Color codes Demarc to CSU/DSU plugs
128K or higher Frame Relay/T1 Plug
The cable from the telco demarc (punch-down block) to the CSU/DSU is just a plain straight-through 8-conductor modular cable with a RJ-48 or RJ-45 plug at the end. For short runs, flat ribbon cable is fine, and one about 6 feet is normally supplied with the CSU. Make sure that the pins are not reversed, looking through the clear plugs you should see the colored wires in the same left-to-right order on both ends. A twisted-pair ethernet cord will also work, and is better for anything over a few feet. It isn't necessary that it be shielded.
An RJ48S biscuit is frequently used to terminate a data circuit. When opening the biscuit, there are 8 color coded wires. Use the internal screws to connect your cable in the following manner:
White/Blue Blue (jack)
Blue Orange (jack)
White/Orange Brown (jack)
Orange Gray (jack)
56K Frame Relay plug
The plug for 56K frame relays differs slightly from the T1 or 128K+. The pairs are on different pins (7 and 8 as opposed to 4 and 5) and the transmit and recieve pairs are swapped.