Computers: Tools for an Information Age

Computers: Tools for an Information Age
Chapter 7 Networking: Computer Connections Copyright © 2003 by Prentice Hall

Copyright © 2003 by Prentice Hall
Objectives Describe the basic components of a network Explain the methods of data transmission, including types of signals, modulation, and choices among transmission nodes Differentiate among the various kinds of communications links and appreciate the need for protocols Copyright © 2003 by Prentice Hall

Objectives Describe various network configurations List the components, types, and protocols of a local area network Appreciate the complexity of networking Describe some examples of networking Copyright © 2003 by Prentice Hall

Data Communications Systems
Computer systems that transmit data over communications lines such as telephone lines or cables History Centralized data processing in early days Distributed data processing began in late 1960s Networks of personal computers began in 1980s Copyright © 2003 by Prentice Hall

Centralized Data Processing
Places all hardware, software, and processing in one location Very inconvenient and inefficient Input data had to be physically transported to computer Processed material had to be delivered to users

Distributed Data Processing
Uses computers that are at a distance from central computer Local computers had access to central computers Some processing done on local computers, some on central computers Accommodates remote access and remote processing Typical application is a business with many locations Return Copyright © 2003 by Prentice Hall

Network Uses communications equipment to connect two or more computers and resources Distributed data processing systems are networks Local area network (LAN) designed to share data and resources among several users in office or building Return Copyright © 2003 by Prentice Hall

Putting Together a Network
There are great variety of components to choose from Basic Components Sending device Communications link Receiving device Another component is Modem which is needed to convert computer data to signals that is carried by communication channels and vice versa Copyright © 2003 by Prentice Hall

Digital and Analog Transmission
A computer produces digital signals which are presence or absence of electric pulse Some communication lines accept digital transmission Telephone lines through which these digital signals are sent requires analog signals Digital transmission Analog transmission Modem Copyright © 2003 by Prentice Hall

Converting Digital to Analog
Carrier wave can be altered Amplitude (height) of wave Can be increased to represent 1 Frequency (number of times a wave repeats during a cycle) of wave Conversion from digital to analog called modulation Conversion from analog to digital called demodulation Reconstructing the original digital message at the other end Return Copyright © 2003 by Prentice Hall

Types of Modems The way modem connects to phone line varies Most are directly connected to the phone system by a cable External modem separate from computer Can be used with a variety of computers Internal modem inserted into computer Standard on most computers today PC Card modem slides into slot on laptop Roughly credit card size Cable connects modem to standard phone jack Copyright © 2003 by Prentice Hall

Modem Data Speeds Measured in bits per second (bps) Early modems transmitted at 300 bps Fastest current modems transmit at 56,000 bps Because of Federal Communications Commission (FCC) restrictions 56K modem receiving speeds are limited to 53,000 bps while transmission speeds top out at 31,200 bps Actual speed depends on line conditions and other variables Copyright © 2003 by Prentice Hall

Other Communications Devices
ISDN DSL Cable modems Cellular modems Copyright © 2003 by Prentice Hall

Integrated Systems Digital Network (ISDN)
Another line of transmission could be used for digital transmission Special type of telephone circuit and special modem Can move data at 128,000 bps Benefits include being able to connect a PC, telephone and FAX machine to a single ISDN line and use them simultaneously Drawbacks Expensive, especially at installation Not available in all areas Copyright © 2003 by Prentice Hall

Digital Subscriber Line (DSL)
Uses advanced electronics to send data over telephone line at very high speeds Just like traditional analog modems DSL spreads the analog signals over a large range of frequencies Lacks industry standard so manufacturers proposing their own variations Wide range of costs and speeds Always on - no need to dial a connection Can use phone while online Drawbacks You must be within three miles of telephone company’s switching office That office must have DSL equipment

Cable Modems Uses coaxial cable already in place for your TV Very fast transmission speed, especially for downloading Always on: no need to dial a connection Drawbacks All users share a cable segment’s capacity As more users in neighborhood go online, speed decreases No security for individual users or data Anyone on a cable segment can view all the data travelling Purchase a firewall program for security Return Copyright © 2003 by Prentice Hall

Coordinating Sender and Receiver
Sending data to remote location only works if receiving device is ready to accept it Two approaches to keeping devices in step: Asynchronous transmission Synchronous transmission

Asynchronous Transmission
Also called start/stop transmission Start signal transmitted at the beginning of each group of bits Stop signal sent at end of each group Each group typically consists of one character Receiving device gets start signal and sets up timing mechanism to accept the group of bits Used for low-speed communications

Synchronous Transmission
Large block of characters transmitted together The sending and receiving devices are synchronized by having their internal clocks put in time via a bit pattern transmitted at the beginning of message Error-check bits transmitted at the end of each message make sure all characters received Much faster, but equipment is more expensive and complex

