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Chapter 6

Communications and Networks

After completing this chapter, students will be able to:

• Define the term communications
• Describe several uses of communications technology
• Describe the basic components of a communications system
• Describe the various transmission media used for communications channels
• Describe the different types of line configurations
• Describe how data is transmitted
• Describe the functions performed by communications software
• Describe commonly used communications equipment
• Explain the difference between local and wide area networks
• Explain the use of communications protocols

What is Communications?

Communications, sometimes called data communications or telecommunications, refers to the transmission of data and information between two or more computers, using a communications channel such as a standard telephone line.

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Examples of Communications

• Electronic Mail (e-mail)
• Voice Mail
• Facsimile (Fax)
• Telecommuting
• Videoconferencing
• Groupware
• Electronic Data Interchange (EDI)
• Global Positioning System
• Bulletin Board Systems (BBSs)
• Online Services
• The Internet

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What are the basic components of a communications system?

1. Computers (or sometimes just input or display devices) that originate and receive the data involved.
2. A communications channel over which data is sent.
3. Communications equipment that assists in sending and receiving the data.
4. Communications software that helps control the functions of the system.

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Transmission Media

1. Twisted-pair cable: oldest and most common transmission medium; consists of strands of insulated copper.
   advantages: low cost
   disadvantages: relatively slow and susceptible to electrical interference that can garble data and make it generally unsuitable for high-speed data transmission
   
2. Coaxial cable: consists of copper wire surrounded by several layers of insulation.
   advantages: can transmit larger amounts of data at faster speeds than twisted-pair wire and does not suffer from electrical interference.
   disadvantages: size - layers of insulation make cable thick and harder to install
   
3. Fiber-optic cable: created by binding together hundred to thousands of strands of smooth, clear glass fiber that are as thin as human hair. Data is transformed into pulses of light emitted by a laser device about the size of a pinhead and can be transmitted at blinding speeds.
   advantages: faster, smaller, lighter and more durable than wire-based; unaffected by magnetic or electrical fields.
   
4. Microwave systems: broadcast data as high-frequency radio waves, much like radio signals; can transmit large volumes of data over long distances extremely quickly.
   disadvantages: signals travel in a straight line, do not curve so microwave stations (earth stations) need to be placed close to one another (max. of 25 - 30 miles) and positioned in high places to help ensure a relatively unobstructed transmission path.
   
• Communications satellites: act as relay stations for earth stations, receiving data, amplifying it and then retransmitting it to another earth station. First launched in the 1960s.
  advantages: relatively inexpensive, high quality transmission medium for long-distance and overseas telecommunications.
  
5. Wireless transmission: radio and light waves: uses one of three techniques to transmit data (carrier-connect radio, infrared light beams, or radio waves).
   
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Line Configurations

Point-to-point lines: direct line between sending and receiving device

1. switched line: uses a regular telephone line to establish a communications connection. Each time a connection is made, the telephone company switching stations select the line to be used for the call. The process of establishing the communications connection between the sending and receiving communications equipment is sometimes referred to as a handshake.
2. dedicated line: a line connection that is always established

Multidrop lines: commonly used to connect multiple devices along a single line to a main computer (host computer).

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Characteristics of Communications Channels

Types of Signals

1. Digital: uses separate, on-off electrical pulses to create a square rather than a continuous wave; pulse transmitted represents 1 bit, absence of pulse interpreted as 0 bit. (can transmit data faster and more accurately than analog signals; computers can store and process digital signals and communicate digitally)
2. Analog: take the form of a continuous wave within a certain frequency range (traditionally TVs, telephones and radios)

Most existing communication infrastructure is set up to use only analog systems (telephones lines, coaxial cable, and microwave). This is solved by using a modem (can translate computers digital signals into analog signals and then reconvert them to digital signals after they have been transmitted over an analog system). However, a disadvantage of the modem is that it can not deliver the full benefits of digital signals.

