2 Communication

In this chapter, you will learn about

★ the benefits of networking devices

★ the characteristics of a local area network (LAN) and a wide area

network (WAN)

★ client-server and peer-to-peer models in networking

★ the differences between thin client and thick client

★ bus, star, mesh and hybrid networking topologies

★ public and private cloud computing

★ the differences between wired and wireless networks

(including types of cable and wireless technologies)

★ the hardware required to support a LAN

★ the function of routers

★ Ethernet and how data collisions are detected and avoided

★ bit streaming (including differences between real-time and

on-demand streaming of data)

★ the differences between the internet and the World Wide Web (WWW)

★ the hardware needed to support the internet

★ IP addresses (including IPv4, IPv6, public IP addresses and private IP

addresses)

★ the use of the uniform resource locator (URL) to locate a resource on

the world wide web

★ the role of the domain name service (DNS).

WHAT YOU SHOULD ALREADY KNOW

Try these three questions before you read this

chapter.

1 a) Explain the following terms associated

with devices connected to a network/

internet.

i) MAC address

ii) IP address

b) Explain the main differences between a

MAC address and an IP address and why it

is necessary to have both associated with a

device connected to the internet.

c) What is the purpose of an internet service

provider (ISP)?

d) Explain the function of an internet browser.

In what ways is this different to an ISP?

2 A college is about to form a network from

20 stand-alone computers. Describe the

hardware and software that might be needed

to produce this simple computer network.

3 a) Mobile phones and tablets can be

configured to access the internet from any

location. Describe the software required to

allow this to happen.

b) Describe some of the benefits and

drawbacks (when compared to a desktop

PC) of accessing website pages from a

mobile phone.

2.1 Networking

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2 CommuniCation

Key terms

ARPAnet – Advanced Research Projects Agency Network.

WAN – wide area network (network covering a very

large geographical area).

LAN – local area network (network covering a small

area such as a single building).

MAN – metropolitan area network (network which

is larger than a LAN but smaller than a WAN, which

can cover several buildings in a single city, such as a

university campus).

File server – a server on a network where central files

and other data are stored. They can be accessed by a

user logged onto the network.

Hub – hardware used to connect together a number

of devices to form a LAN that directs incoming data

packets to all devices on the network (LAN).

Switch – hardware used to connect together a number

of devices to form a LAN that directs incoming data

packets to a specific destination address only.

Router – device which enables data packets to be

routed between different networks (for example, can

join LANs to form a WAN).

Modem – modulator demodulator. A device that

converts digital data to analogue data (to be sent down

a telephone wire); conversely it also converts analogue

data to digital data (which a computer can process).

WLAN – wireless LAN.

(W)AP – (wireless) access point which allows a device

to access a LAN without a wired connection.

PAN – network that is centred around a person or their

workspace.

Client-server – network that uses separate dedicated

servers and specific client workstations. All client

computers are connected to the dedicated servers.

Spread spectrum technology – wideband radio

frequency with a range of 30 to 50 metres.

Node – device connected to a network (it can be a

computer, storage device or peripheral device).

Peer-to-peer – network in which each node can share

its files with all the other nodes. Each node has its own

data and there is no central server.

Thin client – device that needs access to the internet for

it to work and depends on a more powerful computer

for processing.

Thick client – device which can work both off line and

on line and is able to do some processing even if not

connected to a network/internet.

Bus network topology – network using single central

cable in which all devices are connected to this cable so

data can only travel in one direction and only one device

is allowed to transmit at a time.

Packet – message/data sent over a network from node to

node (packets include the address of the node sending the

packet, the address of the packet recipient and the actual

data – this is covered in greater depth in Chapter 14).

Star network topology – a network that uses a central

hub/switch with all devices connected to this central

hub/switch so all data packets are directed through this

central hub/switch.

Mesh network topology – interlinked computers/

devices, which use routing logic so data packets are

sent from sending stations to receiving stations only by

the shortest route.

Hybrid network – network made up of a combination of

other network topologies.

Cloud storage – method of data storage where data is

stored on off-site servers.

Data redundancy – situation in which the same data is

stored on several servers in case of maintenance or repair.

Wi-Fi – wireless connectivity that uses radio waves,

microwaves. Implements IEEE 802.11 protocols.

Bluetooth – wireless connectivity that uses radio waves

in the 2.45 GHz frequency band.

Spread spectrum frequency hopping – a method of

transmitting radio signals in which a device picks one of

79 channels at random. If the chosen channel is already

in use, it randomly chooses another channel. It has a

range up to 100 metres.

WPAN – wireless personal area network. A local wireless

network which connects together devices in very close

proximity (such as in a user’s house); typical devices

would be a laptop, smartphone, tablet and printer.

Twisted pair cable – type of cable in which two wires

of a single circuit are twisted together. Several twisted

pairs make up a single cable.

Coaxial cable – cable made up of central copper core,

insulation, copper mesh and outer insulation.

Fibre optic cable – cable made up of glass fibre wires

which use pulses of light (rather than electricity) to

transmit data.

Gateway – device that connects LANs which use

different protocols.

Repeater – device used to boost a signal on both wired

and wireless networks.

Repeating hubs – network devices which are a hybrid of

hub and repeater unit.

Bridge – device that connects LANs which use the

same protocols.

Softmodem – abbreviation for software modem; a

software-based modem that uses minimal hardware.

NIC – network interface card. These cards allow

devices to connect to a network/internet (usually

associated with a MAC address set at the factory).

2.1 Networking

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2.1 Networking

2.1.1 Networking devices

One of the earliest forms of networking, circa 1970 in the USA, was the

Advanced Research Projects Agency Network (ARPAnet). This was an early

form of packet switching wide area network (WAN) connecting a number of

large computers in the Department of Defense. It later expanded to include

university computers. It is generally agreed that ARPAnet developed the

technical platform for what we now call the internet. Figure 2.1 shows the vast

area this network covered.

LBL

AMES

LLL

SRI

UTAH

XEROX

DOCB

GWC

ILLINOS

RML

LONDON

NORSAR

ABERDEEN

ETAC

ARPA

MITRE

SDAC

BELVOIR

NBS

HARVARD

BBN

CCA

MIT-MAC

MIT-IPC

LINCOLN

CASE

RADC

CARNEGIE

STANFORD

TYMSHARE

FNWC

HAWAI

UCSD

UCSB

UCLA

SDC

RAND

USC

USC–ISI

▲ Figure 2.1 ARPAnet coverage, 1973

As personal computers developed through the 1980s, a local network began to

appear. This became known as a local area network (LAN). LANs tended to

be much smaller networks (usually inside one building) connecting a number

of computers and shared devices, such as printers. WANs typically consist of

a number of LANs connected via public communications networks (such as

telephone lines or satellites). Because a WAN consists of LANs joined together,

it may be a private network, and passwords and user IDs are required to access

it. This is in contrast to the internet which is a vast number of decentralised

networks and computers which have a common point of access, so that anyone

with access to the internet can connect to the computers on these networks.

This makes it intrinsically different to a WAN.

WNIC – wireless network interface cards/controllers.

Ethernet – protocol IEEE 802.3 used by many wired LANs.

Conflict – situation in which two devices have the same

IP address.

Broadcast – communication where pieces of data are

sent from sender to receiver.

Collision – situation in which two messages/data from

different sources are trying to transmit along the same

data channel.

CSMA/CD – carrier sense multiple access with collision

detection – a method used to detect collisions and

resolve the issue.

Bit streaming – contiguous sequence of digital bits sent

over a network/internet.

Buffering – store which holds data temporarily.

Bit rate – number of bits per second that can be

transmitted over a network. It is a measure of the data

transfer rate over a digital telecoms network.

On demand (bit streaming) – system that allows

users to stream video or music files from a

central server as and when required without

having to save the files on their own computer/

tablet/phone.

Real-time (bit streaming) – system in which an event

is captured by camera (and microphone) connected

to a computer and sent to a server where the data

is encoded. The user can access the data ‘as it

happens’ live.

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2 CommuniCation

In recent years, another type of network – a metropolitan area network

(MAN) – has emerged. MANs are larger than LANs as they can connect together

many small computer networks (e.g LANs) housed in different buildings within

a city (for example, a university campus). MANs are restricted in their size

geographically to, for example, a single city.

In contrast, WANs can cover a much larger geographical area, such as a country

or a continent. For example, a multi-national company may connect a number

of smaller networks together (e.g. LANs or MANs) to form a world-wide WAN.

This is covered in more detail later.

Here are some of the main benefits of networking computers and devices

(rather than using a number of stand-alone computers):

» Devices, such as printers, can be shared (thus reducing costs).

» Licences to run software on networks are often far cheaper than buying

licences for an equivalent number of stand-alone computers.

» Users can share files and data.

» Access to reliable data that comes from a central source, such as a file

server.

» Data and files can be backed up centrally at the end of each day.

» Users can communicate using email and instant messaging.

» A network manager can oversee the network and, for example, apply access

rights to certain files, or restrict access to external networks, such as the

internet.

There are also a number of drawbacks:

» Cabling and servers can be an expensive initial outlay.

» Managing a large network can be a complex and difficult task.

» A breakdown of devices, such as the file servers, can affect the whole network.

» Malware and hacking can affect entire networks (particularly if a LAN is part

of a much larger WAN), although firewalls do afford some protection in this

respect.

