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Chapter 7

Telecommunications

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What is Telecommunications?

Often used interchangeably, the terms Data Communications and Telecommunications both relate to the process of transmitting data from one location to another.

A data communications system consists of hardware and software that makes it possible for two or more computer systems to transmit/receive data over data communications media, or communications channels.

The data being transmitted over the communications channels can take the form of text, video and audio.

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Types of Communications

Media

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Guided Media

One of the older communications channels consists of a pair of insulated copper wires twisted together to form a twisted-pair cable. Your telephone line is probably twisted-pair cable.

Coaxial cable consists of a copper wire surrounded by a thick band of insulation, a conducting wire mesh, and a layer of insulating/water-proofing rubber or plastic.

Fiber optic cable transmits data through tiny threads of glass or plastic in the form of light pulses.

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Unguided Media

Terrestrial Microwave systems transmit data between stations through the atmosphere in the form of high-frequency radio waves.

Satellite microwave systems transmit data between earth stations. A satellite is a solar-powered electronic device located in geosynchronous orbit approximately 22,300 miles (35,784 km) above the earth. It contains of transponders (small, specialized radios) that receive signals transmitted from an earth station, amplify the signals, and then transmit them back to earth (or to other satellites).

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Terrestrial Microwave System

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Satellite Microwave System

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Communications Media Characteristics

Twisted-pair Data rate - 4 Mbps Repeater spacing - 2-3 km (D), 5-6 km (A) Susceptible to electromagnetic interference Not particularly secure

Coaxial cable Data rate - 500 Mbps Repeater spacing - <1 km (D), 1-10 km (A) Less susceptible to interference Still not particularly secure

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Communications Media Characteristics (continued)

Microwave Data rate - 10-300 Mbps (2-20 GHz) Repeater spacing - line of sight (10-100 km) Susceptible to radio wave interference and

signal attenuation Not secure

Satellite Data rate - 10-100 Mbps (1-10, 12-14 GHz) Repeater spacing - orbit Same susceptibility as terrestrial microwave Not secure

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Communications Media Characteristics (continued)

Fiber Optic Cable Data rate - 2 Gbps Repeater spacing - 10-100 km Not susceptible interference Very secure

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Multiple Media Channels

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Characteristics of Transmission

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Analog vs. Digital Transmission

There are two types of signals for transmitting data: analog and digital.

With analog transmission, the signals form a continuous wave pattern. Data is encoded by varying the amplitude, frequency and/or phase of the wave.

Distinctly different from analog signals, digital signals can be thought of as series of on and off states of an electric switch or a light.

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Modems and Multiplexers

A modem (modulator/demodulator) converts digital signals into their analog equivalents for transmission over communications channels and reconverts them into digital signals for processing.

A multiplexer combines the digital signals of several low-speed devices before converting them to analog signals for transmission. They also convert returning signals back into digital signals and divides them for processing.

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Modem Operations

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Multiplexer Operations

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Measuring Transmission Speed

Transmission speed is measured in two basic ways, baud rate and bits per second.

Baud rate is the number of times that a modem can change signal states per second.

Bits per second is exactly what it says.

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Transmission Modes

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Transmission Channel Bandwidth

A channel’s bandwidth is the range of frequencies that it can transmit. There is a direct, but rather fuzzy, relationship between bandwidth and data rate.

Twisted-pair voice-grade channels have a bandwidth of only a few KHz, and can handle bit rates of around 19.2 Kbps.

Broadband channels, like coaxial cable, microwave, and fiber optics, are more suitable for ISDN (integrated services digital network) and other applications requiring high-speed transmission of large volumes of data.

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Networks

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Networks

When two or more computers are linked by communication media, a network exists.

Networks allow the sharing of information and resources among multiple computers (and their users).

Local area networks (LANs) enable computers to communicate directly over a common physical media, without intermediate switching nodes required.

Wide area networks (WANs) interconnect communications facilities in physically separated locations.

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Network Facilities

Public access networks - are maintained by common carriers AT&T, MCI, Sprint, and Western Union. They provide long distance communications channels to anyone who can pay the fee.

Value added networks - are provided by companies that lease the channel capacity facilities of common carriers, add additional service features, and offer them to the public at a subscription fee. MCI Mail, America Online, CompuServe and Prodigy are examples of such networks.

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Local Area Networks

Local area networks (LANs) are private communications networks that serve the needs of companies located in the same building with two or more computers.

LANs use a dedicated microcomputer, called a file server, that allows other microcomputers to share its resources.

With a file server and high-capacity hard disk, called a disk server, users can access programs and data just as easily as if they were using their own hard drives.

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Types of LANs

Local area networks are classified by their topologies (physical configurations):

Star Bus Ring (Token ring)

The computers linked together in any network are often called workstations, or nodes.

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Star Networks

In a star network, multiple computers and peripheral devices are linked together through a central, or host, computer in a point-to-point configuration. The host computer is typically a minicomputer or mainframe computer.

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Bus Networks

A bus network does not contain a host computer. Instead, all computers are linked by a single line of cable, called a bus.

Bus networks commonly use coaxial or fiber optic cable communication channels.

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Bus Topology

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Token Ring Networks

In order to eliminate errors due to two communications being sent at the same time, the token ring network was developed, in which a pattern of bits, called a token, is passed from one computer to another computer sequentially around the ring.

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Token Ring Topology

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Miscellaneous Telecommunications

Hardware

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Front-End Processors

Front-end processors are usually minicomputers located near the host computer. They are designed to handle communications between the communications sources and the host computer, thus relieving the host computer of those chores.

Back-end processors handle communications between the host computer and its primary hardware components, such as printers, disk drives and tape drives.

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Front-End Processor

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Gateways, Bridges andCommunications Software

A gateway is a hardware device and its software that allows communication between dissimilar networks.

A bridge is a device that allows two similar networks to communicate.

In order for one computer to communicate with another one (directly or by modem), they must both use the same communications protocol, or transmitting/receiving procedures. Communications software provides the necessary standardization.

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Telecommunications in Use

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Voice and Electronic Mail

Voice mail systems are the computer’s version of an answering machine service.

Electronic mail (E-mail) is a fast and generally inexpensive way to send, receive, store, and forward messages electronically.

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E-Mail Systems