Black Box Explains... RJ-48.
An RJ-48 plug is often mistaken for RJ-45. On the outside, the two look identical—both are housed in a miniature 8-position jack. The difference is in the pairing of the wires.
In RJ-48, two of the wires are for transmit, two are for receive, and two are for the drain. The last two wires are reserved for future use.
There are three subsets within RJ-48: RJ-48C, RJ-48X, and RJ-48S.
RJ-48C and RJ-48X are very similar. Both use lines 1, 2, 4, and 5 and connect T1 lines. The RJ-48C is more common. The difference is that RJ-48X connectors have shorting bars.
RJ-48S uses lines 1, 2, 7, and 8. It connects 56K DDS lines.