Networked computers

Networked computers form an infrastructure which enables internal and external

communications to take place. The infrastructure includes the following:

Hardware

» LAN cards

» routers

» switches

» wireless routers

» cabling

Software

» operation and management of the network

» operation of firewalls

» security applications/utilities

Services

» DSL

» satellite communication channels

» wireless protocols

» IP addressing.

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2.1 Networking

Networks can be categorised as private or public.

Private networks are owned by a single company or organisation (they are

often LANs or intranets with restricted user access, for example, passwords and

user ids are required to join the network); the companies are responsible for the

purchase of their own equipment and software, maintenance of the network and

the hiring and training of staff.

Public networks are owned by a communications carrier company (such as

a telecoms company); many organisations will use the network and there are

usually no specific password requirements to enter the network – but sub-

networks may be under security management.

WANs and LANs

Local area networks (LANs)

LANs are usually contained within one building, or within a small geographical

area. A typical LAN consists of a number of computers and devices (such as

printers) connected to hubs or switches. One of the hubs or switches is usually

connected to a router and/or modem to allow the LAN to connect to the

internet or become part of a wide area network (WAN).

Wireless LANs (WLANs)

Wireless LANs (WLANs) are similar to LANs but there are no wires or cables.

In other words, they provide wireless network communications over fairly

short distances (up to 100 metres) using radio or infrared signals instead of

using cables.

Devices, known as wireless access points (WAPs), are connected into

the wired network at fixed locations. Because of the limited range, most

commercial LANs (such as those on a college campus or at an airport) need

several WAPs to permit uninterrupted wireless communications. The WAPs use

either spread spectrum technology (which is a wideband radio frequency with

a range from a few metres to 100 metres) or infrared (which has a very short

range of about 1 to 2 metres and is easily blocked, and therefore has limited

use; see Section 2.1.5 Wired and wireless networking).

The WAP receives and transmits data between the WLAN and the wired network

structure. End users access the WLAN through wireless LAN adapters which are

built into the devices or as a plug in module.

WAP

▲ Figure 2.2 Wireless local area networks (WLAN)

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2 CommuniCation

Wide area networks (WANs)

Wide area networks (WANs) are used when computers or networks are situated

a long distance from each other (for example, they may be in different cities or

on different continents). If a number of LANs are joined together using a router

or modem, they can form a WAN. The network of automated teller machines

(ATMs) used by banks is one of the most common examples of the use of a WAN.

Because of the long distances between devices, WANs usually make use of a

public communications network (such as telephone lines or satellites), but they

can use dedicated or leased communication lines which can be less expensive

and more secure (less risk of hacking, for example).

A typical WAN will consist of end systems and intermediate systems, as shown

in Figure 2.3. 1, 3, 7 and 10 are known as end systems, and the remainder are

known as intermediate systems. The distance between each system can be

considerable, especially if the WAN is run by a multi-national company.

1 2 3

4 5 6

8 97 10

▲ Figure 2.3 A typical WAN

The following is used as a guide for deciding the ‘size’ of a network:

WAN: 100 km to over 1000 km

MAN: 1 km to 100 km

LAN: 10 m to 1000 m

PAN: 1 m to 10 m (this is not a commonly used term – it means personal area

network; in other words, a home system)

2.1.2 Client-server and peer-to-peer networking models

We will consider two types of networking models, client-server and peer-to-peer.

Client-server model

server

internet

clients

Client sends a request to the

server and the server finds

the requested data and

sends it back to the client.

A system administrator manages

the whole network; clients are

connected through a network;

allows data access even

over large distances.

▲ Figure 2.4 Client-server model

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2.1 Networking

» The client-server model uses separate dedicated servers and specific

client workstations; client computers will be connected to the server

computer(s).

» Users are able to access most of the files, which are stored on dedicated

servers.

» The server dictates which users are able to access which files. (Note: sharing

of data is the most important part of the client-server model; with peer-to-

peer, connectivity is the most important aspect.)

» The client-server model allows the installation of software onto a client’s

computer.

» The model uses central security databases which control access to the

shared resources. (Note: passwords and user IDs are required to log into the

network.)

» Once a user is logged into the system, they will have access to only those

resources (such as a printer) and files assigned to them by the network

administrator, so offers greater security than peer-to-peer networks.

» Client-server networks can be as large as you want them to be and they are

much easier to scale up than peer-to-peer networks.

» A central server looks after the storing, delivery and sending of emails.

» This model offers the most stable system, for example, if someone deletes

a shared resource from the server, the nightly back-up would restore the

deleted resource (this is different in peer-to-peer – see later).

» Client-server networks can become bottlenecked if there are several client

requests at the same time.

» In the client-server model, a file server is used and is responsible for

– central storage and management of data files, thus enabling other

network users to access files

– allowing users to share information without the need for offline devices

(such as a memory stick)

– allowing any computer to be configured as the host machine and act as

the file server (note that the server could be a storage device (such as

SSD or HDD) that could also serve as a remote storage device for other

computers, thus allowing them to access this device as if it were a local

storage device attached to their computer).

Examples of use of client-server network model

A company/user would choose a client-server network model for the following

reasons.

» The company/user has a large user-base (however, it should be pointed out

that this type of network model may still be used by a small group of people

who are doing independent projects but need to have sharing of data and

access to data outside the group).

» Access to network resources needs to be properly controlled.

» There is a need for good network security.

» The company requires its data to be free from accidental loss (in other

words, data needs to be backed up at a central location).

An example is the company Amazon; it uses the client-server network

model. The user front-end is updated every time a user logs on to the

Amazon website and a large server architecture handles items such as order

processing, billing customers and data security; none of the Amazon users are

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2 CommuniCation

aware that other customers are using the website at the same time – there is

no interaction between users and server since they are kept entirely separate

at all times.

Peer-to-peer model

node

▲ Figure 2.5 Peer-to-peer model

On a peer-to-peer network, each node joins the network to allow

» the provision of services to all other network users; the services available

are listed on a nominated ‘look up’ computer – when a node requests a

service, the ‘look up’ computer is contacted to find out which of the other

network nodes can provide the required service

» other users on the network to simply access data from another node

» communication with other peers connected to the network

» peers to be both suppliers and consumers (unlike the client-server

model where consumers and resources are kept entirely separate from

each other)

» peers to participate as equals on the network (again this is different

to the client-server model where a webserver and client have different

responsibilities).

The peer-to-peer model does not have a central server. Each of the nodes

(workstations) on the network can share its files with all the other nodes, and

each of the nodes will have its own data.

Because there is no central storage, there is no requirement to authenticate

users.

This model is used in scenarios where no more than 10 nodes are required (such

as a small business) where it is relatively easy for users to be in contact with

each other on a regular basis. More than 10 nodes leads to performance and

management issues.

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2.1 Networking

Peer-to-peer offers little data security since there is no central security system.

This means it is impossible to know who is authorised to share certain data.

Users can create their own network node share point which is the only real

security aspect since this gives them some kind of control. However, there are

no real authentication procedures.

Examples of peer-to-peer network model

A user would choose the peer-to-peer network model for one or more of

following reasons:

» The network of users is fairly small.

» There is no need for robust security.

» They require workstation-based applications rather than being server-based.

An example would be a small business where there is frequent user

interaction and there is no need to have the features of a client-server

network (for example, a builder with five associated workers located in their

own homes who only need access to each other’s diaries, previous jobs,

skills-base and so on – when the builder is commissioned to do a job they

need to access each other’s computer to check on who is available and who

has the appropriate skills).

Thin clients and thick clients

The client-server model offers thin clients and thick clients. These can often

refer to both hardware and software.

Thin client

A thin client is heavily dependent on having access to a server to allow

constant access to files and to allow applications to run uninterrupted. A

thin client can either be a device or software which needs to be connected

to a powerful computer or server to allow processing to take place (the

computer or server could be on the internet or could be part of a LAN/MAN/

WAN network). The thin client will not work unless it is connected at all

times to the computer or server. A software example would be a web browser

which has very limited functions unless it is connected to a server. Other

examples include mobile phone apps which need constant access to a server

to work. A hardware example is a POS terminal at a supermarket that needs

constant access to a server to find prices, charge customers and to do any

significant processing.

Thick client

A thick client can either be a device or software that can work offline or

online; it is still able to do some processing whether it is connected to a server

or not. A thick client can either be connected to a LAN/MAN/WAN, virtual

network, the internet or a cloud computing server. A hardware example is a

normal PC/laptop/tablet since it would have its own storage (HDD or SSD),

RAM and operating system which means it is capable of operating effectively

online or offline. An example of software is a computer game which can run

independently on a user’s computer, but can also connect to an online server to

allow gamers to play and communicate with each other.

Table 2.1 highlights some of the pros and cons of using thick client or thin

client hardware.

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2 CommuniCation

Pros Cons

Thick clients

n more robust (device can carry out

processing even when not connected

to server)

n clients have more control (they can

store their own programs and data/

files)

n less secure (relies on clients to keep

their own data secure)

n each client needs to update data and

software individually

n data integrity issues, since many

clients access the same data which

can lead to inconsistencies

Thin clients

n less expensive to expand (low-powered

and cheap devices can be used)

n all devices are linked to a server (data

updates and new software installation

done centrally)

n server can offer protection against

hacking and malware

n high reliance on the server; if the

server goes down or there is a break

in the communication link then the

devices cannot work

n despite cheaper hardware, the start-up

costs are generally higher than for

thick clients

▲ Table 2.1 Summary of pros and cons of thick and thin client hardware

Table 2.2 highlights the differences between thick and thin client software.