Transmission Modes

1. Asynchronous: data is transmitted 1 byte (character) at a time; relatively slow and not appropriate when large volumes of data need to be transmitted quickly
2. Synchronous: transmits number of characters at a time in blocks; uses timing signals to synchronize the sending equipment with the receiving equipment; requires more expensive equipment than asynchronous transmission, but is faster because there are fewer intervening bits slowing down the transmission of data

Direction of Transmission

1. Simplex: data can travel in only one direction
2. Half-Duplex: allows data to travel in both directions but in only one direction at a time (i.e. marine radio)
3. Full-Duplex: data can be sent in both directions simultaneously; just like speaking and listening at the same time over the telephone

Transmission Rate

The amount of data that can be sent through a communications channel is directly related to the frequency of the signals then channel carries. Frequency refers to the number of times a wave repeats itself, or how many times it completes a cycle, per second.

More waves per second = more data being sent

A channels transmission rate is also a function of its bandwidth (the difference between the highest and lowest frequencies that the channel can carry).

Wider range of frequencies that channel has available = more data transmitted at one time

Measured by bits per second (bps) or by baud rate (the number of times per second that the signal being transmitted changes in some predetermined manner.

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Communications Software

Terminal emulation: (included in most communications software packages) enables your microcomputer to trick the mainframe into thinking it is communicating with a terminal

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Communications Equipment

1. Modems: make it possible to transmit digital signals over transmission media that are designed for analog signals.
   A modem attached to a computer on the sending end converts the computers digital signals into analog signals through a process called MODULATION.
   DEMODULATION refers to the modem attached to the computer on the receiving end which then converts the analog signal into digital signals.
   
   MOdulation + DEModulation (modem)
   
2. Multiplexors: combines two or more input signals from several devices into a single stream of data and transmits it over a communications channel
3. Front-end processors: handles communications management for the main computer (host computer) in a large telecommunications network; generally performs a number of tasks such as error control, polling (checking to see if data is ready to be sent) and routing data
4. Network-interface cards: circuit card that fits in an expansion slot of a computer or other device (printer) so the device can be connected to the network
5. Wiring Hubs: allows devices to be connected to the server; acts as a central connecting point for cables that run to the server and each of the devices on a network
6. Gateways: allows users on a LAN to communicate with a mainframe or dissimilar network
7. Bridges: connects two or more LANs based on similar technology
8. Routers: used to route messages through several connected LANs or to a WAN

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Communications Networks

Local Area Networks (LANs)

Combination of hardware, software and communications channels that connect two or more computers within a limited area.

LAN Applications

1. hardware resource sharing
2. software resource sharing
3. information resource sharing
   

Servers

1. File-server: high-speed, high-capacity microcomputer or workstation that helps manage the network, processes communications and allows users to share data. programs and peripheral devices
2. Client Server Networks: file server distributes programs and data to other microcomputers; more speed and power but adds expense and complication
3. Peer-to-Peer Networks: computers can communicate directly with one another and do not need to rely on a central host computer or file server to control the network; provide basic network services such as file and print sharing; much less expensive and less difficult to administer than those set up with file servers; not good for more than 25 computers, can cause network to be bogged down

Network Operating Systems (NOS)

System software that makes it possible to implement and control a local area network and allows users to use the files, resources, and other services on that network (such as e-mail)

Tasks include:

1. administration
2. file management
3. printer management
4. security

Wide Area Networks

Cover a large geographic region by using one or more of a variety of communications channels (i.e. Internet)

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Network Configurations

LAN topologies: physical layout, or shape, of a network

1. Star Network: host computer connected to a number of smaller computer and other devices (star pattern)
2. Bus Network: links computers and other devices along a single communications channel; composed of twisted-pair wire, coaxial cable or fiber-optics cable (Often use a proprietary technology called Ethernet). Broadcast through a communications channel in both directions, to the entire network. Network software makes sure it only gets where it is intended (communications channel can only handle one message at a time). If the message (packet) runs into another message at the same time, a collision occurs. Carrier sense multiple access with collision detection (CSMA/CD) is used to detect collisions and retransmit data.
3. Ring Network: links all network devices via a communication channel that forms a closed loop. Each computer can communicate directly with any other computer through the ring. Avoids data collisions by creating an electronic signal called a token that circulates around the network and must be attached to a message being sent to other devices. (A disadvantage is that it is sensitive to single link failure. Any link goes down, and it shuts down the network).

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Communications Protocols

1. Ethernet: most widely used network protocol for LANs; based on a bus topology; packet of data can be sent in both directions along the bus whenever a node (any device directly connected to the network) is ready to transmit
2. Token Ring: Avoids data collisions by creating an electronic signal called a token that circulates around the network and must be attached to a message being sent to other devices.

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