16.1 Basic Concepts of Computer Networks

Computer Networks



A system of interconnected computers, peripherals and
communication devices that can exchange information and share
resources.
Networking is widely used in government departments,
corporations, educational institutions and homes.
Internet - the world’s largest computer network connecting
billions of computers together around the world
The Internet is the largest
computer network.
16.1 Basic Concepts of Computer Networks

Computer Networks



A system of interconnected computers, peripherals and
communication devices that can exchange information and share
resources.
Networking is widely used in government departments,
corporations, educational institutions and homes.
Internet - the world’s largest computer network connecting
billions of computers together around the world
The Internet is the largest
computer network.
16.2 Local Area Networks (LANs) and Wide
Area Networks (WANs)

LANs and WANs



Networks are categorized according to the area they cover.
Local area network (LAN)

A network that connects computers within a
relatively limited area
Wide area network (WAN)

A network that spans a wide geographical area
16.2 Local Area Networks (LANs) and Wide
Area Networks (WANs)

Local Area Network (LAN)


Consists of networked computers, or workstations, shared
peripheral devices, communication devices and links.
Two popular types of LANs:

Peer-to-peer (P2P) networks

Client/server networks
16.2 Local Area Networks (LANs) and Wide
Area Networks (WANs)

Local Area Network (LAN)

Peer-to-peer network






A network of workstations without a server
All the workstations can communicate directly with each other.
Each computer can access the resources which are shared by all the
other peer workstations.
Usually has a small number of workstations.
Cost of setting up is relatively low.
Usually found in small companies and homes.
Sharing
Internet
access
Sharing printing
services
16.2 Local Area Networks (LANs) and Wide
Area Networks (WANs)

Local Area Network (LAN)

Client/server network

Client workstations are connected to a central server, or a host
computer.

Clients can access the resources stored on the server.

The set-up cost is relatively high.


It provides better security controls and network services in a largescale network environment.
Ideal for medium or large organizations.
A client/server network
16.2 Local Area Networks (LANs) and Wide
Area Networks (WANs)

Wide Area Network (WAN)




Refers to a communication network
that spans a large geographical area,
across regions or continents.
Consists of two or more remote LANs.
LANs are connected via different
communication channels
 Telephone lines
 Leased lines
 Microwave
 Satellites
 Optical fibre cables
The Internet is the largest WAN in LANs are interconnected
the world.
to form a WAN
16.2 Local Area Networks (LANs) and Wide
Area Networks (WANs)

Wide Area Network (WAN)




Refers to a communication network
that spans a large geographical area,
across regions or continents.
Consists of two or more remote LANs.
LANs are connected via different
communication channels
 Telephone lines
 Leased lines
 Microwave
 Satellites
 Optical fibre cables
The Internet is the largest WAN in LANs are interconnected
the world.
to form a WAN
16.3 Network Services

Network services


When a computer is connected to a network, it can provide more
extensive services.
Major services provided by a network are:







Internal communication
Hardware sharing
Centralized data management
Software sharing
Internet access service sharing
Network conferencing
Information backup
16.3 Network Services

Internal communication

In both LAN and WAN, users can communicate with each other
by e-mail, instant messaging or file transfer.
Communication through a network
16.3 Network Services

Hardware sharing

In a LAN,



Users are allowed to share hardware resources such as hard disks,
printers and scanners.
Shared equipment can be better monitored and utilized.
Helps save the cost of extra hardware.
A network printer is monitored by print quota software
16.3 Network Services
Sharing a
printer on a
network

In a WAN,


Sharing of hardware is less common except for the hard disk storage
space.
After logging onto a file server, a user can upload and download files
from a dedicated folder on the file server.
16.3 Network Services

Centralized data management








All data centralized in one network server
All authorized users are able to share the latest and identical
information all the time.
Allows many users to work on the same project at the same time.
Increasing productivity and efficiency
Better than duplicate and scatter important data files and
sensitive information onto many different computers
No data update problems.
Reduce management costs
Less vulnerable to security attacks
16.3 Network Services
Data centralized on a network server
16.3 Network Services

Centralized data management

Once the network allows the Internet access:




Possible for a remote user to access a network server
Risk of attack from hackers
Apply a number of security measures to protect the network from
intrusions through the Internet.
Examples:



