INTRODUCTION TO TRANSMISSION MEDIA
The basic functions of media are to carry a flow of information, in the form of bits and bytes, through a LAN. Networking media are considered Layer 1 components of LANs. You can build computer networks with many different media types. Each media has advantages and disadvantages.
A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wire is twisted together in a helical form just like a DNA Molecule. Twisting is done because two parallel wires constitute a fine antenna. When the wires are twisted, the waves from different twists cancel out, so the wires radiate less effectively.
Twisted pair can be used for transmitting either analog or digital signals. The bandwidth depends upon thickness of wires and the distance traveled, but several megabits per seconds can be achieved for a few kilometers in many cases.
Twisted pair cabling comes in several varieties, two of which are important for computer network. Category 3 twisted pair consists of two insulated wires gently twisted together. Four such wires are typically grouped in a plastic sheath to protect the wires and keep them together. The more advanced Category 5 twisted pair have more twists per centimeter which results in less crosstalk and better quality signal over long distances making them more suitable for high speed computer communication.
The most common application of twisted pair is the telephone system. Nearly all telephones are connected to telephone company office by a twisted pair. Twisted pair can run several kilometers without amplification but for longer distances repeaters are needed.
Coaxial cable has better shielding than twisted pair, so it can span far distances at higher speeds. Two kind of coaxial cable are widely used. One kind, 50 ohm cable is commonly used when it is intended for digital transmission from the start. The...