Ethernet Cables: why are they such pains in the buttocks to wire up?

aFatFish · Apr 20, 2016

So, I've just successfully wired my first Ethernet cable, using 568B configuration.

The below are some genuine questions from a newbie who was doing this for the first time (please don't flame me if some of this seems sarcastic or stupid):

1) Why are they wires twisted? Why not ship them straight?

2) Why have different pairs in the 4 different 'channels' to what they'll end up being once connected? e.g. stripy green should be next to solid blue in 568B, so why not ship the cable like this and make life easier? Instead, stripy green and solid green are next to each other and have be separated (and keep moving about like little rascals)

3) Why do the colours jump around? Why is the end configuration not: stripy orange, orange, stripy green, green, stripy blue, blue, stripy brown, brown? This is a logical progression and easy to remember (same order as a rainbow).

Instead the order goes stripy orange, orange, stripy green, THEN blue, stripy blue, THEN green, stripy brown, brown. Why on earth is it all jumbled? Surely the very first person who decided to make this cable couldn't have made the order complicated by design?

4) Why did the order of the cable colours change from 568A to 568B? Why was the cable order of B suddenly better than A?

5) Why now change them again to 568C?

6) Is there a trick to stopping the wires moving about when pushing them in to the Ethernet RJ45 plug head? I've pulled them as straight as possible and resorted to using some pliers, as otherwise the wires kept moving about when inserting them in to the RJ45 plug. Is there a tool that helps keep the wires in the correct order (other than my hands!)?

I really appreciate someone explaining the logic to madness, plus any pro tips on how to make this easier next time!

Thank you,

Tom.

WyomingKnott · Apr 20, 2016

I'll give it a try

1) The wires are twisted in pairs because that helps reduce interference between pairs and from outside sources. TP (twisted pair) is necessary to keep signals clean enough for the high signaling rate, unlike untwisted telephone wire.

2) Stripey-green and solid-green are twisted around each other because they form a single signal pair. Again, this is for noise rejection.

3) Ah-hah! This is the key question, and it comes from how the cabled evolved. First there were two-pin telephone cables. Then came four-pin, two-line telephone cables. To keep things compatible, the center pair was kept together and the second pair sent to the outside. You could plug an old two-pin connecting wire between two four-pin jacks, and instead of losing half of each pair you would have one intact line, by convention the primary line. There was no six-pin connector, and when the eight-pin connector came around they tried to accommodate your idea. But they could not change the pattern of the existing four pins, in order to keep backwards compatibility.

4, 5) Who knows? I think that A and B were originally used on different continents, but I may be wrong. A cable wired A on both ends will work in a B-wired system, so it doesn't matter much.

6) I personally strip off too much of a length of insulation, untwist the wire pairs, and lay them out flat in the correct order. They do indeed squirm around like little rascals.Then I trim the flat set straight across the end, as short as possible. This way, I insert all of them at once into the jack and if I spaced them properly they go in as a unit.

I have a cable tester and test every cable I make before I try to use it. Unlike more experience cablers, I can't count on getting it right every time.

The tolerances for how much wire can be untwisted, how much insulation can be stripped, and how good the contact has to be are pretty tight by the time you get to Gigabit Ethernet. These things are a pain because of how much signal engineers have managed to pack into the wiring, and how fussy it is about the quality of the end-to-end run. I've seen old houses with phone wires run over lamp cord. The phone application is simply less demanding than high-speed data.

EDIT: As to your request for not flaming you, if you are going to use the alias a Fat Fish then we are going to want to put you on the grill!

JackNaylorPE · Apr 20, 2016

Never thought about any of those questions... well at least the 1st 5 I ....

1. Noted the proper order of cables left to right color wise.
2. Stripped the cable
3. Stripped the wires
4. Placed then in correct order
5. Held between thumb and knuckle on forefinger
6. Inserted in connector.

Had no issues with "moving about"

Only issue I had running cables was drilling thru the 8" thick, post and beam construction, locust wood beams (aka ironwood).

I would guess the twisting of the wires helps eliminate interference / noise / magnetic fields. Did a yahoo search:

Noises are generated in signal lines by magnetic fields from the environment. So the noise current in data lines is the result of that magnetic field. In the straight cable, all noise current is flowing in the same direction, just like in an ordinary transformer coil. When the cable is twisted, in some parts of the signal lines the direction of the noise current is the opposite from the current in other parts of the cable. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.

aFatFish · Apr 20, 2016

WyomingKnott :

I'll give it a try

1) The wires are twisted in pairs because that helps reduce interference between pairs and from outside sources. TP (twisted pair) is necessary to keep signals clean enough for the high signaling rate, unlike untwisted telephone wire.

2) Stripey-green and solid-green are twisted around each other because they form a single signal pair. Again, this is for noise rejection.