Thin client software Thick client software

n always relies on a connection to a remote

server or computer for it to work

n can run some of the features of the

software even when not connected

to a server

n requires very few local resources (such as

SSD, RAM memory or computer processing

time)

n relies heavily on local resources

n relies on a good, stable and fast network

connection for it to work

n more tolerant of a slow network

connection

n data is stored on a remote server or

computer

n can store data on local resources

such as HDD or SSD

▲ Table 2.2 Differences between thin and thick client software

ACTIVITY 2A

1 A company has 20 employees working on the development of a new type

of battery for use in mobile phones. Decide which type of network model

(client-server or peer-to-peer) would be most suitable. Give reasons for

your choice.

2 Another company is made up of a group of financial consultants who

advise other companies on financial matters, such as taxation and

exporting overseas. Decide which type of network model (client-server or

peer-to-peer) would be most suitable. Give reasons for your choice.

2.1.3 Network topologies

There are many ways to connect computers to make complex networks. Here we

will consider

» bus networks

» star networks

» mesh networks

» hybrid networks.

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2.1 Networking

Bus networks

A bus network topology uses a single central cable to which all computers and

devices are connected. It is easy to expand and requires little cabling. Data can

only travel in one direction; if data is being sent between devices then other

devices cannot transmit. Terminators are needed at each end to prevent signal

reflection (bounce). Bus networks are typically peer-to-peer. The disadvantages

of a bus network include:

» If the main cable fails, the whole network goes down.

» The performance of the network deteriorates under heavy loading.

» The network is not secure since each packet passes through every node.

The advantages of a bus network include:

» Even if one node fails, the remainder of the network continues to function.

» It is easy to increase the size of the network by adding additional nodes.

▲ Figure 2.6 Bus network topology

In bus network topology, each node looks at each packet and determines

whether or not the address of the recipient in the package matches the node

address. If so, the node accepts the packet; if not, the packet is ignored.

These are most suitable for situations with a small number of devices with light

traffic occurring. For example, a small company or an office environment.

Star networks

A star network topology uses a central hub/switch and each computer/device

is connected to the hub/switch. Data going from host to host is directed

through the central hub/switch. Each computer/device has its own dedicated

connection to the central node (hub/switch) – any type of network cable can

be used for the connections (see Section 2.1.5 Wired and wireless networking).

This type of network is typically a client-server. The disadvantages of a star

network include:

» The initial installation costs are high.

» If the central hub/switch fails, then the whole network goes down.

The advantages of a star network include:

» Data collisions are greatly reduced due to the topology.

» It is a more secure network since security methods can be applied to the

central node and packets only travel to nodes with the correct address.

» It is easy to improve by simply installing an upgraded hub.

» If one of the connections is broken it only affects one of the nodes.

How packets are handled depends on whether the central node is a switch or

a hub. If it is a hub, all the packets will be sent to every device/node on the

star network – if the address in the packet matches that of the node, it will be

accepted; otherwise, it is ignored (this is similar to the way packets are handled on

a bus network). If the central node is a switch, packets will only be sent to nodes

where the address matches the recipient address in the packet. The latter is clearly

more secure, since only nodes intended to see the packet will receive it.

hub/switch

▲ Figure 2.7 Star network

topology

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2 CommuniCation

Star networks are useful for evolving networks where devices are frequently

added or removed. They are well suited to applications where there is heavy

data traffic.

Mesh networks

There are two types of mesh network topologies: routing and flooding.

Routing works by giving the nodes routing logic (in other words, they act like

a router) so that data is directed to its destination by the shortest route and

can be re-routed if one of the nodes in the route has failed. Flooding simply

sends the data via all the nodes and uses no routing logic, which can lead to

unnecessary loading on the network. It is a type of peer-to-peer network, but

is fundamentally different. The disadvantages of a mesh network include:

» A large amount of cabling is needed, which is expensive and time

consuming.

» Set-up and maintenance is difficult and complex.

The advantages of a mesh network include:

» It is easy to identify where faults on the network have occurred.

» Any broken links in the network do not affect the other nodes.

» Good privacy and security, since packets travel along dedicated routes.

» The network is relatively easy to expand.

▲ Figure 2.8 Mesh network topology

There are a number of applications worth considering here:

» The internet and WANs/MANs are typical uses of mesh networks.

» Many examples include industrial monitoring and control where sensors are

set up in mesh design and feedback to a control system which is part of the

mesh, for example

– medical monitoring of patients in a hospital

– electronics interconnectivity (for example, systems that link large screen

televisions, DVDs, set top boxes, and so on); each device will be in a

location forming the mesh

– modern vehicles use wireless mesh network technology to enable the

monitoring and control of many of the components in the vehicle.

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2.1 Networking

EXTENSION ACTIVITY 2A

There appear to be similarities between the peer-to-peer network model

and mesh network model.

Describe the differences between the two models.

Hybrid networks

A hybrid network is a mixture of two or more different topologies (bus and star,

bus and mesh, and so on). The main advantages and disadvantages depend on

which types of network are used to make up the hybrid network, but an additional

disadvantage is that they can be very complex to install, configure and maintain.

Additional advantages include:

» They can handle large volumes of traffic.

» It is easy to identify where a network fault has occurred.

» They are very well suited to the creation of larger networks.

▲ Figure 2.9 Hybrid bus and star network

Note that the handling of packets in hybrid networks will depend on which of

the above topologies are used to make up the hybrid structure.

One of the typical applications of hybrid networks is illustrated by the

following example, involving three hotel chains, A, B and C.

Suppose hotel chain A uses a bus network, hotel chain B uses a star network

and hotel chain C uses a mesh network.

At some point, all three hotel chains are taken over by another company. By

using hybrid network technology, all three hotel chains can be connected

together even though they are each using a different type of network. The

system can also be expanded easily without affecting any of the existing

hotels using the network.

There are many other examples; you might want to explore the various

applications for each type of network topology.

2.1.4 Public and private cloud computing

Cloud storage is a method of data storage where data is stored on offsite

servers – the physical storage covers hundreds of servers in many locations.

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2 CommuniCation

The same data is stored on more than one server in case of maintenance or

repair, allowing clients to access data at any time. This is known as data

redundancy. The physical environment is owned and managed by a hosting

company.

There are three common systems, public cloud, private cloud and hybrid cloud.

Public cloud is a storage environment where the customer/client and cloud

storage provider are different companies.

Private cloud is storage provided by a dedicated environment behind a

company firewall. Customer/client and cloud storage provider are integrated

and operate as a single entity.

Hybrid cloud is a combination of private and public clouds. Some data resides

in the private cloud and less sensitive/less commercial data can be accessed

from a public cloud storage provider.

Instead of saving data on a local hard disk or other storage device, a user can

save their data ‘in the cloud’. The pros and cons of using cloud storage are

shown in Table 2.3.

Pros of using cloud storage Cons of using cloud storage

n customer/client files stored on the cloud can be

accessed at any time from any device anywhere in the

world provided internet access is available

n no need for a customer/client to carry an external

storage device with them, or use the same computer to

store and retrieve information

n provides the user with remote back-up of data to aid

data loss and disaster recovery

n recovers data if a customer/client has a hard disk or

back-up device failure

n offers almost unlimited storage capacity

n if the customer/client has a slow or unstable internet

connection, they would have problems accessing or

downloading their data/files

n costs can be high if large storage capacity is required

n expensive to pay for high download/upload data

transfer limits with the customer/client internet service

provider (ISP)

n potential failure of the cloud storage company is

possible – this poses a risk of loss of all back-up data

▲ Table 2.3 Summary of pros and cons of using cloud storage

Data security when using cloud storage

Companies that transfer vast amounts of confidential data from their own

systems to a cloud service provider are effectively relinquishing control of their

own data security. This raises a number of questions:

» What physical security exists regarding the building where the data is housed?

» How good is the cloud service provider’s resistance to natural disasters or

power cuts?

» What safeguards exist regarding personnel who work for the cloud service

company? Can they use their authorisation codes to access confidential data

for monetary purposes?

Potential data loss when using cloud storage

There is a risk that important and irreplaceable data could be lost from the

cloud storage facilities. Actions from hackers (gaining access to accounts or

pharming attacks, for example) could lead to loss or corruption of data. Users

need to be certain sufficient safeguards exist to overcome these risks.

The following breaches of security involving some of the largest cloud service

providers suggest why some people are nervous of using cloud storage for

important files:

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2.1 Networking

» The XEN security threat, which forced several cloud operators to reboot

all their cloud servers, was caused by a problem in the XEN hypervisor

(a hypervisor is a piece of computer software, firmware or hardware that

creates and runs virtual machines).

» A large cloud service provider permanently lost data during a routine

back-up procedure.

» The celebrity photos cloud hacking scandal, in which more than 100 private

photos of celebrities were leaked. Hackers had gained access to a number of

cloud accounts, which then enabled them to publish the photos on social

networks and sell them to publishing companies.

» In 2016, the National Electoral Institute of Mexico suffered a cloud security

breach in which 93 million voter registrations, stored on a central database,

were compromised and became publicly available to everyone. To make

matters worse, much of the information on this database was also linked to

an Amazon cloud server outside Mexico.

Cloud software

Cloud storage is, of course, only one aspect of cloud computing. Other areas

covered by cloud computing include databases, networking, software and

analytical services using the internet.

Here we will consider cloud software – you can research for yourself how

databases and analytical services are provided by cloud computing services.

