Why is propagation delay of ethernet cables unaffected by cable length?

Question

I did some searching on how propagation delay in ethernet cables is affected by the cable length. The answer seems to be that the delay is negligible until the cable length reaches a kind of threshold value which imposes a limit on the maximum length of ethernet before it becomes too unreliable. However, propagation delay is distance/speed, so technically a longer cable should have a longer delay. I am assuming the cables have the same diameter.

My guess is that since information is continuously sent in a stream, while it takes longer to initiate the transfer, the rate of data transfer is unaffected. However, the latency would still be slower, which would be a problem. Why is the cable length rarely discussed?

Answer 1

No, there is no threshold as you put it; any length of cable produces a delay and that delay is proportional to cable length. Different cables propagate slower of course and this is largely down to the dielectric of the material between the two conductors (or centre and screen in coax). The higher the capacitance, the slower the speed of propagation.

Regarding maximum length that can be used for a certain data rate yes, there is a "kind of" threshold - basically data gets misshaped the further it has to travel down a cable due to cable losses (resistive and dielectric). At some cable-length and at some data-rate the cable can be deemed to be at the "point of no return" in that statistically the number of data errors incurred are too many to warrant further error correction/detection. Time to get a better cable or different modulation scheme!

Cable length is quite commonly discussed where I work for the reasons highlighted above.

Answer 2

As Andy aka already wrote, any length of cable produces a delay. However by limiting the length you know what the maximum delay is and you can correct for that. Ethernet does this by specifying a minimum frame size in combination with a maximum cable length.

Thus ensure that even two computers on the ends of the cable (and thus the furthest away from each other as allowed) will see that the other is transmitting before they finish their own frame.

Imagine the following setup with two computers and ancient coax:

Both computer #1 and #3 want to transmit something.
Both check the cable and see noone else transmitting.
Both start to transmit their own small frame which will travel down the cable at limited speed.

Neither computer 1 or computer 3 will see the others signal while they are transmitting.
Both will receive a readable frame from the other.

however computer 2 will see an unidentifiable mess rather than one or more valid frames.

The solution employed by Ethernet is to limit cable length to about 200 meters and make a framesize of at least 64 char mandatory. (If the frame is shorter it will be padded).

That way any transmitting from one edge of the network will reach the other side of the network. Any computer (well, any NIC) wanting to transmit needs to check if the line is clear and stays clear from the start of transmission until it has send at least the min. frame size.

Finally getting to your question on Why is the cable length rarely discussed? (in combination with the tag Ethernet):

From ancient times the min. frame size and max. cable length have been defined for Ethernet. This is a standard. Even with UTP we still adhere to those standards. This makes the values always the same and not worth discussing.

Answer 3

Any length of cable imposes a delay which is roughly proportional to it's length. There will also be a degradation in signal integrity, again getting worse with length.

If the signal integrity degrades too much then the receiver will not be able to correctly interpret the signals. For coax and twisted pair Ethernet this is what limits the length of a single "segment".

Large propagation delays can cause incorrect collision detection in a CSMA/CD system. Specifically there are problems if the propagation delays grows larger than the minimum packet length. With a single segment of coax or twisted pair Ethernet that meets the length requirements for signal integrity you won't run into this problem. However you can run into it in two cases.

1. chains of repeaters/hubs (NOT switches): With 10 megabit ethernet the rule was you should have no more than 4 repeaters between the most distant segments in a collision domain. With 100 megabit Ethernet this reduced to 2[1].
2. fibre optic links: The fibre physical layers can maintain signal integrity over a much longer distance. Therefore long fibre links cannot be correctly operated in half duplex mode.

Full duplex links do not use CSMA/CD. Switch ports running in half duplex mode do use CSMA/CD but the collision domain does not cross the switch.

So basically as long as you make sure the length limits for the physical layer are respected, make sure you use switches rather than hubs and make sure any long distance fibre links are running in full duplex mode then you are fine.

[1] With gigabit the rules stayed the same as with 100 megabit because they increased the minimum packet size for half duplex gigabit links but it was irrelevant in practice because gigabit hubs are like hens teeth.