CS Unit 5 (Communication and

the internet)
Type IGCSE

Chapter21(Network)
Text in red —→ Definition or key point or main point
Text in green —→ Common sense or Knowledge point / need to know
Other Text —→ General text

The networking of digital devices has had a profound impact on our lives and
the way in which we work, communicate with each other.

What is Network?
A network is a collection of two or more computers tat are connected together
for the purpose of sharing resources and data. It include all types of digital
devices.

Many of networks has server and client. A server is a powerful computer that
provide the network with services where as the client request the services.
Types of server:
▪File server
▪ Print server
▪ Email server
▪ Web server

▪ Database server
▪ DNS server

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 Why are network used?
A network can support multiple users accessing multiple services at the same
time. The network allows multiple users:

to read/write personal files on a central server (provide storage and back-

up)

to access shared files among several users

to download data or updates to computer programs

to send data to a shared printer

to access the internet

to communicate with

What is Internet?
Internet is a worldwide system of interconnected networks that enables
information to be exchanged and shared.

How does the internet and world wide web differ?

Internet is not the same things as World Wide Web. The internet is the
interconnected network of computers that provides many online services to
users. World Wide Web is just one use of network of the internet.

Advantages and Disadvantages Of Computer

Network
Advantages Disadvantages

More storage Security risk - Data loss by unauthorized access, virus

Data Back-up Cost - To buy networks components and maintenances

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 Files share between users Complexity - need expertise to set up and maintain

users can communicate

users can play multiplayer

games remotely

Type of Networks Base on:

Geographical Area
There are three types: LAN, WAN and PAN

LAN: A local area network is a network that covers a relatively small

geographical area, often a single site or home. Most common type of
LAN is ethernet LAN. But home system use wireless local area network
(WLAN) which is wireless.

WAN: A wide are network (WAN) covers a large geographical area,

usually organizations or company. Each sites has one or more LANs and
form WAN. It allow employees on different sites to communicate and
share data.

PAN: A person are network is a network communication between your

own computer devices, such as laptops, phones, tablet, printer…. The
PAN connect the local devices to allow the data sharing. The devices
usually connect using Wi-Fi or Bluetooth

Network Topology
Network Topology is the arrangement of connections between networked
devices. There are four main network topologies.

Bus Topology : A bus network consists of a single cables to which each

networked device is connected. At each end of cable, there is a
terminator to absorb signals that reach end. Bus topology is widely used
in modern cars and some aircraft. (Controlled Area network)

Advantages Disadvantages

Very Cheap to install - one cable is If cable is damaged or cut, whole

needed network fails

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 Advantages Disadvantages

Easy to connect nodes to network Hard to find where a fault on cable

Having one cable increase data

Easy To add Extra devices
collosions

More devices added ——→ Slower the

Node failures doesn’t affect the other
network
devices or network
All data sent is received by all devices

Ring Topology: A ring network is bus topology but in a closed loop or

ring. Each network device has ‘in’ and ‘out’ connection. Message sent
on ring network all travel in the same direction so no collisions. Data is
passed from one device to next until it reaches.

Advantages Disadvantages

Adding extra doesn’t affect the If cable is damaged or cut, whole

performance of network network fails

If any nodes fail, the ring is broken and

Easy To add Extra devices
failed

Adding extra need to shut down the

No collisions
whole network

More expensive than bus

Star Topology: A star network is a network with main central point

(server) in which networked devices are connected to using either hub
or switch. A central device receives and direct messages to correct
recipients (client).

Advantages Disadvantages

Each node is installed separately so no If switch is damaged or fail, whole

failures due to nodes network fails

Easy To add Extra devices without

Expensive to buy cables, switch
needing to shut

Bottlenecking can occurs if too much

Easy to locate faults and which
traffic pass through central point,
devices
making network slow

Network is efficient as messages are

only sent to device needing them

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 Mesh Topology: There is no central point in mesh topology, instead,
each networked devices (node) is directly connected to every other
devices in network. Only two type exist: Fully connected or Partially
connect. Mesh network can be either wired or wireless. Wired >
Wireless (in cost)

Advantages Disadvantages

Failure preserved - if one device fails, Difficult and expensive to install wired
message can re-routed mesh

Very High performance due to each

Can be difficult to manage due to
device is connected to multiple
multiple connections
devices

In wireless mesh, each node can

extend within the range.

Function / Model
There are only two Models of network: Client-Server Network and Peer-To-
Peer Network

1. Client-Server Network: In this network, there is one server to provides

services to the client computers. They all are connected through a
network using switch or hub. Client-Server Network are usually
Centralized.
Q: How does they work?

