· Web view3 Cyclic redundancy check (CRC) is a method of checking for errors in data transmission using a division process. The data is divided into predetermined lengths and divided

Heinemann Information Processes and Technology: HSC Course


The answers below are, in many cases, only a brief guide to a full answer. Some of

the answers in the learning activities have not been provided as they involve the

student investigating, forming opinions, working in a group or completing practical

tasks on the computer.

Chapter 3: Communication systems

Exercise 3.1

1 The five basic components of all communication systems are:

Data source―produces the data to be sent.

Transmitter―encodes the data into a signal suitable for a transmission

medium.

Transmission medium―a channel in which the signal is transmitted to

the destination.

Receiver―decodes the signal back into the original data or an

approximation of the data.

Destination―the receiver of the information.

2 A protocol is a set of rules that governs the transfer of data between computers. It

defines how the information is transmitted and how the errors are detected.

Handshaking is an agreement about which protocol to use, that results in the

exchange of information. It is a series of signals that flow between devices during

data transmission.

3 The OSI reference model is an internationally accepted standard for protocols. It

divides data communication into seven layers. Each layer expresses the standard

using a protocol.

Copyright © Pearson Australia (a division of Pearson Australia Group Pty Ltd)

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4 Handshaking involves sending signals to indicate the type of protocol to be used.

The transmitting device will send a signal and wait for an appropriate response.

When two devices successfully handshake, then a connection is made.

5 Software flow control uses a special code sent with the data. It is used for long

distance communication.

6 The bandwidth is the capacity of the channel or transmission medium.

7 Baud rate is the maximum number of data symbols or electrical signals that can

transmitted in one second. Bits per second (bps) is the number of bits that can be

transmitted in one second. Baud rate is different from bps as a data symbol can

contain more than one bit of data.

8 Parity checking is a method of checking for errors in data transmission using an

additional bit called a parity bit. This bit is used only for the purpose of

identifying whether the bits being moved have arrived successfully. When an odd

parity is chosen, the number of 1s in the nine bits must be odd. This means that if

the number of 1s in the first eight bits is even, then the parity bit must be 1 to

make it odd. If the number of 1s in the first eight bits is odd, the parity bit must be

0 to keep it odd. The sender and receiver can also decide to send an even parity;

then the number of 1s in the nine bits must be even.

9 Cyclic redundancy check (CRC) is a method of checking for errors in data

transmission using a division process. The data is divided into predetermined

lengths and divided by a fixed divisor (prime number). The remainder of the

calculation is attached and sent with data. When the data is received, the

remainder is recalculated.

10 Five common parameters are bps, data bits, parity, stop/start bits and flow control.

LEARNING ACTIVITIES1. a Sending a letter:

Data source―William writing the letter.

Transmitter―inserting the letter into an envelope and putting it into a post

office box.

Transmission medium―road system (rail/air transport?).


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Receiver―receiving the envelope in the letter box and opening the letter.

Destination―friend reading the letter.

b Using a fax machine:

Data source―Julia drawing a diagram.

Transmitter―sending the diagram using the fax machine.

Transmission medium―telephone line.

Receiver―receiving the diagram from their fax machine.

Destination―partner reading the directions to the conference venue.

2 The parity bit would be 1 for an even parity. Parity checking would not have

detected the error as there are two errors in the received character, and this results

in the expected even parity.

3 Cyclic redundancy check (CRC) is a method of checking for errors in data

transmission using a division process. The data is divided into predetermined

lengths and divided by a fixed divisor (prime number). The remainder of the

calculation is attached and sent with data. When the data is received the remainder

is recalculated. Another method of error checking is checksum. Checksum is a

method of checking for errors in data transmission by counting the number of bits

in a data packet. A data packet is created by dividing the data into smaller groups.

The count of the bits in a data packet is attached to the data packet. It is used by

the receiver to check whether all the bits have arrived successfully. If the count

matches, it is assumed that a complete transmission was received. CRC is a more

accurate method of detecting errors than checksum.

4 Time = (400 000 000 x 8)/28 800 = 111 111 seconds.

Characters per second = 28 800/10 = 2880 (start/stop bits are added to the 8-bit

ASCII).

