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Discuss Ethernet point no internet in the Computer and Networking Forum area at ElectriciansForums.net

jcs8215248

Active EF Member
B is crossover which is for network wiring..
A is straight through for device to device communication e.g: pc to pc
In fact A and B colour coding wiring schemes are both equally valid. The important things are to use the SAME standard at both ends of the cable and especially to keep each of the pairs of colour matched wires twisted up to the point where they are connected. A simple cable tester tests DC continuity but not that the wire pairs are correctly matched (e,g Orange+White stripe must pair with White+Orange stripe etc. If they are not matched the cable will not work.

If you wire to the A standard at one end and B standard at then other end you have a "cross over" cable. Just to add to the fun - many computer devices (especially network switches) can detect a crossover cable and compensate for it.

There is no distinction between data and any other use for Cat5/Cat 6 cables. Telephone wiring is less critical and uses different colours (but can be passed over CAT5/6 cabling)

[I am a (retired) computer network designer]
 

jcs8215248

Active EF Member
B is crossover which is for network wiring..
A is straight through for device to device communication e.g: pc to pc
In fact A and B colour coding wiring schemes are both equally valid. The important things are to use the SAME standard at both ends of the cable and especially to keep each of the pairs of colour matched wires twisted up to the point where they are connected. A simple cable tester tests DC continuity but not that the wire pairs are correctly matched (e,g Orange+White stripe must pair with White+Orange stripe etc. If they are not matched the cable will not work.

If you wire to the A standard at one end and B standard at then other end you have a "cross over" cable. Just to add to the fun - many computer devices (especially network switches) can detect a crossover cable and compensate for it.

There is no distinction between data and any other use for Cat5/Cat 6 cables. Telephone wiring is less critical and uses different colours (but can be passed over CAT5/6 cabling)

[I am a (retired) computer network designer]
 

Simon47

Active EF Member
There's (from memory) EIA 258A which splits all the pairs but makes sense for multi-line phones. Then there's EIA 258B which is the same layout as EIA 568A which makes sense for data connections.
I must correct myself here - not bad for a first post eh :oops: Teach me to spout off from memory before the first coffee of the day.

258A (and it's an AT&T standard rather than TIA/EIA) doesn't split the pairs, I'm thinking of another standard whose number I don't recall offhand - and strangely can't find either. AT&T 258A is the same as TIA/EIA 568B which is by far the most commonly used in the computer/networking world. TIA/EIA 568A is the same as 568B except that the orange and green pairs are swapped around. It doesn't matter whether 568A or 568B is used - as long as it's the same at both ends. There is a tiny difference in that the twist rate of the pairs is different, which makes the electrical length of the pairs very slightly different - but I can't think of any situation where that would make any difference.

It's splitting the pairs that does the harm - using one wire from one pair with one wire from a different pair. It'll work for low frequency and DC (as used by the basic 5 led test units) - but it won't work for high frequency data like ethernet.

As an aside, it does cause problems in the telecom world - simply because the runs are so much longer. A few jobs ago we had an office in Milan with a Frame Relay link back to us (at head office). The link was unreliable and we sent a considerable amount shipping out a spare router, getting the other one tested (no fault), and so on. Eventually the telecoms company ran some longer tests and found a split pair in the service - once that was fixed we had no more problems.
 

w0z

Regular EF Member
B is crossover which is for network wiring..
A is straight through for device to device communication e.g: pc to pc
That is not true, however you can make a crossover by wiring to "A" at one end and "B" at the other. If you wired the sockets A to A or B to B either would work.
 

Simon47

Active EF Member
Actually, traditionally ...
Device-switch/hub requires a straight through cable as switches/hubs have their ports connected in crossover pinout (MDI-X). Device-device needs a crossover cable as a straight through cable would connect transmit-transmit and receive-receive. Switches/hubs usually had a switch so one port could be set to device mode (MDI-II) for connecting to another switch/hub.
Well that used to be the case, it generally isn't these days. To start with, gigabit uses all 4 pairs - switching ports between transmit and receive as required. But before that, for many years, devices have increasingly had auto detection and correction (auto MDI-X) which would just take care of it. Still not 100% universal, but I think few people will need to worry about it now.
 

Simon47

Active EF Member
Interesting, I've never seen blue and brown pairs crossed - they've always been wired straight through. In the context of Ethernet, crossover makes no sense for gigabit, and slower speeds (100M and 10M) only use the 1-2 and 3-6 pairs which are crossed over if you wire one end to 568A and the other to 568B.
If you are using the commonest arrangements of PoE, then you really do not want to cross over the 4-5 and 7-8 pairs as then you swap + and - for your power supply. That will at best "just not work", at worst it will let the magic smoke out of your equipment :eek:
 

SWD

Gender neutral
Electrician's Arms
POE you want standard B wiring, I have 3 or 4 POE switches as well as a Hikvision NVR, once the house project kicks in I will invest in a decent Cisco Switch.
 

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