Simplex, Half-Duplex, and Full-Duplex Transmission
Depends upon the permissible directions of traffic flow Simplex transmission sends data in one direction only Example: television broadcasting Half-duplex transmission sends data in both directions, but only one way at a time Example: bank teller sends data about a deposit; after data received, a confirmation returns Full-duplex transmission allows transmission in both directions at same time Example: a telephone conversation Typically used for high-speed data communication Copyright © 2003 by Prentice Hall

Types of Communications Media
Wire pairs Coaxial cables Fiber optics Microwave transmission Satellite transmission Wireless transmission Copyright © 2003 by Prentice Hall

Setting Standards Protocol - a set of rules for the exchange of data between a terminal and a computer or two computers Pre communication agreement on how data is to be sent and receipt acknowledged Handled by hardware and software related to networks Needed to allow computers from different vendors to communicate Software development is a problem as different vendors use different protocols. Standards help Transmission Control Protocol/Internet Protocol (TCP/IP) permits any computer to communicate with the Internet

Wide Area Network (WAN)
Networks can be classified by the geographical area they cover Can span the world or link computers across town Metropolitan Area Network (MAN) - networks that cover a single city Components Communications services WAN hardware WAN software Copyright © 2003 by Prentice Hall

Communications Services
Provided by common carriers Companies licensed by FCC to provide these services Two general categories Switched Dedicated Return Copyright © 2003 by Prentice Hall

Switched Service Also called a dial-up connection Establishes a temporary connection between two points when call is placed When call is ended, connection is broken Most common is public telephone system Return Copyright © 2003 by Prentice Hall

Dedicated Service Provides permanent connection between two or more locations Companies can build their own dedicated service (microwave, fiber, etc.) Can also lease circuits from common carrier Called leased lines Most common leased lines T1 and T3 high-capacity digital lines T1 has a capacity of 1.54 Mbps T3 has the capacity of 43 Mbps Quite expensive and justified for high volume continuous traffic DSL or ISDN lines Copyright © 2003 by Prentice Hall

WAN Hardware Normally controlled by one or more host processors (typically mainframe computers) Host computers connect to WAN through front-end processor (itself a computer) Relieves host computer of some communications tasks, freeing it for processing application programs Also performs error detection and recovery function Multiplexer combines data from several slow-speed devices into a single data stream for transmission over a high speed circuit such as T1 line On the other end, the multiplexer breaks the high speed stream into its component parts Copyright © 2003 by Prentice Hall

WAN Software Over a WAN, a PC sends data to a mainframe
A PC can communicate with a mainframe only if the PC imitates a terminal Terminal emulation software allows PC to emulate mainframe terminal Micro to mainframe link If a PC is being used as a terminal then File transfer (FTP) software Allows users to upload/download files Download - to retrieve a file from another computer Upload - to send a file to another computer

Local Area Network (LAN)
A collection of computers that share hardware, software, and data Hook personal computers together through communication media so that each personal computer can share the resources of the others Typically personal computers Typically within an office or building Copyright © 2003 by Prentice Hall

Network Cable Provides a way to connect to network Low-cost LANs connected with twisted pair wire Many connected by coaxial or fiber optic cable Wireless LANs uses low power radio wave transmission Easy to set up and reconfigure, as no cables to connect or disconnect Copyright © 2003 by Prentice Hall

Network Interface Card
Connects each computer to wiring in the network Handles sending, receiving, and error checking of transmitted data Can be a circuit board that fits in one of the computer’s internal expansion slot Notebook computers may have the NIC circuitry built in or may use a PC card Wireless NIC acts as a transmitter/receiver allows user to connect through wireless access point

Routers Bridge allows connection of similar networks (those using the same protocol) A hardware software combination that recognizes the messages on a network and passes messages addressed to nodes on other networks Router directs communications traffic when several networks connected together If network traffic clogged, router can redirect traffic to another route IP switch used in place of router when networks use the Internet protocol Less expensive and faster than routers

Gateway Collection of hardware and software resources Lets a node communicate with a computer on another dissimilar network Primary function is converting protocol among networks Return Copyright © 2003 by Prentice Hall

Wireless Access Points
Also called base stations Connects to a wired network and provides wireless transmit/receive capabilities over a radius of several hundred feet Computers with wireless NICs can connect to the network through these access points

Client/Server Network
Server computer controls network Often has several hard drives, fastest printer Client computer requests services from server Thin client has little or no storage Processing approaches Client/server File server Copyright © 2003 by Prentice Hall

Network Uses Electronic mail ( ) Facsimile (fax) technology Groupware Teleconferencing Electronic data interchange Electronic fund transfers Computer commuting The Internet Copyright © 2003 by Prentice Hall

Computers: Tools for an Information Age

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