Software applications can be delivered to a user’s computer on demand using

cloud computing services. The cloud provider will both host and manage

software applications – this will include maintenance, software upgrades and

security for a monthly fee. A user will simply connect to the internet (using

their web browser on a computer or tablet or mobile phone) and contact their

cloud services supplier. The cloud services supplier will connect them to the

software application they require.

The main advantages are that the software will be fully tested and it does not

need to reside on the user’s device. However, the user can still use the software

even if the internet connection is lost. Data will simply be stored on the

local device and then data will be uploaded or downloaded once the internet

connection is restored.

Cloud-based applications can, therefore, perform tasks on a local device. This

makes them fundamentally different to web-based apps which need an internet

connection at all times.

2.1.5 Wired and wireless networking

Wireless

Wi-Fi and Bluetooth

Both Wi-Fi and Bluetooth offer wireless communication between devices. They

both use electromagnetic radiation as the carrier of data transmission.

Bluetooth sends and receives radio waves in a band of 79 different frequencies

(known as channels). These are all centred on a 2.45 GHz frequency. Devices

using Bluetooth automatically detect and connect to each other, but they do

not interfere with other devices since each communicating pair uses a different

channel (from the 79 options).

When a device wants to communicate, it picks one of the 79 channels at

random. If the channel is already being used, it randomly picks another

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2 CommuniCation

EXTENSION ACTIVITY 2B

Frequency and wavelength are linked by the equation:

f =

where f = frequency (m), λ = wavelength (Hz), and

c = velocity of light (3 × 10

m /s).

Confirm the frequency values in Table 2.3 using the wavelengths given.

Table 2.5 compares radio waves, microwaves and infrared. (Please note: the

‘>’ symbol in the table means ‘better than’).

Bandwidth infrared > microwaves > radio waves

(infrared has the largest bandwidth)

Penetration radio waves > microwaves > infrared

(radio waves have the best penetration)

Attenuation radio waves > microwaves > infrared

(radio waves have the best attenuation)

▲ Table 2.5 Comparison of radio waves, microwaves and infrared

channel. This is known as spread spectrum frequency hopping. To further

minimise the risks of interference with other devices, the communication pairs

constantly change the frequencies (channels) they are using (several times a

second). Bluetooth creates a secure wireless personal area network (WPAN)

based on key encryption.

Bluetooth is useful when

» transferring data between two or more devices which are less than 30

metres apart

» the speed of data transmission is not critical

» using low bandwidth applications (for example, sending music files from a

mobile phone to a headset).

As mentioned earlier in the chapter, Wi-Fi also uses spread spectrum

technology. However, Wi-Fi is best suited to operating full-scale networks,

since it offers much faster data transfer rates, better range and better security

than Bluetooth. A Wi-Fi-enabled device (such as a computer or smart phone)

can access, for example, the internet wirelessly at any wireless access point

(WAP) or ‘hot spot’ up to 100 metres away.

As mentioned, wireless connectivity uses electromagnetic radiation: radio

waves, microwaves or infrared. The scale of frequency and wavelength of

magnetic radiation is shown in Table 2.4.

radio waves microwaves infrared visible light ultra violet X-rays gamma rays

Wave length (m) 10

−1

−3

−5

−7

−9

−11

Frequency (Hz) 3 MHz 3 GHz 300 GHz 30 THz 3 PHz 300 PHz 30 EHz

▲ Table 2.4 Frequency and wavelength of magnetic radiation

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2.1 Networking

Penetration measures the ability of the electromagnetic radiation to pass

through different media. Attenuation is the reduction in amplitude of a

signal (infrared has low attenuation because it can be affected by, for

example, rain or internal walls). Thus, we would expect infrared to be

suitable for indoor use only; the fact that it can be stopped by walls is

seen as an advantage since this stops the signal causing interference

elsewhere. Microwaves seem to offer the best compromise, since they

support reasonable bandwidth, and have reasonable penetration and

attenuation.

Additional notes on the use of satellites

The use of microwaves and radio waves was previously mentioned as a method

for allowing Wi-Fi connectivity in networks. These methods are perfectly

satisfactory for short distances – the electromagnetic waves carry the signals –

but the curvature of the Earth prevents such methods transmitting data

globally.

The electromagnetic radiation from antenna

A is transmitted but is unable to reach antenna

B due to the Earth’s curvature.

▲ Figure 2.10

To overcome this problem, we need to adopt satellite technology:

The signal is boosted by the satellite

orbiting Earth and is then beamed back

to Earth and picked up by antenna B.

The signal is beamed from

antenna A to a satellite

orbiting Earth.

▲ Figure 2.11

The communication between antennae and satellite is carried out by radio

waves or microwave frequencies. Different frequency bands are used to

prevent signal interference and to allow networks spread across the Earth to

communicate through use of satellites (many satellites orbit the Earth – refer

to Section 2.2.2 for more information on use of satellite technology with

networks).

Wired

There are three main types of cable used in wired networks (see Figure 2.12).

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2 CommuniCation

Twisted pair cables

Twisted pair cables are the most common cable type used in LANs. However,

of the three types of cable, it has the lowest data transfer rate and suffers

the most from external interference (such as electromagnetic radiation).

However, it is the cheapest option. There are two types of twisted pair cable:

unshielded and shielded. Unshielded is used by residential users. Shielded is

used commercially (the cable contains a thin metal foil jacket which cancels

out some of the external interference).

Coaxial cables

Coaxial cables are the most commonly used cables in MANs and by cable

television companies. The cost of coaxial cables is higher than twisted pair

cables but they offer a better data transfer rate and are affected less by

external interference. Coaxial cables also have about 80 times the transmission

capacity of twisted pair. Coaxial suffers from the greatest signal attenuation,

but offers the best anti-jamming capabilities.

Fibre optic cables

Fibre optic cables are most commonly used to send data over long distances,

because they offer the best data transfer rate, the smallest signal attenuation

and have a very high resistance to external interference. The main drawback is

the high cost. Unlike the other two types of cable, fibre optics use pulses of

light rather than pulses of electricity to transmit data. They have about 26 000

times the transmission capacity of twisted pair cables.

Fibre optic cables can be single- or multi-mode.

Single-mode uses a single mode light source and has a smaller central

core, which results in less light reflection along the cable. This allows the

data to travel faster and further, making them a good choice for CATV and

telecommunications.

Multi core allows for a multi-mode light source; the construction causes higher

light reflections in the core, so they work best over shorter distances (in a LAN,

for example).

Wired versus wireless

Numerous factors should be considered when deciding if a network should use

wired or wireless connectivity, as listed below.

pairs

conductor

insulator

cable jacket

copper mesh

copper wire

outside insulation

insulation

optical fibres

flexible buffer

tube

water blocking

binders

ripcord

jacket

Aramid strength

yarns

▲ Figure 2.12 (left to right) Twisted pair cable, coaxial cable, fibre optic cable

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2.1 Networking

Wireless networking

» It is easier to expand networks and is not necessary to connect devices

using cables.

» Devices have increased mobility, provided they are within range of the WAPs.

» Increased chance of interference from external sources.

» Data is less secure than with wired systems; it is easier to intercept

radio waves and microwaves than cables so it is essential to protect data

transmissions using encryption (such as WEP, WPA2).

» Data transmission rate is slower than wired networks (although it is improving).

» Signals can be stopped by thick walls (in old houses, for example) and signal

strength can vary, or ‘drop out’.

Wired networking

» More reliable and stable network (wireless connectivity is often subjected to

interference).

» Data transfer rates tend to be faster with no ‘dead spots’.

» Tends to be cheaper overall, in spite of the need to buy and install cable.

» Devices are not mobile; they must be close enough to allow for cable

connections.

» Lots of wires can lead to tripping hazards, overheating of connections

(potential fire risk) and disconnection of cables during routine office cleaning.

Other considerations

» If mobile phones and tablets are connected to the network, it will need to

offer Wi-Fi or Bluetooth capability.

» There may be regulations in some countries regarding which wireless

transmission frequencies can be used legally.

» Permission from authorities and land owners may be required before laying

cables underground.

» There are numerous competing signals in the air around us; it is important to

consider this when deciding whether to go for wired or wireless connectivity.

2.1.6 Hardware requirements of networks

In this section we will consider a number of hardware items needed to form a

LAN network and the hardware needed to form a WAN. Please note

» the concept of the WLAN and the hardware needed to support it have been

covered in earlier sections

» the hardware items hub and gateway have been included in this section to

complete the picture; however, knowledge of these two items is not required

by the syllabus.

Hub

Hubs are hardware devices that can have a number of devices or computers

connected to them.

computer

data packet sent to network

HUB

data sent out to all

computers on the

network

▲ Figure 2.13 Hub flow diagram

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2 CommuniCation

They are often used to connect a number of devices to form a local area

network (LAN), for example a star network (see Section 2.1.3). A hub’s

main task is to take any data packet (a group of data being transmitted)

received at one of its ports and then send the data to every computer

in the network. Using hubs is not a very secure method of data distribution

and is also wasteful of bandwidth. Note that hubs can be wired or

wireless devices.

Switch

Switches are similar to hubs, but are more efficient in the way they distribute

the data packet. As with hubs, they connect a number of devices or computers

together to form a LAN (for example, a star network).

However, unlike a hub, the switch checks the data packet received and works

out its destination address (or addresses) and sends the data to the appropriate

computer(s) only. This makes using a switch a more secure and efficient way of

distributing data.

computer

data packet sent to network

SWITCH

data sent out only to

the appropriate

computers on the

network

▲ Figure 2.14 Switch flow diagram

Each device or computer on a network has a media access control (MAC) address

which identifies it uniquely. Data packets sent to switches will have a MAC

address identifying the source of the data and additional addresses identifying

each device which should receive the data. Note that switches can be wired or

wireless devices.