5 The school needs to investigate the parameters used to establish the

communication link such as the bps, data bits, parity, stop/start bits and flow

control. The data could be secured using parity checking, checksum or CRC.

6 Student investigation.


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Exercise 3.2

1 Teleconferencing is the use of an electronic transmission to allow a meeting to

occur at the same time in different locations.

2 Advantages―teleconferencing simulates a face-to-face meeting and reduces costs.

Money is saved by not buying airfares, hotel rooms and meals. Teleconferencing

also saves people time and energy involved in travelling to attend a face-to-face

meeting.

Disadvantages―teleconferencing does remove the inter-personal relationship

achieved through a face-to-face meeting. The physical contact and informal

discussions are often needed to clarify ideas and develop partnerships.

3 Messaging systems are used to send messages to people in different locations who

can receive the message at a different time.

4 Voice mail works by the sender dialling a voice mail number and recording a

message. The message is digitally stored on a computer system and can only be

retrieved by the intended receiver of the message. To retrieve a message, you dial

into the voice mail system and enter an account number and password. The

receiver of the message can use any telephone to enter the voice mail system.

After the receiver enters the voice mail system, he or she can listen to the

message, delete it, save it or send it to another person.

5 The first part of an email address is the name of the account. It is often a person’s

username and is chosen when a user signs up with an ISP. The second part of an

email address is the domain name. It is the location of the person’s account on the

Internet. The domain name identifies the specific computer, or server, where the

email messages are stored.

6 The header contains four main parts: email address, carbon copy, blind carbon

copy and the subject.

7 Messages are written off-line to save money if the ISP charges by the hour.

8 A mailing list is a group of people who may want to receive the same messages. It

may be people working on the same project or a group of people with the same

interests.


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9 E-commerce provides 24-hour availability, global reach, the ability to interact and

to provide customer information, and a multimedia environment.

10 Electronic banking allows customers to view their account balances and

transaction histories, transfer money between accounts and pay bills using Bpay. It

provides banking services 24 hours a day but cannot cater for cash or cheque

withdrawals.

LEARNING ACTIVITIES1. a Teleconferencing. Participants―people involved in the meeting, IT personnel.

Data/information―video data of meeting. Information technology―computer

system, video camera, communication software, telephone line or leased line.

The purpose of teleconferencing is to allow a meeting to occur at the same

time in different locations using an electronic transmission.

b Messaging systems. Participants―sender and receiver of the message.

Data/information―message (either voice, fax, or email). Information

technology―telephone, computer system, fax machine, email software,

telephone line. The purpose of a messaging system is to send messages to

people in different locations who may receive the message at a later time.

c Electronic commerce. Participants―people who are buying and selling the

goods, IT personnel. Data/information―Web site, credit card number.

Information technology―computer system, Web browser, authoring software.

The purpose of electronic commerce is the buying and selling of goods and

services via the Internet.

2 This table compares the advantages and disadvantages of three messaging

systems:

Advantages DisadvantagesTraditional system

Convenient―telephone, fast, express feelings more clearly using the spoken word

Receiver needs to be at the telephone to accept the message

Voice mail Convenient―telephone, express feelings more clearly using the spoken word

People need to check voice mail box regularly, insecurity of messages in the voice mail system

Electronic mail Communicates complex information, such as different data types, relatively cheaply,

Access to a computer system, junk email, people need to check mail box regularly, security of


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fast, person does not have to be present to receive the message, sending the same message to groups of people

ISP for storing messages

3 Teachers need to assess students’ skills when completing each practical task.


5 Information published on the Internet is protected by copyright. Text obtained

from a Web site should not be reproduced without acknowledgment. Images

should not be downloaded and used for other purposes without permission.

6 Downloading is the action of receiving a file from another computer. Data from a

Web page is usually downloaded using the browser’s ‘Save As’ command. Many

files are downloaded in a compressed format, such as zip or hqx. These files

require the user to decompress, or expand, them. Factors that affect the speed of

transmission are the bandwidth, modem speed and processing speed of the

computer.

