RJ-48 - technical definition

(Registered Jack-48) A telephone connector that holds up to eight wires. It uses the same plug and socket as RJ-45 but has different pinouts. RJ-48C is commonly used for T1 lines and uses pins 1, 2, 4 and 5. RJ-48X is a variation of RJ-48C that contains shorting blocks in the jack so that a loopback is created for troubleshooting when unplugged. RJ-48S uses pins 1, 2, 7 and 8, and is commonly used for 56 Kbps digital lines (DDS). See RJ-45.