A: Client access data or files from server by the following process:

1. client make connection to server using it address (IP)

2. then, client make a service request to server

3. if the request is valid, server will send requested data to client

2. Peer-To-Peer Network: In this network, each devices/computers can act

as client and server. There is no central point in this network. Peer-To-
Peer Network are usually Decentralized.

Communication Media

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 There are many different devices that can be use to connected devices in a
network.

Network Devices Network Media

Wired
→Hub
→Ethernet Cable
→Switch
→Coaxial Cable
→Router
→Fiber Optic Cable
→Modern
→Repeater
Wireless
→Bridge
→Wi-fi
→Wireless Access Point
→Bluetooth
→Gateways
→Mobile Broadband (4G,5G)

Ethernet Cable: Twisted pairs of copper wires to transmit data

Coaxial : A single copper conductor surround by insulation and shielding

Fiber-optic Cable: Made of glass or plastic which carry light signals at light
speed so faster data transmission

Switch and Hub : Switch and Hub are devices that receives incoming data
packets and redirects them to their destination on a local area network (LAN).
The difference is that Switch only sent to intended computers where as Hub
sent to all devices.

Router: Router store the address of all devices that are connect to it in a
network. It route the data from one network to another based on their IP
address.

Modern : Modern convert digital data to analog signals. Modulates (D to A) or

Demodulate (A to D)
Gateway: Connect two different types of network and acts as ‘gate’

Wireless Access Point : allow wireless device to connect to wired network .

They convert data they receive through cables into a wireless signals.

Bridges : connects two or more network segments, making them function as a

single network.
Repeater : extend the range of network

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 Difference between Wired and Wireless

Wired Wireless

Not require physical

Has physical connection connection instead use
signal connection

No need for a cable to

Data Transmission is faster connect devices or to the Advantages
internet

Not easy to intercept or Allows user to use their

Advantages
eavesdrop on data own device

Less susceptible to A wider range of devices

interference than wireless can communicate within Advantages
connectivity the range

Data transmission is slower

Expensive to install and
Interference can often Disadvantages
reconfigure
affect performance

Walls and physical objects

Requires many cables / can affect performance
Disadvantages
expensive Data need to encrypted to
prevent interception

Network Data Speed

The speed that data can be transmitted through a communications medium is
measured in bits per second (bps, b/s, bit/s). Note that network data speed is
1000

Calculating the time needed to transmit a file

If you have a network connection of 12 Mbps and you want to download a
50 MB file, how long would it take?

1. Convert the file size from megabytes to bits.

• File size in megabytes = size x 1024 x 1024 x 8
• 50 MB = 50 x 1024 x 1024 x 8 bits = 419,430,400 bits

2. Convert the transmission speed from Mbps to bits per second.

• Transmission speed in Mbps = size x 1000 x 1000
• 12 Mbps = 12 x 1000 x 1000 = 12 000 000 bps

3. Divide bits by bits per second.

• Time = size of file (in bits) / network speed (in bits per second)

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 • 419,430,400/12 000 000 = 34.95 seconds

IP address and MAC address

Domain Names
Domain names are used to identify one or more IP address.
Domain Name Services (DNS)

DNS is a internet service that translate domain names into IP address. It

has databse of domain names and IP address.

How does it work?

1. User enters a domain names into browser on client computer

2. A client request a DNS server to resolve the domain name

3. If one DNS server doesn’t know how to translate a particular domain

name, it asks another one, and so on.

4. If it hasn’t got that domain name in its database it will ask another server.

5. If one DNS server found the domain names, it will return IP address to
client and the client will connect the host using that IP address.

Data Communication (Protocols)

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 Data communication mean exchanges of data between devices using
transmission media in a network. Five main components in data
communication:

1. Source or Sender

2. Destination or Receiver

3. Channel or media

4. Message

5. Protocol

Protocols
Protocol is a set of rules that control how the communication be between
devices in a network. Without it, different computers wouldn’t be able to
communicate.
A general protocol follows these process:

Identifying : how computer will be identify (IP or MAC)

Encoding and Formatting : Data is converted into signals and formatted

before transmit.

Routing : what route/path the data will take to get its destination

Sequencing : How the data is di-sort and sort/ sequence in correct

order

Error checking : how error will be found and dealt with

Acknowledgement : confirmation that destination has received the

message correctly

Error Handling : what to do if data isn’t received properly

Synchronization : how the speed of the sender and receiver can be

synchronized

Protocol Stacks : Protocol stack means a collection of protocols that work

together. Examples: Internet, TCP/IP and OSI Protocol Suite

Email Protocols

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 Email are sent and received using set of standard protocols

Network Protocols
TCP : Transmission Protocol is used when you access web pages,
send/receive email or upload/download files
IP : Internet Protocol deal with addressing system to identify individual
devices and splitting data into packet and adding packet header with details

TCP/IP Protocols Suite

Data sent using TCP/IP is broken into packets. Each packets of data consists
of a small section of data and packet header. A packer header contain
details of

sender

receiver

how many packets the data is split into

number of this packet

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 Once the receiver computer receives all packets, it will reconstruct data
using the information in the header into original data. A network that moves
data in packets is called packet switched network.