Exercise 3.3

1 Simplex mode allows transmission in one direction only, from the sender to the

receiver. Half-duplex mode allows transmission in both directions but not at the

same time. Full-duplex mode allows transmission in both directions at the same

time.

2 Protocols define how a link is established, how data is transmitted and how errors

are detected and corrected.

3 Ymodem is a file transfer protocol whose data is transferred in blocks 1024 bytes

long. It includes such features as transfer of file names, increased reliability of

error checking and increased data transfer. ZModem is a file transfer protocol

developed to rectify the limitations associated with YModem. It uses a variable

block size and supports CRC. It provides for high speed, packet and network

communication environments.

4 An X.25 protocol is a standard packet switching protocol. It is used for low-speed

applications, such as credit card verifications and ATM transactions.


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5 LANs connect computers within a building or group of buildings on one site.

They are in a small geographical area, and the computers are linked together by

coaxial cable or fibre optic cables. WANs connect computers over hundreds or

thousands of kilometres. They often consist of a mainframe computer called the

host and a number of terminals.

6 A private leased line offers higher transmission speeds and more accurate transfer

of data than those available through the public switched telephone network

(PSTN).

7 Packet switching is a technique that divides messages into small data packets,

transmits the packets and later rejoins the packets to form the original message.

8 A bus topology is an arrangement where all the devices are attached to a direct

line called the bus. Each device has a unique identity and can only recognise those

signals intended for it. A ring topology is an arrangement where all devices are

attached so that the path is in the shape of a continuous circle. Each device in the

ring has a unique address.

9 An Ethernet allows data to be transmitted simultaneously to all nodes on the

network in both directions. Addressing information allows each node to recognise

and receive individual data packets. A token ring operates by continually passing

special data packets called tokens between nodes on the network. Workstations

with data to send capture a free token and attach data along with addressing

information.

10 In CSMA/CD, all nodes have the ability to sense signals on the network. When a

node wishes to transmit, it ‘listens’ to the bus for signals. When there is no signal

on the bus, it transmits. However, occasionally a collision will occur if two nodes

sense a clear bus at the same time. When a collision is detected, each device stops

transmitting and retransmits at another time.

11 Four different types of servers are:

A file server is a controlling computer in a network that stores the

programs and data shared by users.


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A print server is a computer in a network that controls one or more

printers and stores data to be printed.

A mail server is a computer in a network that provides email facilities.

It stores incoming mail for distribution to users and forwards outgoing mail to

appropriate devices.

A Web server is a computer that provides a connection to the Internet.

All the Internet traffic is directed through this server.

12 A router determines where to send a data packet based on its current

understanding of the networks. A router maintains a table of the available routes

and their conditions. It uses this table, together with distance and cost algorithms,

to determine the best route for a given data packet.

13 A bridge is a combination of hardware and software to link two similar networks.

A gateway is a combination of hardware and software to link two different types

of networks.

14 A hub is a central connecting device in a network. Data arrives at the hub from

one or more devices and is forwarded out using just one cable.

15 Three methods of wire transmission are:

Twisted-pair cable consists of two thin insulated copper wires, twisted

to form a spiral. Twisting reduces the amount of interference from other

cabling.

Coaxial cable has a single copper wire surrounded by an insulator,

grounded shielding and an outer insulator. The shielding allows data to be

transmitted with little distortion.

Fibre-optic cable uses a laser of light to carry data in small glass fibres

about the diameter of a human hair. Fibre-optic cable is free from

electromagnetic and radio interference, very secure and can transmit data at

high speeds without errors.

16 Baseband networks use the entire capacity of the cable to transmit only one signal

at a time. Broadband networks divide the cable so that several signals can be

transmitted at the same time.


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17 Microwave is a high-frequency radio signal sent through space in a straight line

from one antenna to another.

18 A network administrator’s responsibilities include network security, installing new

applications, distributing software upgrades, monitoring daily activity, enforcing

licensing agreements, developing a storage system and providing routine backups.

19 The tasks performed by a network operating system include:

Administration―adds, removes and organises users; installs hardware

devices and applications; and carries out maintenance operations, such as

backup.

