Cat-5 Cable is a descendant of a long line of phone cabling. Originally, when phones were wired through a building, the cables were single pair, blue/blue white. When subsequent bandwidth demands required an additional pair they were split and laid to either side of the blue/blue white pair, thus [green] [blue] [blue white] [green white]. Again, as needs increased the additional two pairs were added to either side. Since many companies already had plenty of this type of wiring installed it was co-opted for network use when Coax was no longer feasible. To reduce signal noise between the pairs, the wires are twisted inside the cable in specific lengths. This is the main difference between Cat-3 and Cat-5 cable, Cat-5 has more twists and therefore can carry a higher frequency signal farther with less distortion. 10/100 Ethernet only uses two pairs of the 4 available and pins 1 2 3 6 of the RJ45 jack. Often to reduce wiring costs, a company will split the cables at the box and use 2 pairs for data and the other two pairs for either phone or additional data. Gigabit over copper, however, requires the full 4 pairs and Cat-5 or higher. Technically, as long as pins 1 2 3 6 at one end were going to pins 1 2 3 6 at the other end the cable will work, although you can get signal interference over long distances. The standard is adhered to mainly for ease of use, as a technician doesn't have to figure out which wire is carrying which signal at each company they go to. Donald Grayson Systems Administrator SportPaint, Inc.
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Donald Grayson