Wire and Cable
The medium is the actual path for the electromagnetic energy of the link or channel of the communication system. Through the medium, the energy representing the data of the sender can reach the receiver. This path can take many forms : an electrical conductor such as wire, vacuum or optical fiber.
Copper is an inexpensive metal that is easily made into wire, Fortunately, it is also an excellent conductor of electricity. When separate wires are collected into bundles and are given special protective jacketing, the term 'cable' is often used.
The main types of wire and cable used in data communications are (a) Twisted pair (b) Multiconductor flat cable and (c) Coaxial cable and are represented in Fig
Twisted pair cable is the simplest and lowest cost cable. It consists of two insulated wire twisted around each other in a continuous spiral as shown in Fig a. The wire is twisted to minimise the external noise. Twisted pair is used between telephones and the central office. It is difficult to use, when many signals must be brought from one place to another.
The multiconductor flat cable consists of many parallel wires in a common plastic jacket as shown in Fig. b. A cable of this type can have any number from 10 to about 50 wires. All the wires are grouped mechanically and they can be used with a single connector at each end. Flat cable is more expensive than twisted pair.
For some applications, coaxial cable (coax) must be used. It consists of a solid-centre conductor surrounded by a plastic insulator such as teflon. Over the insulator, is a second conductor, a tubular braid or shield made of fine wires. An outer plastic insulation protects and insulates the braid. It is fairly expensive to buy and can be difficult to install, because of its mechanical stiffness and thickness.