How should I terminate this cable?

October 17, 2008 8:31 PM

In need of some networking advice, how to terminate a cable on the switch side. As always more information after the jump.

I am in charge of networking a switch to ten drops. I am currently deployed and the only thing I could get for the rooms is a dual port surface mounted boxes. I will only be using one port per room. The instructions say to terminate at the box in this fashion


T568B
White/Orange
Orange
White/Blue
Blue
White/Green
Green
White/Brown
Brown

I need to figure out how I will terminate the switch end of the cable. Straight Through, Crossover, or what?

posted by hxc to computers & internet (15 comments total)

Straight, but don't use the instructions, check the other ends and copy that exactly. Then its irrelevant what the color/order is, since it's straight through anyway.

Otherwise you're counting on whoever made the other ends following the same instructions as above, and there are at least three common standards for that (T568B, T568A, and some other one I forget that cost me about six hours of going insane in Berlin once.)
posted by rokusan at 8:48 PM on October 17, 2008

568B on both ends.
posted by rhizome at 9:53 PM on October 17, 2008

Sorry, I misread the question and didn't realize that even the terminal ends are under your control. So yes, follow the T568B plan you already have on both ends, straight through.

And as soon as you have two rooms connected to each other via the switch, test it well before continuing. :)
posted by rokusan at 10:07 PM on October 17, 2008

Thanks guys for the speedy help!
posted by hxc at 10:19 PM on October 17, 2008

It definitely needs to be the same at both ends, but isn't T568B supposed to be:

1) Orange/White
2) Orange
3) Green/White
4) Blue
5) Blue/White
6) Green
7) Brown/White
8) Brown

...on both ends? I'm not sure whether the actual pinout really matters as long as it's the same on both sides, but if you've already terminated the room-side jacks, I guess you'd better check the order and make sure you do it the same way on the switch side.
posted by Juffo-Wup at 10:26 PM on October 17, 2008

Then its irrelevant what the color/order is, since it's straight through anyway.

Shouldn't you be following the spec to make sure signalling pairs are twisted together properly? Also, I believe Juffo-Wup has the correct order, not the OP. ( I have never heard of a randomly wired straight cable causing a problem in the real world, but there must be some reason for the funky ordering. )
posted by ghost of a past number at 11:13 PM on October 17, 2008

The reason the order is not correct for an RJ-45 jack in the OP's post is because that order is for the surface mounted boxes, which are punched in or screwed in, not crimped into a jack.
posted by doomtop at 8:04 AM on October 18, 2008

"If 100-base-T you wish to do, split the green and reverse the blue."
posted by Wild_Eep at 8:54 AM on October 18, 2008

Order depends on how the jack is wired.. in terms of pins, yes, 1&4 and 2&3 should be reversed pairs. They may have adjusted the order on the punch-down block, though, to compensate for this and making punchdown easier.
posted by TravellingDen at 1:35 PM on October 18, 2008

Ignore me - I have that completely wrong.
Apologies to all.
posted by TravellingDen at 1:37 PM on October 18, 2008

Not to hijack your question, but I have always wondered why straight thru had to be wired in any specific format. As long as both of ends are the same why would it matter?
posted by Raichle at 6:23 PM on October 19, 2008

Signals are transmitted across pairs of wires. Twisting signalling pairs together is supposed to minimize electromagnetic interference. The first pair uses the two central pins in the jack ( 4-5 ), then the rest are symmetrically arranged. The pairs would have been 1-8, 2-7, 3-6 and 4-5, but this makes crimping the jack unwieldy due to the large distances, hence the standard scheme (1-2, 3-6, 4-5, 7-8).

I read this on the internet, so it must be true.
posted by ghost of a past number at 10:02 PM on October 19, 2008

must be some reason for the funky ordering.

The reason for the specific funky ordering is twofold. One, so that for (pre-Gigabit) ethernet, the pins that a phone line (the center pair) would be wired to use will be non-connected at an ethernet card, to avoid damage when some person plugs the wrong cable into the wrong hole somewhere.

The other reason is to have some standard that people can agree on so that when one person does the patch panel in the wiring closet, and another person does the jacks in an office they wire it correctly.

I try to get the electrical contractors to deal with the data wiring whenever possible (IOW, when they aren't complete yahoos), and they usually get it right thanks to standards.

The kicker is that different manufacturers wire their jacks differently, so you can never be sure without looking which colors go where to get the 568B pinout at the jack pins. That's why you must use specific colors when you're using jacks.

If you're just making an Ethernet cable yourself, it doesn't matter as long as it's consistent on both ends. But again, when using jacks, you never know how they will be wired, so you pretty much have to use 568B (or A, if that's what you prefer and it's marked on the jack, but you'll confuse people who come later if you use A)
posted by wierdo at 7:43 AM on October 20, 2008

>> If you're just making an Ethernet cable yourself, it doesn't matter as long as it's consistent on both ends.

This is incorrect. For short distances, it may not matter, but for longer runs, it does matter and here's why:

Take a short length (a couple feet) of CAT5e or CAT6 cable (not a patch cable) and cut it off so you have a piece to work with. Take the sheathing off so you are left with 4 twisted pairs of wire. Now unravel each pair carefully and gently stretch and smooth them out with your fingers. Now compare the lengths of each color pair: Each color is different. Orange and Green are the longest because they have the FASTEST twists. Blue is next, then brown is the slowest twist and therefore the shortest. Guess which pairs the signals are transmitted over? The green and the orange pairs. The different rates of twist also help minimize cross-talk between the pairs (signal leaking from one pair to another).
posted by Witherwings at 8:24 AM on October 20, 2008

Witherwings, the twist in brown and blue exceeds the Cat3 spec. In the real world, you can do 100 or 200 foot Fast Ethernet runs over Cat3, despite the spec calling for Cat5. I've had to do it in cases where the client wanted 100Mbps but wouldn't shell out for a new run.

Obviously, if you don't keep the pairs together, you won't have good luck no matter what speed you're using. ;)

Not that I ever recommend doing anything that doesn't follow relevant standards, both for reliability reasons and the principle of least surprise.
posted by wierdo at 8:39 AM on October 20, 2008