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> but the diagrams don't say what those other lines were for, so if anyone knows... Telephones. Telephones are why. Those other two pairs were often used for voice communication. If you had four-pair station cabling, the pairs were provisioned on the modular jack from the inside out. So line one was the blue/blue-white pair on the inner pins, line two was the orange/orange-white pair on the next two pins, and so on. Ethernet comes along and lots of places where you'd want a network connection already had a phone jack with two pairs unused, so for signal integrity reasons those are moved to the outside and used for data, leaving the inner two pairs where they were to be used for voice. But why 4 pairs in the first place? Just about the time that Ethernet was transitioning from coax to twisted pair, the digital PBX was taking over from key systems (1A2) and reduced the number of wires required for a business telephone from 25 pairs (or more ... secretarial sets often had 100 or more pairs) per station down to 4 (for HORIZON[0]) and later two pairs (DIMENSION and eventually Merlin, Definity, etc.). So if you're wiring a new building, you can just run one CAT-3[1] cable to each desk and use the first two pairs for voice and the second two for data[2]. [0] OK, for the pedants out there, HORIZON wasn't ever very popular and really pre-dated Ethernet, but the telecom world moves kinda slow
[1] Wasn't really CAT-3 until the early 90s
[2] Not on the same jack, but by using pins 1, 2, 7, and 8 for data, you can plug the wrong cable in without risk of hurting the phone or your computer's network card |
But 10BASE-T uses pins 1, 2, 3, 6, thus green and orange pair.