Firewalls
Virtual private networks (VPNs)
Dedicated leased lines
A hardware firewall
16.3 Network Services

Centralized data management
In a virtual private network (VPN), virtual communication
linkages between LANs are created on the Internet.
16.3 Network Services

Software sharing


Application software can be shared simultaneously among
computers on a network.
A network licence:





Allows multiple legal access to the software.
Usually sets a limit on the number of users who use the program
simultaneously in the network
Cost is usually lower than the total cost of individual software
licences.
No need to carry out software installation on each computer
More efficient
16.3 Network Services

Internet access service sharing



Users can access the Internet via a computer network with a
single Internet service account.
Can reduce the cost of Internet access
Helps monitor, control and protect network communications with
the external parties through the Internet.
Sharing of Internet access
16.3 Network Services

Network conferencing


The technology that enables users in two or more locations to
communicate with each other via two-way audio and video
signals
Video signals must be compressed before they are sent through a
computer network.
Video conferencing
16.3 Network Services

Information backup


Information on individual computers can be backed up and
stored on a server via the network.
These backup files can be used to recover lost information in an
out-of-order computer in a few minutes
Recover information with
backup files via a computer
network.
16.3 Network Services

Information backup


Information on individual computers can be backed up and
stored on a server via the network.
These backup files can be used to recover lost information in an
out-of-order computer in a few minutes
Recover information with
backup files via a computer
network.
16.4 Connecting to a Computer Network

Network Interface Card (NIC)

An expansion board for connecting a computer to a network

Media access control (MAC) address


Each NIC’s unique hardware number

Identify the computer on the network.

Burnt onto the NIC by the manufacturer
Classified by:

Types of connectors

Expansion buses

Size of its bandwidth
16.4 Connecting to a Computer Network

Network Interface Card (NIC)
Criterion for classification
Example
Connector
BNC and RJ-45
Expansion bus
ISA, PCI, PCMCIA and USB
Bandwidth
10 Mbps, 100 Mbps and 1,000 Mbps
ISA network card
PCI network card
PCMCIA network
card for notebook
computer
USB network adapter
16.4 Connecting to a Computer Network

Network Interface Card (NIC)
BNC
Different types of connectors
RJ-45
16.4 Connecting to a Computer Network

Network connecting devices

Switch



Provide a central connection point for cables on a network.
In a typical network configuration, one or more computers are
connected to a switch via network cables.
Connected to a server with typical data transmission speeds of 10
Mbps, 100 Mbps and 1 Gbps
A switch
16.4 Connecting to a Computer Network

Network connecting devices

Hub

Similar to switch

Can only handle low transmission traffic

Its efficiency drops significantly under heavy network traffic.

Cost is very close to switch
A hub
16.4 Connecting to a Computer Network

Network connecting devices

Repeater

A device that amplifies the received signals

Removes unwanted noise.

Sends the amplified signals to the connected device.

Overcomes the problem of signal attenuation due to the long
distance transmission.
Repeater
16.4 Connecting to a Computer Network

Network connecting devices

Bridge

Often used to interconnect two or more LANs of the same type
Bridge
16.4 Connecting to a Computer Network

Network connecting devices

Router



A device that translates signals from
different networks
Usually responsible for the
connectivity between different types
of LANs and between a LAN and the
Internet.
Home-use routers = switching routers
 Work as both a router and a
switch
 Users can connect two or more
computer to the broadband
modem and form a small network.
 Not very reliable and durable
 Incapable of handling large
volumes of data traffic
Router
16.4 Connecting to a Computer Network

Network connecting devices

Modem

A dial-up modem is used when a computer is connected to other
remote computers via telephone lines.

Converts signals between digital and analogue form.

Can be installed internally or externally.