When Data is sent, it pass through Four different Layers:

Application Layer

Transport Layer

Internet Layer

Link (Network Layer)

Web Browsing Protocols

HTTP : the rules to be followed by web server and web browser when
sending and requesting data.

HTTPS : More secure version of http. Especially it ensure communication

between financial information
File Transfer Protocols

FTP: rules that must follow when files are being transmitted between
devices in network. It is often used when sending web pages and other files
by web developers.

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 Benefits of Networking Layers

1. it makes the overall model easier to understand by dividing into

functional parts

2. Each layer perform specialized tasks

3. Different layer can be combine different ways

4. One layer can be developed or changed without affecting other layers

5. It make it easier to identify and correct networking errors and problem.

6. It provide a universal standard for hardware and software manufactures

to communicate

Mobile Broadband Communications

A mobile or cellular network is also a wireless network distributed through

cells where each cell(tower) includes a fixed location transceiver known as
base station. They all work together to cover up over large geographical
area.

When user moves out of range, the signals fails and the base station make a
request to transfer control to another base station nearby without notifying
subscriber. This is called handover.

2G was first mobile communication. It can send text messages and

introduced multimedia messaging service (MMS)

3G increase data transmission speeds and gave wireless access.

4G provide much higher data speeds and rise the popularity of

mobile gaming. It also got much higher capacity (more
users/subscriber)

5G is smarter, faster, and more efficient than 4G. It peaks speeds of

100 Gbps, 100 times faster than 4G

Chapter22(NetworkSecurity)
It is essential that all data stored on networks is kept confidential and secure.

Network security

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 Network Security covers a wide range of activities that protect data from
threats to its confidentiality, correct ness (integrity) and availability.
Data confidentiality involves restricting access to data and ensuring it is
protected form unauthorized individuals. It is important that the network is not
hacked, intercepted or stolen by criminals or competitors. Otherwise, they can
use the data for illegal purposes or business advantage. Some ways to ensure
this is done by:

Access Control Measures : Ensure that only authorized person can

access/modify specific files/data

Stopping Misuse : Stopping users not to misuse the files which are
beyond their permission

Encrypting data : Encrypt/Scramble data so that when unauthorized

person gain access, they won’t be able to read it without encryption key

Data Correctness is important because Data is useless unless it is correct.

Store data must not change or modify without authorization.
Data Availability is as well as needed. A network is useless if the data cannot be
accessed when it is needed. So it is important to protect from failures caused
by criminals,, virus or denial of service (DoS) - an attack on a network that
attempts serval request at the same time in oder to shut down/ slow down the
network.

Why network Security is important?

1. Required for the running of the organization

2. Private and Confidential

3. Financial Valuables

Authorization and Validation

Authorization is the process of checking the identify of a user of a computer
system or network where as Validation is the checking of username and
password meets up with the rules.

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 Ways to secure A Network
There are many different ways to secure a network.

Firewall
A network security system that monitor and controls data that is ‘in’ and ‘out’
of the network to network. It inspects the incoming and outgoing data and
uses set of rules to decide whether to allow the data or not. It can be
hardware- or software- base

Access Control
A method to control that particular user can gain access to particular file.
Access control decides if user should get read-only access/ or read/write
access.

Physical Security
It ensures that critical parts of network can only be physically accessed by
authorized people. It protect against theft of equipment. For examples,
locked/authorized person room, installing burglar alarm, security tagging/
record entry and exist. Physical security is very important because anyone
with physical access to the server can more easily bypass the security
provided by the authentication system or access control system.

Cloud storage and security

fjasklf

Contemporary Storage and security

One type of this storage is network-attached storage (NAS). It is a hardware

device that is connected to a network to provide file storage for any device
connected to that network.
Security risk of NAS:

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 Not changing device’s default password

Not updating software running on the NAS

Another widely type is USB flash drive. They are easy to transport, cheap for
storage amount and convenient to use. The disadvantages is that people put
sensitive information there and it is easily lost. Anyone find it can access. But to
secure this, encryption or setting password is need.

Cyber Attack
A cyber attack is any kind of attack on a computer system or network intention
of

gain access to data

modify and delete data

making system unavailable for use

physically damage a device

Type of Cyber Attack

Social Engineering
Attacks that rely on exploiting human behavior.