Repeater

When signals are sent over long distances, they suffer attenuation or signal

loss. Repeaters are devices which are added to transmission systems to

boost the signal so it can travel greater distances. They amplify signals on

both analogue (copper cable) and digital (fibre optic cable) communication

links.

Repeaters can also be used on wireless systems. These are used to boost

signals to prevent any ‘dead spots’ in the Wi-Fi zone. These devices plug into

electric wall sockets and send out booster signals. They are termed non-logical

devices because they will boost all signals which have been detected; they are

not selective.

Sometimes, hubs contain repeaters and are known as repeating hubs. All

signals fed to the hub are boosted before being sent to all devices in the

network, thus increasing the operational range.

There are two main drawbacks of repeating hubs:

1 They have only one collision domain. When the signals are boosted and

then broadcast to devices, any collisions which might occur are not resolved

there and then. One way to deal with this problem is to make use of

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2.1 Networking

jamming signals – while this manages the collisions, it also reduces

network performance since it involves repeated broadcasts as the

collisions are resolved.

2 The devices are referred to as unmanaged since they are unable to manage

delivery paths and also security in the network.

Bridge

Bridges are devices that connect one LAN to another LAN that uses the same

protocol (communication rules). They are often used to connect together

different parts of a LAN so that they can function as a single LAN.

BRIDGE

computer

server

computer

SWITCH

LAN

computer

server

computer

SWITCH

LAN

▲ Figure 2.15 Bridge flow diagram

Bridges are used to interconnect LANs (or parts of LANs), since sending out

every data packet to all possible destinations would quickly flood larger

networks with unnecessary traffic. For this reason, a router is used to

communicate with other networks, such as the internet. Note that bridges can

be wired or wireless devices.

Router

Routers enable data packets to be routed between the different networks for

example, to join a LAN to a WAN. The router takes data transmitted in one

format from a network (which is using a particular protocol) and converts the

data to a protocol and format understood by another network, thereby allowing

them to communicate via the router. We can, therefore, summarise the role of

routers as follows. Routers

» restrict broadcasts to a LAN

» act as a default gateway

» can perform protocol translation; for example, allowing a wired network

to communicate with a wireless (Wi-Fi) network – the router can take an

Ethernet data packet, remove the Ethernet part and put the IP address into

a frame recognised by the wireless protocol (in other words, it is performing

a protocol conversion)

» can move data between networks

» can calculate the best route to a network destination address.

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2 CommuniCation

ROUTER

computer

server

computer

SWITCH

LAN

LAN or WAN

internet

▲ Figure 2.16 Router flow diagram

Broadband routers sit behind a firewall. The firewall protects the computers

on a network. The router’s main function is to transmit internet and

transmission protocols between two networks and allow private networks to

be connected.

The router inspects the data package sent to it from any computer on any

of the networks connected to it. Since every computer on the same network

has the same part of an internet protocol (IP) address, the router is able to

send the data packet to the appropriate switch and it will then be delivered

using the MAC destination address (see next section). If the MAC address

doesn’t match any device on the network, it passes on to another switch on

the same network until the appropriate device is found. Routers can be wired

or wireless devices.

Gateway

A gateway is a network point (or node) that acts as an entrance to another

network. It is a key point for data on its way to or from other networks. It

can be used to connect two or more dissimilar LANs (LANs using different

protocols). The gateway converts data packets from one protocol to another.

Gateways can also act as routers, firewalls or servers – in other words, any

device that allows traffic to flow in and out of the networks. Gateways can be

wired or wireless devices.

All networks have boundaries so that all communication within the network is

conducted using devices such as switches or routers. If a network node needs

to communicate outside its network, it needs to use a gateway.

Modems

Modern computers work with digital data, whereas many of the public

communication channels still only allow analogue data transmission. To allow

the transmission of digital data over analogue communication channels we

need to use a modem (modulator demodulator). This device converts digital

data to analogue data. It also does the reverse and converts data received

over the analogue network into digital data which can be understood by the

computer.

Wireless modems transmit data in a modulated form to allow several

simultaneous wireless communications to take place without interfering

with each other. A modem will connect to the public infrastructure (cable,

telephone, fibre-optics or satellite) and will supply the user with a standard

Ethernet output which allows connection to a router, thus enabling an internet

connection to occur.

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2.1 Networking

modem router

laptop

smart phone

tablet

internet

▲ Figure 2.17 Wireless modem flow diagram

While the router will allow the creation of a network in a home, for

example, the modem allows for the connection to the external networks

(for example, the internet). Routers and modems can be combined into one

unit; these devices have the electronics and software to provide both router

and modem functions.

Another example of a modem is a softmodem (software modem), which uses

minimal hardware and uses software that runs on the host computer. The

computer’s resources (mainly the processor and RAM) replace the hardware of a

conventional modem.

Table 2.6 shows the differences between routers and gateways.

Routers Gateways

n forward packets of data from one

network to another; routers read each

incoming packet of data and decide

where to forward the packet

n convert one protocol (or data format)

to another protocol (format) used in a

different network

n can route traffic from one network to

another network

n convert data packets from one protocol

to another; they act as an entry and

exit point to networks

n can be used to join LANs together to

form a WAN (sometimes called brouters)

and also to connect a number of LANs

to the internet

n translate from one protocol to another

n offer additional features such as

dynamic routing (ability to forward

data by different routes)

n do not support dynamic routing

▲ Table 2.6 Differences between routers and gateways

EXTENSION ACTIVITY 2C

Draw a diagram to show how a gateway could be used to connect together

three LANs which are using different protocols. Include all the hardware

devices and cables needed.

Network interface card (NIC)

A network interface card (NIC) is needed to allow a device to connect to

anetwork (such as the internet). It is usually part of the device hardware

andfrequently contains the MAC address generated at the manufacturing

stage.

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2 CommuniCation

Wireless network interface card/controller (WNIC)

Wireless network interface cards/controllers (WNICs) are the same as the

more ordinary NICs, in that they are used to connect devices to the internet

or other networks. They use an antenna to communicate with networks via

microwaves and normally simply plug into a USB port or can be internal

integrated circuit plug in.

As with usual NICs, they work on layers 1 and 2 of the OSI model (refer to

Chapter 14 for more details). WNICs work in two modes.

Infrastructure mode requires WAPs (wireless access points) and all the data

is transferred using the WAP and hub/switch; all the wireless devices connect

to the WAP and must use the same security and authentication techniques.

Ad hoc mode does not need to have access to WAPs; it is possible for devices

to interface with each other directly.

2.1.7 Ethernet

Ethernet is a protocol used by many wired LANs. It was adopted as a standard

by the Institute of Electrical and Electronic Engineers (IEEE) and Ethernet is

also known as IEEE 802.3. A network using Ethernet is made upof:

» a node (any device on the LAN)

» medium (path used by the LAN devices, such as an Ethernet cable)

» frame (data is transmitted in frames which are made up of source address

and destination address – the addresses are often the MAC address).

Conflicts

When using Ethernet, it is possible for IP addresses to conflict; this could show

up as a warning such as that in Figure 2.19.

▲ Figure 2.19 IP address conflict error

This may occur if devices on the same network have been given the same

IP address; without a unique IP address it is not possible to connect to a

network. This is most likely to occur on a LAN where dynamic IP addresses

may have been used. Dynamic IP addresses are temporary and may have been

assigned to a device on the network, unfortunately, another device using

static IP addresses may already have the same IP address. This can be resolved

by re-starting the router. Any dynamic IP addresses will be re-assigned, which

could resolve the issue.

Collisions

Ethernet supports broadcast transmission (communications where pieces of

data are sent from sender to receiver) and are used to send messages to all

devices connected to a LAN. The risk is that two messages using the same data

▲ Figure 2.18 Wireless

network interface

card/controller (WNIC)

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2.1 Networking

channel could be sent at the same time, leading to a collision. Carrier sense

multiple access with collision detection (CSMA/CD) was developed to try and

resolve this issue. Collison detection depends on simple physics: when a frame

is sent it causes a voltage change on the Ethernet cable. When a collision is

detected, a node stops transmitting a frame and transmits a ‘jam’ signal and

then waits for a random time interval before trying to resend the frame.

CSMA/CD protocol will define the random time period for a device to wait

before trying again.

Figure 2.20 shows how data collisions can be dealt with using transmission

counters (which keep track of how many times the collision detection routine

has been entered – there will a defined limit as part of the CSMA/CD protocol)

and random time periods.

assemble

frame

is line

idle?

wait for

allocated time

Yes

start to send

frame

END

set transmission

counter = 1

collision

detected?

stop transmission and

send jam signal

increment transmission

counter

abort

transmission

max

transmission

counter?

wait for allocated time period

then re-start transmission

frame

sent?

continue to

send

another

frame?

▲ Figure 2.20 How data collisions can be dealt with using transmission counters

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2 CommuniCation

EXTENSION ACTIVITY 2D

Review Figure 2.20. As it stands, it is possible for an endless loop to be

established.

Suggest a modification to the flow diagram to ensure it terminates if there is

a problem with the data channel, or to prevent the data transmission holding

up the computer for an unacceptable time period.

