1. Physical Lines:
Three types of physical lines are used in telecommunications systems today: twisted-wire, coaxial cable, and fiber optic cable.
Twisted-wire pairs, in which strands of wire are twisted in twos, is the communications technology that has been in use the longest. The telephone system, which carries most of the data transmitted in this country and abroad, still consists heavily of twisted-wire pairs.
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In some cases, several thousand pairs may be placed into single cables, which might connect switching stations within a city. In contrast, only a few pairs are needed to connect a home phone to the closest telephone pole.
Generally, each twisted-wire pair in a cable can accommodate a single phone call between two people or two machines. Since the bulk of the phone system was set up many years ago for voice transmission, it is not the ideal medium for computerized telecommunications.
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Nonetheless, the phone system is fast enough to accommodate many types of computer applications. And since twisted-wire pairs can be manufactured and installed at a very low cost, they are still a common means of linking two points in a communications system.
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Coaxial cable, the medium employed by cable television, was developed primarily to deal with a phenomenon called crosstalk that occurs in twisted wire-pairs during high-speed transmission.
Crosstalk results when a conversation taking place in one twisted-wire pair interferes with a conversation occurring in another. For example, on many phone lines one can sometimes hear background conversations.
With voice communication, this problem isn’t serious. However, crosstalk can inhibit the high-speed transmission required for television reception and some other sophisticated types of communication.
Thus, coaxial cable was developed to fill the need for a fast, relatively interference-free transmission medium. Coaxial cable is also used extensively by the phone companies, typically as a replacement for twisted-wire pairs in important links within the phone network.
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One of the most promising developments in cable technology is fiber optics. An innovation whose potential is just beginning to be realized, fiber optic cable consists of thousands of clear glass fiber strands, each approximately the thickness of a human hair.
Transmission is made possible by the transformation of data into light beams, which are sent through the cable by a laser device at speeds on the order of billions of bits per second.
Each hairlike fiber has the capacity to carry a few television stations or a few thousand two-way voice conversations.
The principal advantages of fiber optics over wire media include speed, size, weight, resistance to tapping, and longevity. For example, it is common for a fiber optic cable to have ten times the data-carrying capacity and one-twentieth the weight of a standard coaxial cable.
Fiber optic cable is especially suitable in situations involving heavy point-to-point transmission between two stations.
2. Microwaves:
Microwaves are high-frequency radio signals. Text, graphics, voice, and video data can all be converted to microwave impulses. Microwave transmission works by what is known as a line-of-sight principle.
The transmission stations need not actually be within sight of each other; however, they should have a relatively unobstructed path along which to communicate.
When one microwave station receives a message from another, it amplifies it and passes it on. Because of mountains and the curvature of the earth, microwave stations often are placed on tall buildings and mountaintops to ensure an obstacle-free transmission path.
Microwave signals can be sent in two ways: via terrestrial stations or by way of satellite. Both technologies can transmit data in large quantities and at much higher speeds than can twisted-wire pairs.
Terrestrial microwave stations must be no more than 25 to 30 miles apart to communicate with each other directly.
This limitation arises because the moisture at the earth’s surface causes interference, which impedes communication between stations. As you can imagine, it is quite impractical to build all the repeater stations needed to connect distant locations.
Communications satellites were developed to reduce the cost of long-distance transmission via terrestrial repeater stations as well as to provide a cheaper and better overseas communications medium than under seas cable.
Communications satellites are placed into an orbit about 22,300 miles above the earth. Because they travel at the same speed as the earth’s rotation, they appear to remain stationary over a given spot.
Both communications satellites and terrestrial microwave stations are most appropriate for transmitting large amounts of data one way at a time. Thus, they are ideal for applications such as television and radio broadcasting.
Because of the long transmission distances involved, they are not intended for rapid-response, interactive communications. Microwave networks are different than networks supporting cellular phones, in which a radio wave is used to reach a cell station that ties into the public phone network.