File management ―gives users access to the remote hard disks on the

server and provides a file system and the ability to manage a network

directory.

Applications―handles request from uses to share data and

applications.

Resource management―allows network devices, such as printers and

modems, to be shared; assigns users to printers; and orders print jobs.

Security―monitors and restricts access to network resources.

20 Logon procedure involves the user being identified by means of a user ID and a

password. The user ID usually conforms to a limited length, such as eight

characters; and the password often contain at least one digit.

21 The main purpose of an intranet is to share information and computing resources

among the employees of an organisation.

LEARNING ACTIVITIES1 The type of transmission is asynchronous. The type of error detection is parity

checking. It uses an additional bit called a parity bit. Gloria is sending data using

an odd parity. Therefore the parity bit will be a 0 or 1 to make the total number of

bits an odd number. One major deficiency in parity checking is that, if the 7-bit

ASCII contains two errors, the errors will not be detected. An alternative method

that would provide better results would be CRC. It is a method of checking for

errors in data transmission using a division process. The data is divided into


9 Chapter 3 Answers


predetermined lengths and divided by a fixed divisor (prime number). The

remainder of the calculation is attached and sent with data. When the data is

received, the remainder is recalculated.

2 A LAN with seven nodes in a ring topology will have each node in the shape of a

continuous circle. Data transfer is in one direction, moving from node to node

until it arrives at its destination. The problem of multiple users is overcome using

a token ring. The token ring operates by continually passing special data packets

called tokens between nodes on the network. Workstations with data to send

capture a free token and attach data along with addressing information. A busy

token with data cannot be used by other nodes.

3 The benefits of connecting two LANs are the ability to transfer data, improved

communication, and the ability to share hardware devices and application

software. A gateway is a device that is able to link the Ethernet and token ring

protocols. It converts data between these protocols.

4 The diagram should show all the devices in each LAN attached to a direct line

called a bus. Data is transferred in each LAN by assigning each device a unique

identity. Each device only recognises those signals intended for it. Devices check

the bus periodically and retrieve their messages as data travels along the bus. The

problem of multiple users is overcome using the CSMA/CD. When a node wishes

to transmit, it ‘listens’ to the bus for signals. When there is no signals on the bus,

it transmits. A bridge is used to connect these LANs. A bridge examines each data

packet on a LAN and forwards any data packets addressed to the connected LAN.

5 The sketch should show all the devices attached to a direct line called a bus. The

print server and the printer should be attached. The file server is often located at

the end of the bus. The print server is a computer in a network that controls one or

more printers and stores data to be printed. The mail server is a computer in a

network that provides email facilities. It stores incoming mail for distribution to

users and forwards outgoing mail to appropriate devices. Security involves using

passwords, firewalls, backup and data encryption.

6 Student investigation.


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7 An intranet is a private network that often uses a leased line or its own cabling

whereas the Internet uses the PSTN. This dedicated line makes the intranet faster.

Both the intranet and the Internet share a similar interface. An intranet often

provides access to the Internet through firewalls.

Exercise 3.4

1 Five collection devices are ATM terminals for electronic banking, EFTPOS

terminals for a retail store, telephone for voice mail, keyboard for electronic mail

and video cameras for a surveillance system.

2 Encoding involves converting data from its original form into another form for

transmission. Decoding is the reverse process. It converts data from the form used

for transmission back into the original form.

3 Most natural events in the real world, such as temperature, light and pressure, are

in analog form.

4 Digital data is represented in the form of digits, or numbers.

5 A series of 0s and 1s (digital data) is encoded into a continuous wave (analog

signal).

6 The wave shape of the data (analog data) is encoded into a series of 0s and 1s

(digital signal).

7 Client-server architecture works by the client sending a request to a server

according to some protocol and the server responding.

8 The Web browser is a client program that requests services from a Web server to

complete the request. Ideally, a server should provide a user-friendly interface so

that clients are not aware of the information technology that is providing the

service.

9 An EFTPOS terminal displays information about EFTPOS transactions. It

contains a screen to display the name and price of the product being purchased.

When the customer’s bank has been contacted, then the EFTPOS terminal

displays the approval if funds are available.