2.1.8 Bit streaming

Bit streaming is a contiguous sequence of digital bits sent over the internet

or a network that requires a high speed data communication link (such as

fast broadband). Since bit streaming often involves very large files (such as

video) it is necessary for the files to undergo some data compression before

transmission. It is also necessary to have some form of buffering to ensure

smooth playback of the media files.

The data transmission rate from the file server (containing the video, for

example) to the buffer must be greater than the rate at which data is

transmitted from buffer to media player. The larger the buffer, the better the

control over the bit rate being sent to the media player. The media player will

always check to ensure data lies between a minimum value (often referred to as

low water mark) and a maximum value (often referred to as a high water mark).

The difference between the two values is usually about 80% of the total buffer

capacity. The buffer is a temporary storage area of the computer.

source of

data stream

low high

buffer

media

player

bit streaming

from server

▲ Figure 2.21 Bit streaming

Table 2.7 shows the pros and cons of bit streaming.

Pros of bit streaming Cons of bit streaming

n no need to wait for a whole video or

music file to be downloaded before the

user can watch or listen

n no need to store large files on your

device

n allows video files and music files to be

played on demand (as required)

n no need for any specialist hardware

n affords piracy protection (more difficult

to copy streamed files than files stored

on a hard drive)

n cannot stream video or music files if

broadband connection is lost

n video or music files will pause to allow

the data being streamed to ‘catch up’ if

there is insufficient buffer capacity or

slow broadband connection

n streaming uses up a lot of bandwidth

n security risks associated with

downloading files from the internet

n copyright issues

▲ Table 2.7 Pros and cons of bit streaming

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2.1 Networking

Bit streaming can be either on demand or real time.

On demand

» Digital files stored on a server are converted to a bit streaming format

(encoding takes place and the encoded files are uploaded to a server).

» A link to the encoded video/music file is placed on the web server to be

downloaded.

» The user clicks on the link and the video/music file is downloaded in a

contiguous bit stream.

» Because it is on demand, the streamed video/music is broadcast to the user

as and when required.

» It is possible to pause, rewind and fast forward the video/music if required.

Real time

» An event is captured by camera and microphone and is sent to a computer.

» The video signal is converted (encoded) to a streaming media file.

» The encoded file is uploaded from the computer to the dedicated video

streaming server.

» The server sends the encoded live video to the user’s device.

» Since the video footage is live it is not possible to pause, rewind or fast

forward.

ACTIVITY 2B

1 a) Explain the differences between LAN, MAN and WAN.

b) Give three of the benefits of networking computers.

c) Explain the following terms.

i) Thick client

ii) Thin client

2 a) Draw diagrams to show the following network topologies.

i) Bus

ii) Star

iii) Mesh

b) Give one benefit and one drawback of using each type of network

topology.

3 a) Explain the differences between public and private cloud computing.

b) Give two benefits of using cloud computing.

c) Give two drawbacks of using cloud computing.

4 You have been asked by a manager to write a report on whether a

LAN being set up in their new building should use wired or wireless

connectivity. The building has 20 floors.

Explain your arguments for and against using both types of connectivity

and draw a conclusion to help the manager make their decision.

5 a) What is meant by bit streaming?

b) Why is it necessary to use buffers whilst streaming a video from the

internet?

c) Explain the differences between on demand and real time bit streaming.

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2 CommuniCation

2.2 The internet

2.2.1 The differences between the internet and the World

Wide Web

There are fundamental differences between the internet and the World Wide

Web (WWW).

Internet

» The internet is a massive network of networks (although, as explained in

Section 2.1.1, the internet is not a WAN) which are made up of various

computers and other electronic devices.

» It stands for interconnected network.

» The internet makes use of transmission control protocol (TCP)/internet

protocol (IP).

Key terms

Internet – massive network of

networks, made up of computers and

other electronic devices; uses TCP/IP

communication protocols.

World Wide Web (WWW) – collection

of multimedia web pages stored on

a website, which uses the internet to

access information from servers and

other computers.

HyperText Mark-up Language (HTML) –

used to design web pages and to write

http(s) protocols, for example.

Uniform resource locator (URL) –

specifies location of a web page (for

example, www.hoddereducation.co.uk).

Web browser – software that connects

to DNS to locate IP addresses; interprets

web pages sent to a user’s computer so

that documents and multimedia can be

read or watched/listened to.

Internet service provider (ISP) –

company which allows a user to connect

to the internet. They will usually charge a

monthly fee for the service they provide.

Public switched telephone network

(PSTN) – network used by traditional

telephones when making calls or when

sending faxes.

Voice over Internet Protocol (VoIP) –

converts voice and webcam images

into digital packages to be sent over the

internet.

Internet protocol (IP) – uses IPv4 or IPv6

to give addresses to devices connected

to the internet.

IPv4 – IP address format which uses

32 bits, such as 200.21.100.6.

Classless inter-domain routing

(CIDR) – increases IPv4 flexibility by

adding a suffix to the IP address, such as

200.21.100.6/18.

IPv6 – newer IP address format

which uses 128 bits, such as

A8F0:7FFF:F0F1:F000:3DD0:

256A:22FF:AA00.

Zero compression – way of reducing the

length of an IPv6 address by replacing

groups of zeroes by a double colon

(::); this can only be applied once to an

address to avoid ambiguity.

Sub-netting – practice of dividing

networks into two or more sub-networks.

Private IP address – an IP address

reserved for internal network use behind

a router.

Public IP address – an IP address

allocated by the user’s ISP to identify the

location of their device on the internet.

Domain name service (DNS) – (also

known as domain name system) gives

domain names for internet hosts and is

a system for finding IP addresses of a

domain name.

JavaScript

– object-orientated (or

scripting) programming language used

mainly on the web to enhance HTML

pages.

PHP – hypertext processor; an HTML-

embedded scripting language used to

write web pages.

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2.2 The internet

World Wide Web (WWW)

» This is a collection of multimedia web pages and other documents which are

stored on websites.

» http(s) protocols are written using HyperText Mark-up Language (HTML).

» Uniform resource locators (URLs) specify the location of all web pages.

» Web resources are accessed by web browsers.

» The world wide web uses the internet to access information from servers and

other computers.

2.2.2 Hardware and software needed to support

the internet

The fundamental requirements for connecting to the internet are

» a device (such as a computer, tablet or mobile phone)

» a telephone line connection or a mobile phone network connection

(however, it is possible that a tablet or mobile phone may connect to the

internet using a wireless router)

» a router (which can be wired or wireless) or router and modem

» an internet service provider (ISP) (combination of hardware and software)

» a web browser.

The telephone network system, public switched telephone network (PSTN),

is used to connect computers/devices and LANs between towns and cities.

Satellite technology is used to connect to other countries (see later).

In recent years, telephone lines have changed from copper cables to fibre optic

cables, which permits greater bandwidth and faster data transfer rates (and

less risk of data corruption from interference). Fibre optic telephone networks

are usually identified as ‘fast broadband’. As discussed earlier, high speed

broadband has allowed WLANs to be developed by using WAPs.

High speed communication links allow telephone and video calls to be made

using a computer and the internet. Telephone calls require either an internet-

enabled telephone connected to a computer (using a USB port) or external/

internal microphone and speakers. Video calls also require a webcam. When

using the internet to make a phone call, the user’s voice is converted to

digital packages using Voice over Internet Protocol (VoIP). Data is split into

packages (packet switching) and sent over the network via the fastest route.

Packet switching and circuit switching are covered in more detail in Chapter 14.

Comparison between PSTN and internet when making a phone call

Public switched telephone network (PSTN)

PSTN uses a standard telephone connected to a telephone line.

The telephone line connection is always open whether or not anybody is talking–

the link is not terminated until the receivers are replaced by both parties.

Telephone lines remain active even during a power cut; they have their own

power source.

Modern phones are digitised systems and use fibre optic cables (although

because of the way it works this is a big waste of capacity – a 10 minute phone

call will transmit about 10 MB of data).

Existing phone lines use circuit switching (when a phone call is made the

connection (circuit) is maintained throughout the duration of the call – this is

the basis of PSTN).

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2 CommuniCation

Phone calls using the internet

Phone calls using the internet use either an internet phone or microphone and

speakers (video calls also require a webcam).

The internet connection is only ‘live’ while data (sound/video image) is being

transmitted.

Voice over Internet Protocol (VoIP) converts sound to digital packages

(encoding) which can be sent over the internet.

VoIP uses packet switching; the networks simply send and retrieve data as it

is needed so there is no dedicated line, unlike PSTN. Data is routed through

thousands of possible pathways, allowing the fastest route to be determined.

The conversation (data) is split into data packages. Each packet contains at

least the sender’s address, receiver’s address and order number of packet – the

sending computer sends the data to its router which sends the packets to

another router, and so on. At the receiving end, the packets are reassembled

into the original state (see Chapter 14 for more details).

VoIP also carries out file compression to reduce the amount of data being

transmitted.

Because the link only exists while data is being transmitted, a typical 10

minute phone call may only contain about 3 minutes where people are talking;

thus only 3 MB of data is transmitted making it much more efficient than PSTN.

Cellular networks and satellites

Other devices, such as mobile phones, use the cellular network. Here, the

mobile phone providers act as the ISPs and the phones contain communication

software which allows them to access the telephone network and also permits

them to make an internet connection.

Satellites are an important part of all network communications that cover

vast distances. Due to the curvature of the Earth, the height of the satellite’s

orbit determines how much coverage it can give. Figure 2.22 shows how

satellites are classified according to how high they orbit in relation to the

Earth’s surface.

