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* RJ9 and RJ45 are not registered jacks for customer interfaces
† RJ11 and RJ14 use the same colors as RJ25, but fewer pairs
‡ 10-base-T and 100-base-TX twisted-pair Ethernet use only pairs on pins 1 and 2 (for "transmit across", TX) and 3 and 6 (for "receive across", RX) pairs
Cable Handling and Storage
Use care when pulling cables with these small connectors, as their retention and release tabs will snag and break easily. To avoid tangling stored cables, I coil and stored them in resealable transparent plastic bags.
Flush Wall Plate Orientations
According to a former Pacific Bell engineering vice president (and former neighbor of mine), flush wall-mounted modular telephone connectors (such as RJ11W) were intended to be mounted with the release tab on top so that field technicians could – with the aid of an instrument such as a ruler – release connectors from behind desks or other furniture. It is now common practice to orient these connectors with the release tab on the bottom and contacts on top, apparently to protect the contacts from deterioration due to environmental factors. (Similarly, it is now apparently common practice to mount three-conductor Edison power connectors with the ground pin at the top.)
Polarity: Origin of "Tip" and "Ring" Designations
The terms tip and ring refer to the electrical polarity associated with the parts of the quarter-inch diameter cylindrical tip ring-sleeve plugs used by telephone operators. For safety – because the the tip is most likely to touch something other than intended when a connector is handled – the tip is connected to the pair's wire nearest ground potential. To protect copper wires from corrosion caused by electrolysis, the ring's polarity was changed from positive to negative (nominally -48V).
An Ethernet crossover cable, for example, may be constructed by swaping the "transmit across" (TX) and "receive across" (RX) pairs at one end of a cable, but the same polarity must be maintained for each. (Note that crossover cables for 1000base-TX interfaces – "gigabit Ethernet" – must also swap pair 1 with pair 4, and that some 1000base-TX interfaces include automatic MDI/MDI-X configuration that therefore do not require crossover cables.)
See also the A.T.&T. Monopoly Bumper Sticker.
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