An RJ-48 plug is often mistaken for RJ-45. On the outside, the two look identical—both are housed in miniature 8-position jacks. The difference is in the wire pairing. 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, though RJ-48C is more common. Both use lines 1, 2, 4, and 5, and connect T1 lines. RJ-48X connectors, however, have shorting bars. RJ-48S uses lines 1, 2, 7, and 8, and generally connects 56K DDS lines.
Here’s how RJ-48C pinning compares to RJ-48S pinning: