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Twisted copper

The wire that runs from your ISDN equipment (known as Customer Premises Equipment, or CPE) to the Central Office (CO) of the phone company is called the local loop. ISDN uses the same twisted copper pair that ordinary telephone systems use, but ISDN connects to different equipment at either end. When used for ISDN, the local loop is generally limited to a maximum length of about 18,000 feet, but can be extended with amplification equipment known as repeaters. (Replay animation.)

Data going over the wire

Binary data (ones and zeros) is transmitted over copper cable by applying a voltage at one end and receiving it at the other end. Very basically, an electrical voltage of +V represents a one, and -V represents a zero. Each wire in a pair has the same amount of current, but it is applied in opposite directions so that a complete circuit is made end-to-end. Twists in copper pairs are used to reduce noise caused by electricity. The opposing current cancels out the interference.

Plugging in

All ISDN lines require a network terminator (which, in North America and Japan, is often included in your basic ISDN equipment; in Europe, Canada, and other countries it is a separate device).


The Network Terminator 1 (NT1) is actually the physical and electrical end point of your equipment and the interface to the twisted copper pair that comes into your premises from the phone company. If you need the phone company to test your ISDN line, be sure that your NT1 device (or the equipment it is included in) is powered on and plugged into the service.

Getting the most from a copper cable

The efficiency of copper has a great deal to do with how long the cable is. The longer the cable, the more likely you are to find signal distortion caused by these degrading influences:

  • # attenuation - loss of signal strength or amplitude
  • # capacitance - the variation in the amount of energy stored in the wire
  • # delay distortion - resistance, which causes frames to arrive out of sequence
  • # noise - external sounds and other transmissions on adjacent lines mix with the signal and create errors

In almost every case, a shorter cable improves the quality of the signal.

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