35 800 km

5000–12 000 km

500–2500 km

GEO

MEO

LEO

Geostationary Earth Orbit (GEO) provide long distance telephone and computer network communications;

orbital period = 24 hours

Medium Earth Orbit (MEO) used for GPS systems (about 10 MEO satellites are currently orbiting the Earth);

orbital period = 2 to 12 hours

Low Earth Orbit (LEO) used by the mobile phone networks (there are currently more than 100 LEO satellites

orbiting the Earth); orbital period = 80 mins to 2 hours

diagram not to scale

▲ Figure 2.22 Satellite classification

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2.2 The internet

Satellites have the advantage that they will always give complete coverage

and don’t suffer from signal attenuation to the same extent as underground/

undersea cables. It is also difficult to isolate and resolve faults in cables on

the sea bed.

2.2.3 IP addresses

The internet is based on TCP/IP protocols. Protocols define the rules that must

be agreed by senders and receivers on the internet. Protocols can be divided

into TCP layers (see Chapter 14). We will first consider internet protocols (IP).

Internet protocols (IP)

IPv4 addressing

The most common type of addressing on the internet is IP version 4 (IPv4).

This is based on 32 bits giving 2

(4 294 967 296) possible addresses. The

32 bits are split into four groups of 8 bits (thus giving a range of 0 to 255).

For example, 254.0.128.77.

The system uses the group of bits to define network (netID) and network host

(hostID). The netID allows for initial transmission to be routed according to the

netID and then the hostID is looked at by the receiving network. Networks are

split into five different classes, as shown in Table 2.8 below.

Network

class

IPv4 range Number of

netID bits

Number of

hostID bits

Types of

network

A 0.0.0.0 to 127.255.255.255 8 24 very large

B 128.0.0.0 to 191.255.255.255 16 16 medium size

C 192.0.0.0 to 223.255.255.255 24 8 small networks

D 224.0.0.0 to 239.255.255.255 – – multi-cast

E 240.0.0.0 to 255.255.255.255 – – experimental

▲ Table 2.8 The five network classes

Consider the class C network IP address 190.15.25.240, which would be written

in binary as:

10111110 00001111 00011001 11110000

Here the network id is 190.15.25 and the host ID is 240.

Consider the class B network IP address 128.148.12.14, which would be written

in binary as:

10000000 10010100 00001100 00001110

Here the network ID is 128.148 and the host ID is 12.14 (made up of sub-net ID

12 and host ID of 14).

Consider the class A network IP address 29.68.0.43, which would be written in

binary as:

00011101 01000100 00000000 00101011

Here the network ID is 29 and the host ID is 68.0.43 (made up of sub-net ID

68.0 and host ID of 43).

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2 CommuniCation

However, it soon became clear that this IPv4 system provides insufficient

address range. For example, a user with a medium sized network (class B) might

have 284 host machines and their class B licence allows them 2

(65534; note

the value is not 65536 since two values are not assigned). This means several of

the allocated host IDs will not be used, which is wasteful.

Classless inter-domain routing (CIDR) reduces this problem by increasing the

flexibility of the IPv4 system. A suffix is used, such as 192.30.250.00/18, which

means 18 bits will be used for the net ID and the last 14 bits will be used for

the host ID (rather than the normal 24 bits and 8 bits for a class C network).

The suffix clearly increases the flexibility regarding which bits represent the

net ID and which represent the host ID.

EXTENSION ACTIVITY 2E

Network address translation (NAT) removes the need for each IP address to

be unique. Find out how it works.

IPv6 addressing

IPv6 addressing has been developed to overcome some of the problems

associated with IPv4. This system uses 128-bit addressing, which allows for

much more complex addressing structures. An IPv6 address is broken into

16-bit chunks and because of this, it adopts the hexadecimal notation. For

example:

A8FB:7A88:FFF0:0FFF:3D21:2085:66FB:F0FA

Note how a colon (:) rather than a decimal point (.) is used here.

It has been designed to allow the internet to grow in terms of number of hosts

and the potential amount of data traffic. IPv6 has benefits over IPv4, it

» has no need for NATs (network address translation)

» removes risk of private IP address collisions

» has built in authentication

» allows for more efficient routing.

Zero compression

IPv6 addresses can be quite long; but there is a way to shorten them using

zero compression. For example, 900B:3E4A:AE41:0000:0000:AFF7:DD44:F1FF

can be written as:

900B:3E4A:AE41::AFF7:DD44:F1FF

With the section 0000:0000 replaced by ::

The zero compression can only be applied ONCE to an IPv6 address, otherwise

it would be impossible to tell how many zeros were replaced on each occasion

where it was applied. For example, 8055:F2F2:0000:0000:FFF1:0000:0000:DD04

can be rewritten either as:

8055:F2F2::FFF1:0000:0000:DD04

or as:

8055:F2F2:0000:0000:FFF1::DD04

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2.2 The internet

8055:F2F2::FFF1::DD04 is not a legal way of compressing the original address –

we have no way of knowing whether the original address was

8055:F2F2:0000:FFF1:0000:0000:0000:DD04

8055:F2F2:0000:0000:0000:FFF1:0000:DD04

8055:F2F2:0000:0000:FFF1:0000:0000:DD04

It would, therefore, be regarded as ambiguous.

Sub-netting

CIDR is actually based on sub-netting and the two are similar in many ways.

Sub-netting divides a LAN into two or more smaller networks. This helps reduce

network traffic and can also hide the complexity of the overall network. Recall

that the IP address (using IPv4) is made up of the netID and hostID. Suppose

a university network has eight departments and has a netID of 192.200.20

(11000000.11001000.00010100). All of the devices on the university network

will be associated with this netID and can have hostID values from 00000001

to 1111110 (hostIDs containing all 0s or all 1s are forbidden). The university

network will look something like this:

internet

Admin and

finance

Humanities

Maths

Science

Arts

Engineering

Computing

Business

gateway

▲ Figure 2.23 An example of a university network

So, for example, the devices in the Admin and finance department might have

hostIDs of 1, 8, 240, 35, 67, 88, 134, and so on, with similar spreads for the

other seven departments.

It would be beneficial to organise the netIDs and hostIDs so that the network

was a lot less complex in nature. With sub-netting, the hostID is split as follows:

000 00000, where the first 3 bits are netID expansion and the last 5 bits are

the hostIDs.

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2 CommuniCation

Thus, we have eight sub-nets with the same range of hostIDs.

Department netID hostID range

Admin and finance 192.200.20.0 00001 to 11110

Humanities 192.200.20.1 00001 to 11110

Maths 192.200.20.2 00001 to 11110

Science 192.200.20.3 00001 to 11110

Arts 192.200.20.4 00001 to 11110

Engineering 192.200.20.5 00001 to 11110

Computing 192.200.20.6 00001 to 11110

Business 192.200.20.7 00001 to 11110

▲ Table 2.9

Admin and

finance

192.200.20.0

Humanities

192.200.20.1

Maths

192.200.20.2

Science

192.200.20.3

Arts

192.200.20.4

Engineering

192.200.20.5

Computing

192.200.20.6

Business

192.200.20.7

internet

router

▲ Figure 2.24 An example of a university network with netIDs

The devices in the Admin and finance department will have IP addresses

192.200.20.000 00001 to 192.200.20.000 11110

The Humanities department will have IP addresses

192.200.20.001 00001 to 192.200.20.001 11110

And so on for the other departments.

To obtain the netID from the IP address we can apply the AND mask (recall

that 1 AND 1 = 1, 0 AND 0 = 0 or 1 AND 0 = 0). Thus, if a device has an IP

address of

11000000.11001000.00010100.011 00011

we can apply the AND mask

11111111.11111111.11111111.111 00000

which results in the netID value

11000000.11001000.00010100.011 00000 (or 192.200.20.03)

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2.2 The internet

This is the Science department. Consequently, the whole network is more

efficient (for the reasons stated above) and less complex. Compare this to

CIDR 192/200/20/0/27, which extends the size of the netID to 27 bits and has

a hostID of only 5 bits, but would not reduce the complexity of the network.

Private IP addresses and public IP addresses

Private IP addresses are reserved for internal use behind a router or other NAT

device. The following blocks are reserved for private IP addresses.

Class A 10.0.0.0 to 10.255.255.255 16 million possible addresses

Class B 172.16.0.0 to 172.31.255.255 1 million possible addresses

Class C 192.168.0.0 to 192.168.255.255 65 600 possible addresses

▲ Table 2.10

Private IP addresses (which are internal value only) allow for an entirely

separate set of addresses within a network. They allow access to the network

without taking up a public IP address space. However, devices using these

private IP addresses cannot be reached by internet users.

Public IP addresses are the ones allocated by a user’s ISP to identify the

location of their device. Devices using these IP addresses are accessible from

anybody using the internet. Public IP addresses are used by

» DNS servers

» network routers

» directly-controlled computers.

2.2.4 Uniform resource service (URLs)

Web browsers are software that allow users to access and display web pages on

their screens. They interpret HTML sent from websites and display the results.

Web browsers use uniform resource locators (URL) to access websites; these are

represented by a set of four numbers, such as 109.108.158.1.

But it is much easier to type this into a browser using the following format:

protocol://website address/path/filename

Protocol is usually http or https

Website address is

» domain host (www)

» domain name (name of website)

» domain type (.com, .org, .net, .gov, and so on)

» (sometimes) a country code (.uk, .de, .cy, .br, and so on).