Internal modems are either in the form of an expansion card or built
as a part of the motherboard.
External modems are single devices that can connect to the
connection port of a computer system.
Maximum data transmission speed is 56 Kbps.
16.4 Connecting to a Computer Network

Network connecting devices

Modem
External dial-up modem
Cable modem
Internal modem card
DSL modem
16.4 Connecting to a Computer Network

Network connecting devices

Modem

Cable modem




Used when the broadband service is provided via a cable TV line.
Modulates the digital computer network data into radiofrequency signals for transmission.
Demodulates incoming radio-frequency signals into digital
signals.
Digital subscriber line (DSL) modem



Used when broadband service is provided via a residential
telephone cable.
Filters DSL signals from telephone voice signals.
Converts incoming signals that a computer can process and vice
versa.
16.4 Connecting to a Computer Network

Network connecting devices
Hardware components of networking
16.4 Connecting to a Computer Network

Network cables

Twisted-pair cable

Widely used in local area networks.

Looks similar to a telephone cable and has an RJ-45 on both ends.

Wounding two conductors together = eliminate electromagnetic
interference from external sources.
STP cable
UTP cable
16.4 Connecting to a Computer Network

Network cables

Twisted-pair cable

Two types:

Shielded twisted-pair (STP) cable


Has a metal wrapper around each wire to reduce noise and
electromagnetic interference.
Unshielded twisted-pair (UTP) cable

Does not have metal shielding

Low-cost substitution for the STP cable

Maximum data transmission speed of Cat 6 UTP cable is
about 1,000 Mbps within 100 m distance limit.
16.4 Connecting to a Computer Network

Network cables

Coaxial cable

Commonly used for VCR and cable television connections.

Has a BNC connector on both ends.



Higher ability to retain the strength of a transmitting signal over a
long distance than a twisted-pair cable
Consists of:

a copper wire

a sheath of insulation

a wire mesh that conducts and shields against interference

an outside coating
Strengthened insulation allows coaxial cables to transmit more
information than twister-pair cables.
Coaxial
cable
16.4 Connecting to a Computer Network

Network cables

Optical fibre cable


Made of a bundle of glass threads that
transmit light beams.
Advantages:




Significantly greater signal-carrying
capacity
Faster data transmission
Optical fibre cable
Reduced susceptibility to noise or
interference from other devices

Better security for signal transmission

Much thinner, lighter and more durable
Largely replaced coaxial cables in telephone
networks.
16.4 Connecting to a Computer Network

Network cables

Optical fibre cable

Disadvantages:




Cost is very high.
Installation process is much more difficult that that of other
network cables.
Special device is needed to convert digital signals to optical light
beams and vice versa.
Once installed it can hardly be changed.
16.4 Connecting to a Computer Network

Radio connections

Satellite broadband




Can be used to establish a connection to the Internet in remote
areas where wired broadband is not available.
Requires a dedicated satellite and a satellite dish on each end of the
connection.
Average upload and download speeds are around 50Kbps and
500Kbps respectively.
Despite the low transmission speeds, the set-up cost and the
monthly charge can be very expensive
16.4 Connecting to a Computer Network

Radio connection

Satellite broadband
1. Computer sends the
data to the satellite
via the satellite dish.
2. ISP uses its satellite
dish to receive the
radio signal from the
satellite and then sends
the data to the Internet.
Internet connection using a satellite broadband system
16.4 Connecting to a Computer Network

Radio connections

Microwave broadband


Communicates with the ISP by a microwave signal.
Broadband subscriber and the ISP send and receive radio signals
directly to each other.

Consists of a number of access points and subscriber units.

Access points


Mounted on top of buildings since each subscriber unit must
have a line-of-sight to the access point

Practical coverage: up to 1.5 km

Data transmission speed: up to 100 Mbps
May be affected by rainstorms and connection may become unstable
or even breaks up.
16.4 Connecting to a Computer Network

Radio connections

Microwave broadband
Microwave access
point
Microwave
subscriber unit
Internet connection using a microwave broadband
system
16.4 Connecting to a Computer Network

Radio connections

Wi-Fi connections

Wireless fidelity (Wi-Fi)




The network based on the IEEE 802.11 wireless local area
network (WLAN) standard
Data is transmitted by radio waves.
Any two Wi-Fi-enabled devices can create a wireless connection to
communicate with each other.
To transmit data:


The sender translates digital data into radio signals and
transmits it to the receiver
The receiver then translates the received radio signals back to
digital data.
16.4 Connecting to a Computer Network

