Modular or RJ Connectors Guide

RJ-22 (pin) used in telephone handset

RJ-50 10P10C (also known as RJ45 10 pin)

RJ48 configured as 8P8C (same connector as RJ45 but different pinout)

RJ48C is commonly used for T1 lines and uses pins 1, 2, 4 and 5.

RJ48X is a variation of RJ48C that contains shorting blocks in the jack so that a loopback is created for troubleshooting.

RJ48S is typically used for local area data channels/subrate digital services, and carries one or two lines. It uses a keyed variety of the 8P8C modular connector.

RJ48 connectors are fastened to STP (Shielded Twisted Pair) cables, not the standard UTP (Unshielded Twisted Pair) CAT-(1-5).
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RJ-45 (8 pin) relatively uncommon but available in 10 pin

RJ-12 (6 pin or 6 pin Keyed)

RJ-11 (originally 4, today mainly 6 pin)

RJ-21 conenctor or “Champ” connector or “Amphenol” connector (referencing the connector manufacturers)