Phishing : A method to get personal information and payment details

from users by sending a fake message or email which seems to be
from real organization. Users can loss data by replying them or
clicking the link provided and reveal their personal data such as
username and passwords.

Pharming : A method to gain personal information and payment

details from users by directing them to fake version of trusted
websites to trick users into entering personal login details. There are
two main methods when user are directed to pharming site.

Internet traffic: while going to real website is redirected to fake

website

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 pharming sites have very similar URL to real website so when
user mistype the real one it is already directed to fake website

Blagging : Blagging is when someone makes up a store to gain a

person’s interest and steal money.

Shoulder Surfing : Gaining access to confidential information by

directly observing a user, possibly literally looking over their
shoulder, as they complete a task
To prevent Social Engineering Attacks,
Users should always be careful when opening email attachments or
hyperlinks in emails and other messages. Users should not reply or open if :

no sender

text is general

spelling and grammatical errors

attached file is .exe file or zip file

contain message that seem to rush

not recognize the URL

the website has https or https

install latest security updates/ anti-virus software

be cautious when entering PINs/ password in public location

Technical Weakness
Other forms of cyber attack rely on technical weaknesses (vulnerabilities) in
the system.
Unpatched software : Software is very complicated and security issues are
usually found if it is used in real world. The developer should normally
provide updates to fix these issues. If not, some people use this information
to attack unpatched software to gain access to information.
USB devices : discussed above

Eavesdropping: While data are being sent through, some computer system
will intercept data. In this way the data can be stolen or lost. The owner
wouldn’t know if the data is being eavesdrop

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 Hacking : Hacking is breaking into computer system to steal personal data
without the owners’ consent or knowledge.
Virus : Program or program code that can replicate/copy itself with the
intention of deleting or corrupting files or cause the computer to
malfunction.

Protecting against Security Weakness

Security must be considered at the system and software or application design
stage for the software to be as safe as possible against cyber attack.
Consider these question:

Once Design phase is done, they need to look at implementation ( writing

code).
One major issue is “codes are poor quality and don’t consider how safe or
secure program is” . Another major issue is some programming lead to code
vulnerabilities.
These issues can minimized by having regular code reviews. There are two
main types:

1. Review by another programmer, usually more senior and experience

2. Automated review, usually using software to examine the code

Modular testing is important to security too as it check every tiny problems

remain, by reviewing each section of blocks of code

Other Security Measures to cyber attack

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 Used audit trail which record the activities that have taken place on a
computer system and which can’t be changed. it is order by chronological.
Audit trail contain details of who or what made the change so that
technicians can find out what happened during cyber attack.
Used secure operating system as different systems as different purpose.
Keeping a network secure require effective network management,
monitoring and training of it user.

Identifying Vulnerabilities
Ethical hacking is the branch of computer science that relates to cybersecurity
and preventing cyber attacks from being successful. Ethical hacking is
essentially ‘good’ hacking, looking for weaknesses in software and systems so
that they can be improved. Once weaknesses have been found, steps to
remove or reduce its impact can be taken.

Ways to identify vulnerability

Penetration Testing (Pen Testing)
It is likely to try to gain access to all the systems that the organization has
and the attacks will often include looking for technical weaknesses and
trying social engineering methods. The IT systems of an organization are
deliberately attacked to find any weakness. These attacks are authorized by
the organization and are therefore legal. Once the pen testing has been
completed, a report is usually presented to a senior manager within the
organization explaining what issues were found and the likely impact of
them.

Commercial Analysis Tools

Commercial analysis tools or vulnerability scanners can be used to scan a
system for vulnerabilities, either by purchasing the tools or hiring a service.
These tools can only identify known vulnerabilities, look for common issues
and alert the user. But it must be kept up to date to be effective. They can be
used to scan the network from:

Internally : show issues that could exploited by rogue employee or

hacker who physically get in

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 Externally : show vulnerabilities that hacker could exploit from
outside of company network

Reviews of Network and User Policies

All networks should have written policies that document:
▪ who is authorized to carry out various activities on the network
▪ how and when patches to software should be applied
▪ access controls
▪ password requirements, including how complex passwords should be
and how often they should be changed
▪ how security is set up and maintained on the network
▪ what data audit trails should collect and how long they should be kept
for
▪ anything else relevant to the security and maintenance of the network

Before accessing the network, employees must read and sign the network
user policy to confirm they agree to the organization's rules. User policy is
likely to contain detail of:
▪ what use of the network is allowed or not allowed
▪ what will happen to the user if they do something unacceptable
▪ how to report faults, problems and security issues
▪ security information, such as good practice when choosing and using
password

Over time, the hardware, software, and usage of a network may change.
Therefore, network policies should be regularly reviewed. This ensures they
remain current and effective for maintaining network security.

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