Path is the web page (if this is omitted then it is the root directory of the website)

Filename is the item from the web page

For example: http://www.hoddereducation.co.uk/computerscience

2.2.5 Domain name service (DNS)

The domain name service (DNS) (also known as domain name system) gives domain

names for internet hosts and is a system for finding IP addresses of a domain name.

Domain names eliminate the need for a user to memorise IP addresses. The DNS

process involves converting a host name (such as www.hoddereducation.co.uk) into

an IP address the computer can understand (such as 107.162.140.19).

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2 CommuniCation

Often, DNS servers contain a database of URLs with the matching IPaddresses.

DNS server (1)

computer

DNS server (2)

website server

▲ Figure 2.25 An example of the DNS process

① The user opens their web browser and types in the URL

(www.hoddereducation.co.uk) and the web browser asks the DNS server (1)

for the IP address of the website.

② The DNS server can’t find www.hoddereducation.co.uk in its database or its

cache and sends out a request to DNS server (2).

③ DNS server (2) finds the URL and can map it to 107.162.140.19; the IP

address is sent back to DNS server (1) which now puts the IP address and

associated URL into its cache/database.

④ This IP address is then sent back to the user’s computer.

⑤ The computer now sets up a communication with the website server and the

required pages are downloaded. The web browser interprets the HTML and

displays the information on the user’s screen.

2.2.6 Scripting in HTML

This section considers HTML scripting using JavaScript and PHP. While this

extends beyond the syllabus, it is included here to help you understand how

HTML is used to create websites and how web browsers communicate with

servers. It is included here for information and to aid understanding.

A user may wish to develop a web application, which is client-server based, on

their own computer. To do this they would need to:

» download the necessary server software

» install the application on the chosen/allocated server

» use the web browser on their computer to access and interpret the

application web pages.

Each web page would need to be created using HTML. A domain name would

have to be purchased from a web-hosting company. The HTML files would

needto be uploaded to the server which was allocated to the user by the

web-hosting company.

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2.2 The internet

HTML would be used to create a file using tags. For example:

<html>

<body>

<p> Example <p/>

[program code]

</html>

Between the HTML tags the inclusion of JavaScript or PHP can be used.

JavaScript

JavaScript (unlike HTML) is a programming language which will run on the

client-side. What is the difference between running on the client-side and

running on the server-side?

» Client-side – the script runs on the computer, which is making the request,

processing the web page data that is being sent to the computer from the

server.

» Server-side – the script is run on the web server and the results of

processing are then sent to the computer that made the request.

The following short program inputs a temperature and outputs ‘HIGH’ if it is

200 °C or over, ‘OK’ if it is 100 °C or over and ‘LOW’ if it is below 100 °C.

01 <html>

02 <body>

03 <p>Enter the temperature</p>

04 <input id="Temp" value="0"

05 <button onclick="checkReading()>"Enter</button>

06 <script>

07 function checkReading() {

08 var temp, result;

09 temp = document.getElementById("Temp").value;

10 if (temp >= 200) {

11 result = "HIGH"

12 } else if (temp >= 100) {

13 result = "OK"

14 } else {

15 result = "LOW"

16 }

17 alert("The result is " + result)

18 }

19 </script>

20 </body>

21 </html>

PHP

PHP is another language which can be embedded within HTML. However,

when PHP is used it is processed on the server-side. Again, the code will be

sandwiched inside HTML and will be stored as a .php file.

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2 CommuniCation

The following example is similar to the JavaScript example; again temperatures

are input but this time ‘H’, ‘O’ and ‘L’ are output depending on the result. Note

that variables begin with $ and are case-sensitive.

01 <?php

02 if(isset($ _ GET['temp'])) {

03 echo "Result: " . checkReading($ _ GET['temp']);

04 } else {

05 ?>

06 <form action="#" method="get">

07 Enter Temp: <input type="text" name="temp" /><br />

08 <input type="submit" value="Calculate" />

09 </for m>

11 <?php

12 }

13 function checkReading($inputTemp) {

14 $resultChar = "L";

15 if($inputTemp >= 200) $resultChar = "H";

16 else if($inputTemp >= 100) $resultChar = "O";

17 return $resultChar;

18 }

19 ?>

EXTENSION ACTIVITY 2F

Look at the two pieces of code in the previous JavaScript and PHP sections,

then answer these questions.

a) Write down the names of two variables which are used in each piece

of code.

b) In each case, identify which statement(s) correspond(s) to an output.

c) What is the purpose of the statement shown in line:

i) 09 of the JavaScript code

ii) 03 of the PHP code?

e) What is the purpose of line 05 in the JavaScript?

ACTIVITY 2C

1 a) Describe what happens when a telephone call is made using PSTN.

b) Describe what happens when a computer, equipped with microphone

and speakers, is used to make a ‘telephone’ call over the internet.

c) Communication links between continents frequently involve the use of

satellite technology. Explain the differences between GEO, MEO and

LEO satellites.

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2.2 The internet

2 a) Class A computer networks are identified by IP addresses starting

with 0.0.0.0, class B computer networks are identified by IP addresses

starting with 128.0.0.0 and class C computer networks are identified by IP

addresses starting with 192.0.0.0. (Class D networks begin with 224.0.0.0.)

Write these starting IP addresses in binary format.

b) Using the data above, write down the upper IP addresses of the three

network classes A, B and C.

c) A device on a network has the IP address:

10111110 00001111 00011001 11110000

i) Which class of network is the device part of?

ii) Which bits are used for the net ID and which bits are used for the

host ID?

iii) A network uses IP addresses of the form 200.35.254.25/18.

Explain the significance of the appended value 18.

d) Give two differences between IPv4 and IPv6.

3 a) Describe the differences between private IP addresses and public

IP addresses.

b) Identify the protocol, domain name and file name used in the following

URL: https://www.exampleofaurl.co.de/computer_logic.html

c) Describe how DNS is used to retrieve a web page from the website

used in part b).

4 a) Explain the differences between the internet and the world wide web

(www).

b) Hasina wrote,

‘The internet is not necessarily a type of WAN.’

Is Hasina’s statement correct? Give reasons for your answer.

c) Explain these two terms.

i) Web browser

ii) Internet service provider (ISP)

End of chapter

questions

1 Star and mesh are two types of network topology that can be used to make a LAN.

Star network Mesh network

a) i) State one benefit and one drawback of the star network topology. [2]

ii) State one benefit and one drawback of the mesh network topology. [2]

➔

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2 CommuniCation

b) Copy the diagram below and connect each description to either a client-server

or peer-to-peer network. [4]

Type of network Description

Connectivity is the most important aspect of

this type of network

Uses separate dedicated servers and specific

workstations

Client-server

Has no central storage and doesn’t require

authentication of users

Sharing of data is the most important aspect of

this type of network

Has no central server; each workstation shares

its files/data with the others

Peer-to-peer

Performance and management issues can occur

if the number of workstations exceeds ten

Once logged in, a user can only access resources

that the network manager allows them to use

More stable system since there is centralised

backing up of files

2 a) Conventional telephone calls are made using the public service telephone

network (PSTN). The national network uses both copper cables and fibre

optic cables.

i) Explain the difference between copper cabling and fibre optic cabling. [2]

ii) Describe two benefits and two drawbacks of both types of cabling. [4]

b) Satellite technology is often used in long distance communications.

Compare the differences between GEO, MEO and LEO satellites. [3]

c) Some telephones use Bluetooth to connect to the telephone network. Explain

what is meant by:

i) the attenuation of a signal [2]

ii) spread spectrum frequency hopping. [2]

3 a) Explain the term bit streaming. [2]

b) A person watches a film streamed from a website on a tablet computer.

i) Give two benefits of using bit streaming for this purpose. [2]

ii) State two potential problems of using bit streaming for this purpose. [2]

c) Explain the terms on-demand bit streaming and real-time bit streaming. [4]

Cambridge International AS & A Level Computer Science 9608

Paper 11 Q1 November 2015

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2.2 The internet

4 A buffer is 2 MiB in size. The lower limit of the buffer is set at 200 KiB and the

higher limit is set at 1.8 MiB.

Data is being streamed at 1.5 Mbps and the media player is taking data at the rate

600 kbps.

You may assume a megabit is 1 048 576 bits and a kilobit is 1024 bits.

a) Explain why the buffer is needed. [2]

b) i) Calculate the amount of data stored in the buffer after 2 seconds of

streaming and playback.

You may assume that the buffer already contains 200 KiB of data. [4]

ii) By using different time values (such as 4 secs, 6 secs, 8 secs, and so on)

determine how long it will take before the buffer reaches its higher limit

(1.8 MiB). [5]

c) Describe how the problem calculated in part b) ii) can be overcome so that a

30-minute video can be watched without frequent pausing of playback. [2]

5 a) When data is transmitted over a LAN network there is the possible risk of

data collision.

i) Explain the term data collision. [2]

ii) Describe how CSMA/CD is able to detect collisions. [1]

iii) Explain how CSMA/CD can be used to resolve the problem of data

collision. [2]

b) Copy the diagram below and connect each network device to its

description. [5]

Network device Description

gateway

device that analyses packets of data transmitted

from one network to another or analyses data

within a single network

switch

network point (node) that connects two networks

that use different protocols

hub

device that connects LANs that use

the same protocol to allow them to work as a

single network

router

device on a network that redirects data received

to only those destinations on the LAN network

that match the address in the data packet

bridge

device that sends all the received data packets to

every device in the network irrespective of any

data packet addresses

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