ISDN uses unshielded twisted-pair cable instead of more costly coaxial cable. ISDN lines can be run on existing category 3 wiring to a computer as long as distance limitations are not exceeded. Repeaters can be used when distance limitations are exceeded.
ISDN service from the phone company officially ends at what is called the demarcation point, generally referred to as the demarc. The demarc is usually inside the wall of the customer's office or home. You, the customer, are responsible for the inside wiring from the demarc to your ISDN equipment, including the wall jacks. The telephone company or an electrical contractor can install and maintain the inside wiring for an additional charge.
If you are connecting just one computer to an ISDN line, the wiring requirements can be very simple. Most offices are wired with extra sets of telephone wires, and one of those sets can be used for your ISDN line. However, a number of possible wiring pitfalls exist:
Your "extra" wires might already be in use for an analog line or lines.
Your extra wires might be used to power lighted phone buttons.
Your extra wires might not be connected directly to the demarc.
The wiring between the ISDN wall jack and the demarc might be "daisy-chain" rather than direct wiring.
Direct wiring between the ISDN wall jack and the demarc, also known as a home run, is recommended. For more information on wiring issues, consult your telephone company or an electrical contractor.