WIRE PAIRS

One of the most common communications media is the wire pair, also known as twisted pair. Wire pairs are twisted pairs to form cable, which is then insulated. Wire pairs are inexpensive. Further, they are often used because they had already beeen installed in a building for other purposes or because they are already used in a telephone system. However, they are susceptible to electrical interference, or noise Noise is anything that causes distortion in the signal when it is received. High-voltage equipment and even the sun can be sources of noise.

COAXIAL PAIRS

Known for the sending strong signal, a coaxial cable is a single conductor wire within a shielded enclosure laid underground or undersea. These cables can transmit data much faster than wire pairs and are less prone to noise.

FIBER OPTICS

Traditionally, most phone lines transmitted data electrically over wires made of metal, ususally copper. These metal wires had to be protected from water and other corrosive sucbstances. Fiber optics technology eliminates this requirement. Instead of using electricty to send data, fiber optics uses light. The cables are made of glass fibers, each thinner than a human hair, that can guide light beams for miles. Fiber optics transmit data faster than some technologies, yet the materials are substantially lighter and less expensive than wire calbes. It can also send and receive awider assorment of data frequencies at one timne. The range of frequencies that a device can handle is known as its bandwidth; bandwidth is a measure of the capacity of the link. The broad bandwidth of fiber optics translates into promissing multimedia possiblities, since fiber optics is well suited for handling all types of data - voice, pictures, music, and video - at the same time.

MICROWAVE TRANSMISSION

Another popular medium is microwave transmission, which uses what is called line - of - sight transmission of data signals through the atmospheres. Since these signals cannot bend aroun the curvatures of the earth, relay stations - often athenas in high places such as the tops of mountains and buildings - are positioned at points approx. 30 miles apart to continue the transmission. Microwave transmission offres speed, cost - effectiveness and ease of implementations.Unfortunately, in major metropolitan areas tall buildings may interfere with microwave transmission.

SATELLITE TRANSMISSION

The basic components of satellite transmission are earth stations, which send and receive signals, and a satellite component called a transponder. The transponder receives the transmission from an earth station, amplifies the signal, changes the frequency, and retransmits the data to a receivng earth station. This entire process takes a matter of a few seconds.

If a signal must travel a thousand miles, satellites are usually part of the link. A message being sent around the world probably travels by cable or some other physical link only as far as the nearest satellite, which sends it back to earth to another transmission station near the data destination. Communication satellites are launched into space where they are suspended about 22,300 miles above the erath. That is where satellites reach geosynchronous orbit - the orbit allows them to remain positioned over the same spot on the earth.

MIXING AND MATCHING

A network system is not limited to one kind of link and, in fact, often works in various combinations, especially over long distances. An office worker who needs data from a company computer on the opposite coast will most likely