$ Speed of electricity \$\endgroup\$ – m.Alin Oct 25 '12 at 16:36
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    \$\begingroup\$ Can't resist to post this link to "A downloadable nanosecond" which shows you how long it is (at best). blog.jgc.org/2012/10/a-downloadable-nanosecond.html \$\endgroup\$ – jippie Oct 25 '12 at 18:03
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    \$\begingroup\$ Mind you that the delay in cabling can be very different from the delay in the network equipment. Hard to predict which will be the dominating part. \$\endgroup\$ – jippie Oct 25 '12 at 18:06
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    \$\begingroup\$ @jippie That is a great link, thank you very much :) \$\endgroup\$ – jwbensley Nov 18 '12 at 22:14
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It's more complicated than that, the propagation speed is going to be based on both the wire and the dielectric constant of the material around the wire. In your case that would be some plastic and air. I'll leave the detailed explanation for someone else, but for a quick check analysis wiki says the propagation delay of cat 5 is 4.8 to 5.3 ns/meter, and the speed is 0.64 c (where c is the speed of light).

So worst case 5.3 ns/m * 10 meters is 53ns. Then 5.3 ns/m * 100 meters is 530ns so the difference in delay is about 477ns.

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    \$\begingroup\$ Did you mean to say "dielectric constant" instead of "magnetic permeability"? Both affect the propagation velocity, but dielectric constant is much more likely to be significantly different between materials you might use to insulate a wire. \$\endgroup\$ – The Photon Oct 25 '12 at 16:46
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    \$\begingroup\$ Ah yeah you're right I did I'll edit that I've been looking at inductors all day and my wires got crossed. +1 for the catch and thank you ;) \$\endgroup\$ – Some Hardware Guy Oct 25 '12 at 17:17
  • \$\begingroup\$ Ah, your opening sentence has given me a good starting point for further research. Thanks for your answer! :) \$\endgroup\$ – jwbensley Nov 18 '12 at 22:15

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