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Transmission Media

Transmission Media

Communication is one of the most important functions performed by computers. It is easy to understand that a computer must calculate, compare, and store data. It is also easy to see that the computer must input and output data used to communicate with the input and output devices. Also, within the computer, data must be transferred from one locationfor example, from read only memory to random access memory (RAM) , or from permanent storage such as a compact disc to temporary memory, etc. Transferring data, therefore, is just another way of referring to communication.

Communicating from one device to another within the computer is usually done through a bus, which is essentially a set of printed circuit board traces or wires within the computer. Buses are usually local to the device. Very special wire arrangements, called transmission lines, must be used when the required communications travel a significant distance or are executed at a very high speed. For high-speed computers, these transmission line techniques are even used for the internal computer buses.

To understand transmission lines, it is helpful to appreciate what makes a very poor transmission line. When Guglielmo Marconi (18741937) first spanned the Atlantic Ocean with radio waves in 1901, he erected a very long wire as an antenna at both the transmitter site and the receiver site. It was Marconi's goal to radiate energy in the form of radio waves and receive the energy on the other side of the Atlantic. When one desires to transmit signals through long wires, one does not want the radiation and reception of radio waves. A transmission line is the opposite of an antenna; it transmits data from one place to another with no loss of energy by radiating radio waves, called egress , and receives no energy from external radio waves, called ingress .

A very simple transmission line consists of a pair of wires that are kept precisely side by side. Since the wires are often twisted to keep them together, they have earned the title "twisted pair." The twisted pair was one of the first transmission lines originally used, and remains in use for telephone systems. The twisted pair is inexpensive to manufacture and is also used extensively for computer communications.

Twisted pairs are used in many local area networks (LANs) . There is a large selection of standard cable for use in LANs. Some twisted pairs include a shield, which reduces the amount of egress and ingress. The two basic types of twisted pairs are unshielded twisted pair (or UTP) and shielded twisted pair (or STP).

Although twisted pairs can have very little ingress and egress, they are not perfect, particularly at higher frequencies and data rates. The imperfections in transmission lines become more pronounced when the data rates are very high. One transmission line topology that reduces the amount of egress and ingress for very high data rates, extending to the gigabit per second range, is the coaxial transmission line, commonly called coaxial cable or co-ax. In this transmission line design, one conductor is actually a hollow cylinder and the other conductor is placed in the center of the cylinder. Egress and ingress can be reduced to very low levels but coaxial cable is much more expensive than unshielded twisted pairs. Coaxial cable is used for television distribution despite the increased cost because of the very broad frequency range of television signals.

The loss of energy due to radiation and from other losses within the transmission line subtracts from the signal energy in the line. This means that the signal has to be amplified or restored if the transmission line is long. Since there are more losses at higher frequencies, more amplification is required for the higher frequencies. Very high-speed data communications systems will require more amplifiers or repeaters for the same length of cable.

The best transmission line for very high-speed data or wide bandwidth is a glass fiber. It is often difficult to view the glass fiber as a transmission line because the signals within the fiber are not electrical but light waves. But, light waves are electromagnetic waves just like radio. The glass fiber has characteristics exactly like a wire transmission line. However, the transmission rate is generally higher.

Many computer communications applications require a "wireless" communications medium. Of course, in modern terminology it must be understood that wireless also implies "fiber optic-less." Clearly this is the only communications solution for portable and vehicle-mounted devices. Wireless transmission is accomplished through electromagnetic waves, radio, and light. These electromagnetic waves require no physical medium because they are able to flourish through a vacuum better than through any substance. In fact, wireless signals can be partially blocked by common building materials causing difficulties with wireless systems used indoors.

For transmission through short distances, "wireless modems" are used. These modems are low powered radio transmitters and receivers that require no government license. Long distance data communications using radio waves include terrestrial microwave links and satellite data links. These applications involve much higher power transmitters and require government licensing to insure that users do not interfere with other users. Microwave links propagate in straight lines. Depending on the height of the transmitting and receiving antennas, the microwave links are seldom more than 100 kilometers (or approximately 62 miles) apart because of the curvature of the Earth. As with wired communications, repeaters are required to extend microwave communications.

There are two basic types of satellite links, LEO for low earth orbiting and GEO for geostationary orbit. LEO satellites orbit the Earth in less than two hours and are visible to the user for only 20 minutes or so. To provide continuous communications, a number of satellites called a "constellation" is required. Thus when one satellite "sets" or is no longer in view, another satellite can be used for communications. Because the satellites are close to the Earth, typically only 800 kilometers (497 miles) or so, a modest antenna and transmitter power will provide reliable communications. On the other hand, because the user must switch from one satellite to another, a complex system must be employed to switch the communications channel between satellites much like a cellular telephone system in space.

The GEO satellite is always in view and the antenna is pointed at the satellite. Since the satellite never sets, only one satellite is used. Most GEO satellite systems are used by large organizations. This is because the uplink transmitter must use a rather large antenna and needs to be licensed by the government. For small users and individuals, satellite systems are available where the uplink is provided via a conventional telephone line and the downlink is via the satellite. Generally, the uplink data rate required by the individual user is much less than the downlink and this arrangement is acceptable.

For short distance communications, such as from a large room of computers to a LAN, infrared radiation may be used. Low-powered infrared radiation from a light emitting diode (LED) provides the transmitter while the receiver is a phototransistor or diode. This type of infrared technology has been used for many years for remote control devices for consumer entertainment equipment such as television. The range of these systems can be as much as 30 meters (98.5 feet), but the light energy can be blocked easily.

see also Bridging Devices; Satellite Technology; Telecommunications; Wireless Technology.

Albert D. Helfrick

Bibliography

Jamalipour, Abbas. Low Earth Orbital Satellites for Personal Communication Networks. Boston: Artech House, 1998.

Sloan, John P., ed. Local Area Network Handbook. Boca Raton, FL: Auerbach, 2000.

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