Communications channels

If data is to be transmitted, a pathway or channel is required. The communications channel is the link or line through which the data is transmitted. There are three primary types of communications channels in use today— wire and cable, microwave, and satellite.

Wire and Cable

A channel makes use of a variety of wire and cable media types including twisted pair wire, coaxial cable, or fiber-optic cables. Twisted-pair wire is simply two strands of copper wire and is widely used in communications systems, including the phone system. Communications channels that require higher speeds of transmission use coaxial cable or fiber-optic cable.

Microwave

Unlike transmissions that use the various types of wire media, microwave is sent through the air in the form of electromagnetic waves. One problem with microwave transmissions is that they must be sent in a straight line. Therefore, microwave transmission positions must be established in direct line-of-sight configurations. Because the surface of the earth is curved, microwave relay stations must be atop high towers a few dozen miles apart.

Satellite

When data is transmitted via a satellite communications system, the transmission is relayed by a satellite that is placed in orbit 22,300 miles above the earth’s surface. At that altitude, the satellite can rotate at the same speed as the earth, maintaining a constant position above one point on the earth’s surface. Equipment aboard the satellite receives the transmission, amplifies it, and retransmits it back to earth. Although these systems are very expensive to put in place, the amount of information that can be transmitted and the fact that a set of properly placed satellites can almost instantly transmit to any place on earth, is resulting in constant expansion of satellite-based communications.

14. Ответьте на вопросы:

1. What communications channels are used nowadays?

2. Why must microwave relay stations be atop high towers?

3. What wire and cable media types do you know?

4. How is microwave sent?

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CONNECTION OPTIONS

Data communications lines can be connected in two ways, point-to-point and multipoint.

Point-to-Point Connections. Two computer devices may be connected with no other devices on the line. This configuration is known as a point-to-point connection. This type of connection may be switched line, as used by the phone system. The phone company directs the call and establishes the connection. When the transmission is completed, the line is disconnected. Alternatively, the transmission may use a dedicated line that is never disconnected.

Multipoint Connections. Often, several computer devices share the same channel. This is known as a multipoint or multidrop configuration. These systems are designed to establish communications between a number of devices and involve the use of some type of controller to manage the traffic of transmitted data on the shared line.

16. Ответьте на вопросы:

1. What does a phone company do?

2. What type of connection is used by the phone system?

3. What is a multipoint configuration?

4. What does the equipment aboard the satellite do?

5. Why is a satellite communications system useful?