LEARNING ACTIVITIES1 Analog signals are pulses, usually electrical or optical, in the form of a continuous

wave. Digital signals are represented as a series of 0s and 1s. A scanner converts

analog data into a digital data/digital signal. A modem converts digital data to

analog signals and analog signals back to digital data.

2 In the client-server method, the client sends a request for all customers who have a

balance of over $500 to a server according to some protocol and the server

responds by sending the result of the request back to the client. In the file-server

method, the request for data is made directly to the file server without making any

changes to the request.

3 Some of the information processes when using an EFTPOS terminal are:

Collecting―scanning the plastic card for customer details and product

barcode.

Organising―ensuring the data is in the correct format to be used by

the database.

Analysing―searching the bank’s database for the customer’s account

and searching the store’s database for the product.

Storing and retrieving―retaining information about the transaction.

Processing―updating the customer’s account and the store’s account.

Transmitting and receiving―transferring information between the POS

terminal and the customer’s bank.

Displaying―showing the name of the product being purchased and the

approval of funds and providing a receipt for the customer.


Exercise 3.5

1 There is a danger of misinterpretation with messaging systems as communication

often depends on the context and inflection in the speakers’ voices and their body

language.




2 Privacy and confidentiality are a concern with messaging systems as messages

need to be stored. Email and voice mail both store messages on servers, and these

can be accessed by the people who are providing the service. Hackers may also

break the security of these servers. Telephone conversations can be intercepted,

and fax messages can be read by anybody near the machine.

3 Electronic junk mail is unwanted email. It is a problem as people can send an

email message to one person or thousands of people very easily. If a person

receives hundreds of junk mail each day it takes up their valuable time.

4 Businesses are establishing a Web site to promote their goods and services.

Internet provides significant advantages for consumers, such as more information

about products and services, shopping globally and increased competition that has

resulted in lower prices.

5 The increase in Internet business requires more people to be employed in the

information technology industry. It may result in fewer shop fronts and fewer

people employed to provide service at these locations.

6 The developments in communication technology have made trade barriers

between countries irrelevant. People are buying and selling goods on the Internet

from other countries with little thought about trade barriers.

7 IIA has released a code of conduct that deals with censorship of online content.

The code requires ISPs to remove offensive content from their servers and to

block access to classified material hosted on overseas sites. The government has

passed a law requiring ISPs to subscribe to the IIA’s code.

8 Issues arising from Internet banking include:

Security―all banks are determined to make their online banking

services safe from inference and secure customer details.

Changing nature of work―people working for the bank are not

carrying out the services provided by Internet banking.

Branch closure and job loss―with customers using Internet banking,

EFTPOS and ATMs, there is no need to access the facilities provided by a

bank branch. This has resulted in branch closures and job losses.




9 Video on the Internet is providing time and cost savings and the ability to view the

video globally.

10 The main advantages of telecommuting are greater flexibility in work hours and

saving money on transport, clothing, food and time. In particular, it benefits

people who are physically impaired or required to look after small children. The

employer saves on overheads, such as office space and furniture. Disadvantages

include people missing the social and professional contacts offered by an external

place of work. They experience feelings of loneliness and isolation.

Telecommuting can also blur the distinction between work and home life. The

home is no longer a place where the pressures of work can be forgotten, and work

may be interrupted by domestic chores.

LEARNING ACTIVITIES1 Banking has changed drastically with the use of plastic cards, ATMs and

EFTPOS. Internet banking and bill paying are further transforming the way we

use money. Most people agree that society has benefited from these changes;

however, there are some negative issues. The nature of jobs in the banking

industry has changed with fewer people required in the branches and more people

required with IT skills.

2 Security is a problem when Alanna pays for goods and services on the Internet.

Her account details need to be safe from interference. Data encryption is used to

secure data during transmission, and reliable backup procedures are used to ensure

the transaction has been accurately stored.

3 Student opinion. Communication systems are reducing the need for people to

leave their houses. For example, people can complete their shopping at home. On

the other hand, if communication systems are saving time, then people have more

time to explore the world.

