eros which travel across the circuit digitally. 2-Wire VS. 4-Wire Still another issue relating to transmission is whether the cir- cuit is a 2-wire or a 4-wire circuit. Most dedicated point-to-point data communications circuits are 4-wire, having a transmit pair and a receive pair. Data can also be transmitted on a 2-wire dial up circuit usually at a lower speed. Note: On many circuits, the signals travel part of the way on fiber optic cable, but still termi- nates on copper wire at either end. The telephone in your home uses a 2-wire circuit. When ordering services and trying to get two devices to com- municate there are a lot of questions to ask to ensure that things work the way you expected. See The Telecom Tutorials by Jane Laino for a tutorial on Ordering and Provisioning Telecommunications Services www.cmpbooks.com. 246 * The Telecom Handbook CHAPTER 12 Cable This chapter is about the physical transmission medium called cable, which is fundamental to telecommunications systems and services. The most well thought out telecommunications systems will not work as expected unless the cabling infrastructure is prop- erly designed and installed. There are many variables in both the types of cable and the quality of the installation. Transmission qualities are continually improving as the cabling manufacturers keep up with the demands of the marketplace for sending more information at faster speeds. Copper is by far the most commonly used type of cable in on- premises telecommunications installations. Copper is the best metallic conductor of electricity, other than silver and gold. Silver and gold have other limitations making them unsuitable. Telecom- munications signals are considered to be low-voltage electricity. Unshielded Twisted Pair Copper Cable The most prevalent type of copper cable is UTP, which stands for Unshielded Twisted Pair. The wires come in pairs which are twisted around each other. A different rate of twist for pairs of wires within the cable minimizes the effect of outside interference (crosst