3) Ah-hah! This is the key question, and it comes from how the cabled evolved. First there were two-pin telephone cables. Then came four-pin, two-line telephone cables. To keep things compatible, the center pair was kept together and the second pair sent to the outside. You could plug an old two-pin connecting wire between two four-pin jacks, and instead of losing half of each pair you would have one intact line, by convention the primary line. There was no six-pin connector, and when the eight-pin connector came around they tried to accommodate your idea. But they could not change the pattern of the existing four pins, in order to keep backwards compatibility.

4, 5) Who knows? I think that A and B were originally used on different continents, but I may be wrong. A cable wired A on both ends will work in a B-wired system, so it doesn't matter much.

6) I personally strip off too much of a length of insulation, untwist the wire pairs, and lay them out flat in the correct order. They do indeed squirm around like little rascals.Then I trim the flat set straight across the end, as short as possible. This way, I insert all of them at once into the jack and if I spaced them properly they go in as a unit.

I have a cable tester and test every cable I make before I try to use it. Unlike more experience cablers, I can't count on getting it right every time.

The tolerances for how much wire can be untwisted, how much insulation can be stripped, and how good the contact has to be are pretty tight by the time you get to Gigabit Ethernet. These things are a pain because of how much signal engineers have managed to pack into the wiring, and how fussy it is about the quality of the end-to-end run. I've seen old houses with phone wires run over lamp cord. The phone application is simply less demanding than high-speed data.

EDIT: As to your request for not flaming you, if you are going to use the alias a Fat Fish then we are going to want to put you on the grill!

Awesome reply! Thank you

I love understanding why things are the way they are, it helps me retain information...rather than my brain just writing it off as "stupid". So thank you, sincerely, for your response.

PS. Good bants on my username! :rofl:

timmoseus · Apr 20, 2016

1) The wires are twisted to help ensure signal quality and help with tension strength.

2) 568B is not the only way to terminate the ends of the cables, some terminations do not even use all of the four twisted pair. So for everybody's sake, the stripey green is twisted with the solid green, etc. Also, again, to minimize signal degradation, the colored pairs are twisted together because one side usually handles sending while it's mate handles receiving.

3) As stated earlier, there are many ways to terminate a cable end. For example, a cross over cable requires one end to have pairs switched to handle proper communication between like devices. Ignoring color you have wires 1 2 3 4 5 6 7 8 on one end and 1 2 6 4 5 3 7 8 on the other end (just a theoretical example, look up cross over cable for actual pairings). Like devices being switch to switch, or router to router, etc. A regular straight through cable like you have created works for connecting routers to switches, or switches to computers, etc.

4) B is not better than A. The thing that matters most is that if you terminate one end one way, terminate the other end the same way. A is still used. I believe some older devices had a problem with signal loss due to the pairings of A, but again, I still see it out in the wild all the time. Important note, if you are a net admin, pick one and always use that one. That way you know all of your cables are terminated the same way.

5) Again, A, B, C, not much difference. Some devices send stronger signals down different wires and the different configurations are attempts at optimizing signal quality and decreasing cross talk between wires.

6) This last one makes me giggle, sorry. No, there is no tool to hold and push the wires into the head, while keeping them in order. It really is an art form that one learns by doing it hundreds of times. I find that using a curved set of wire snips when cutting off the extra bits of the ends helps to kind of mash the cables together. Then you just have to develop your own technique for pushing them in. I have seen some ends that have little separators to hold the wires while you insert them, but I have also found these to be more difficult than doing things the old fashioned way as now you have the four twisted pair and this funny little plastic bit to push into the plug. Maybe you (and others) will prefer them, I do not though.

As an individual with a cabling certificate, bachelor's degree in networking, and five plus years of experience, the best advice I can give you is to spend the extra money on good comfortable tools, and practice makes perfect.

jsmithepa · Apr 20, 2016

aFatFish :

There are, always logical explanations.

Fortunately for CAT cabling, is perhaps a waste of neurons to retain this information since pre-made CAT cables are so dirt cheap (monoprice.com). Besides, retaining this information may not make you more proficient on your next cable build, it takes practice.

HERE looks like an easier RJ45 "system."

timmoseus · Apr 20, 2016

~jsmithepa That is a great video, and tool.

A little pricey though...

jsmithepa · Apr 22, 2016

timmoseus :

Like said, if one is adamant at DIY'ing it.

Am saying this and I am in IT. One day a contractor happened to be on site and I asked him how they seem to do it so effortlessly. What he showed me, is like learning from a master sushi maker, am not kidding, it takes practice, and the way you move your hands and flex your fingers to handle the wires seems to make the different between easy and pulling hairs.

Anyone reading this, JUST %&^$* buy pre-made and be done with it.

WyomingKnott · Apr 23, 2016

Can't. I pulled cat5 through my house. Premades not that long, and pulling cables with a plug on them already would have been a beeyotch.

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Ethernet Cables: why are they such pains in the buttocks to wire up?

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