4 Some of the changes to society caused by communication systems include ATMs,

EFTPOS, Bpay, voice mail, mobile phones, email, video conferencing,

telecommuting and accessing information from the Internet. Some of the concerns

include privacy, security, accuracy of data, changing nature of work, offensive

material on the Internet, information overload and electronic junk mail.




5 Student opinion. There is a large amount of offensive material on the Internet that

is inappropriate for children. Banning offensive material compromises our free

society.

Chapter review 3

PART A1 A

2 D

3 C

4 C

5 C

6 A

7 B

8 A

9 A

10 B

PART B1 (e) CRC

2 (k) protocol

3 (m) star

4 (c) bps

5 (f) digital

6 (i) handshaking

7 (d) checksum

8 (j) hub

9 (o) v-mail

10 (h) ethernet

11 (g) email

12 (b) baud rate

13 (l) ring

14 (n) switch

15 (a) analog




PART C1 A communication system enables people to send and receive data and

information. The five basic components of a communication system, are the data

source, transmitter, transmission medium, receiver and destination.

The components of a communication system for email are:

Data source―writing the email message.

Transmitter―sending the email using a modem.

Transmission medium―telephone line.

Receiver―opening the email using a modem.

Destination―reading the email message.

2 Parity checking is a method of checking for errors in data transmission using an

additional bit called a parity bit. When an odd parity is chosen, the number of 1s

in the nine bits must be odd. This means that if the number of 1s in the first eight

bits is even, then the parity bit must be 1 to make it odd. If the number of 1s in the

first eight bits is odd, the parity bit must be 0 to keep it odd. The sender and

receiver can also decide to send an even parity; then the number of 1s in the nine

bits must be even. One major deficiency in parity checking is that, if the data

contains two errors, they will not be detected.

3 a Coaxial cable has a single copper wire surrounded by an insulator, grounded

shielding and an outer insulator. It is commonly used in LANs and over

distances less than a few kilometres. The bandwidth for a coaxial cable is 10

MBps. Fibre-optic cable uses a laser of light to carry data in small glass fibres

about the diameter of a human hair. They are replacing the conventional

copper wires. The bandwidth for fibre-optic cables is in excess of 400 MBps.

b A host computer is a computer on a network that carries out functions for

other computers. They serve terminals on the network. A terminal is a device

used to send data to a computer system or to receive data from a computer

system. Terminals often consist of a keyboard for input and a screen for

output.




c Asynchronous transmission is the sending of data by identifying each byte

with special start and stop bits. Synchronous transmission requires all the data

to be sent at the same rate. The same number of bytes is sent each second. This

is synchronised by each device using a clock. Synchronous transmission is

faster and more efficient than asynchronous transmission as there are no extra

bits.

d Error detection is a method of finding errors in transmission. Three common

methods for error detection are parity checking, check sum and cyclic

redundancy check. Error correction is a method of fixing the errors. The most

common form of correction is to retransmit the data.

4 There are many possible network topologies, such as star, bus and ring:

Star topology has a central computer with each device connected

directly to it. The central computer serves as a switch. It receives messages

and sends them to the destination device.

Bus topology is an arrangement in which all the devices are attached to

a direct line called a bus. Each device has a unique identity and can only

recognise those signals intended for it. Devices check the bus periodically and

retrieve their messages as data travels along the bus.

Ring topology is an arrangement in which all devices are attached so

that the path is in the shape of a continuous circle. Each device in the ring has

a unique address. Data flow is in one direction, moving from device to device

until it arrives at its destination.

5 The diagram consists of one computer with digital data entering a modem with

analog signal leaving the modem and travelling to another modem that transfers

the analog signal back into digital data for a second computer.

6 Students identify a social or ethical issue involved in a communication system,

such as privacy, security, accuracy of data, changing nature of work, offensive

material on the Internet, information overload and electronic junk mail. They need

to outline the negative aspects of an issue (such as the security of financial data)

compared to the positive aspects (such as the benefits of Internet banking).






Documents

· Web view3 Cyclic redundancy check (CRC) is a method of checking for errors in data transmission using a division process. The data is divided into predetermined lengths and divided