Ethernet wiring of pairs for a 100mbit configuration

Question

In Ethernet wiring, for a 100mbit configuration, if Pairs C & D are connected at one end instead of pairs A & B, would it function?
Computer: Eth
A ==|
B ==|
C ==|========= A : computing system (100mbit)
D ==|========= B

To elaborate, consider T568B straight cabling, where blue pair at source is connected to orange pair of destination, and brown pair of source is connected to green pair of destination, while orange and green pairs at source are left open (source := switch, destination := 100mbit supporting device). Effectively these are pin connections:
4 ------------- 1
5 ------------- 2
7 ------------- 3
8 ------------- 6

`would it function`? No. Ethernet requires a minimum of two pairs. — Greg Askew, Nov 11 '19 at 18:36
Actually 2 pairs are connected. Pairs C and D from one system to pairs A and B of another system — Rajesh T N, Nov 11 '19 at 18:40
10BASE-T and 100BASE-TX require two pairs on each end. So your proposal will not work. — joeqwerty, Nov 11 '19 at 19:39

score 1 · Accepted Answer · answered Nov 11 '19 at 22:14

For T568 the orange pair is pair 2, and the green pair is pair 3. For T568-A, pair 3 (green) is on pins 1 and 2, and pair 2 (orange) is on pins 3 and 6. For T568-B, pair 2 (orange) is on pins 1 and 2, and pair 3 (green) is on pins 3 and 6. Some 100Base-TX devices can do Auto-MDI/MDX (wired straight-through, both ends T568-A or both ends T568-B), and some may need a crossover (one end T568-A and the other T568-B). Horizontal cable must always be straight-through, but patch cables can be crossover cables.

What you claim is T568-B straight cabling simply is not that. T568-B straight-through would look like:

orange-white 1----------1 orange-white
orange       2----------2 orange
green-white  3----------3 green-white
blue-white   4----------4 blue-white
blue         5----------5 blue
green        6----------6 green
brown-white  7----------7 brown-white
brown        8----------8 brown

T568-A looks like:

green-white  1----------1 green-white
green        2----------2 green
orange-white 3----------3 orange-white
blue-white   4----------4 blue-white
blue         5----------5 blue
orange       6----------6 orange
brown-white  7----------7 brown-white
brown        8----------8 brown

What you propose is not any standard, and it will not work.

A crossover cable would look like:

green-white  1----------3 green-white
green        2----------6 green
orange-white 3----------1 orange-white
blue-white   4----------4 blue-white
blue         5----------5 blue
orange       6----------2 orange
brown-white  7----------7 brown-white
brown        8----------8 brown

10Base-T and 100Base-TX only use pairs 2 and 3, but the cabling standard still requires all four pairs to be wired straight through on horizontal cabling. 1000Base-T and 10GBase-T use all four pairs.

For 10Base-T and 100Base-TX, you have DTE (data terminal equipment) and DCE (data communication equipment) devices. DTE examples are PCs, printers, routers, etc. DCE examples are hubs, switches, modems, etc. Absent any Auto-MDI/MDX, DTE-to-DTE or DCE-to-DCE require having a single crossover in the cable channel. That is disallowed for horizontal cabling (must be straight-through), so you use a crossover patch cable on one end.

Thanks, Ron for the detailed answer. Appreciate it. – Rajesh T N Nov 11 '19 at 22:58 — Rajesh T N, Nov 11 '19 at 22:58

Ethernet wiring of pairs for a 100mbit configuration

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