Radio connections

Wi-Fi connections

Several standards in the 802.11 series have been defined.
802.11a
802.11b
802.11g
802.11n
Data
transmission
speed
Up to 54 Mbps
Up to 11 Mbps
54 Mbps or
above
Up to 600 Mbps
Maximum
signal
coverage
Indoor: 35m
Outdoor: 120m
Indoor: 38m
Outdoor: 140m
Indoor: 38m
Outdoor: 140m
Indoor: 70m
Outdoor: 250m
Radio
frequency
5 GHz
2.4 GHz
2.4 GHz
2.4 GHz and/or
5 GHz
Cost
High
Lowest
Low
Very high
Popularity
Unpopular
Very popular
Very popular
Properties of different WLAN standards
-
16.4 Connecting to a Computer Network

Radio connections

Wi-Fi connections

Widely used in building WLANs.

WLAN

Uses high-frequency radio waves to transmit and receive data.

Providing all the features and benefits of wired LAN technologies.

Without the limitations of cables

Can be connected to a wired LAN by means of an access point.
Access point
16.4 Connecting to a Computer Network

Radio connections

Wi-Fi connections

Access Point


A device that allows a wireless device to communicate to other
wireless devices by radio frequencies
Or communicates to a wired network by converting digital
signals to radio frequencies and vice versa.
A WLAN connects to a wired
LAN by means of an access
point.
16.4 Connecting to a Computer Network

Radio connections

Wi-Fi connections

To connect a device to WLAN

Must be equipped with a wireless network interface card

Placed within the radio signal region on an access point.
Wireless network interface card
16.4 Connecting to a Computer Network

Radio connections

Wi-Fi connections


Advantages:

Cost of building up a network is lower.

Network structure can be modified if required.

A network can be extended to an outdoor area.
Disadvantages:



Effective data transfer rate is highly affected by the surrounding
environment.
The risk of intrusions is higher.
The maximum data transmission speed of a WLAN is significantly
lower than that of a wired network.

The coverage of a wireless router or access point is limited.

The cost of building up a large wireless network can be very high.
16.4 Connecting to a Computer Network
Twisted-pair
cable
Coaxial
cable
Optical fibre
cable
Wireless (IEEE
802.11 standard)
Data
transmission
speed
10 Mbps, 100
Mbps
Up to 299
Mbps
100 Mbps to
several Gbps
Maximum rate ranges
from 11 Mbps to 600
Mbps
Transmission
signal
Electrical
Electrical
Optical
Radio waves
Cost
Cheap
Expensive
Most
expensive
Varied, dependent on
network size
Characteristics of different types of communication links
16.4 Connecting to a Computer Network
Twisted-pair
cable
Coaxial
cable
Optical fibre
cable
Wireless (IEEE
802.11 standard)
Data
transmission
speed
10 Mbps, 100
Mbps
Up to 299
Mbps
100 Mbps to
several Gbps
Maximum rate ranges
from 11 Mbps to 600
Mbps
Transmission
signal
Electrical
Electrical
Optical
Radio waves
Cost
Cheap
Expensive
Most
expensive
Varied, dependent on
network size
Characteristics of different types of communication links
16.5 Methods of Internet Access

Internet service provider



To access the Internet with your computer:

Register an account with an Internet service provider (ISP).

Set up necessary computer equipment.

Install appropriate software and then establish a connection.
An ISP is a company that provides Internet access services.
It usually offers subscribers a communication software package
for establishing a connection with the ISP server.


Includes a username, a password and other necessary information.
Data transmission speed or bandwidths vary from 56 Kbps to 1,000
Mbps.
16.5 Methods of Internet Access

Dial-up connection

Basic equipment:

Dial-up modem

A telephone line

Probably the cheapest option of infrequent Internet users.

It charges are either fixed or based on usage time.

Necessary software for Internet access:

A web browser for viewing web pages

An e-mail program for receiving and sending e-mails

Communication software for establishing a connection with the ISP
16.5 Methods of Internet Access

Dial-up connection
Necessary computer hardware and software for establishing a dial-up
connection to the Internet
16.5 Methods of Internet Access

Integrated Services Digital Network (ISDN)


ISDN works on the same principle as a dial-up connection.
Need an ISDN modem connected to a conventional telephone
line to establish an ISDN connection to the ISP server.

Maximum data transmission speed: 128 Kbps

Data is transmitted as digital signals instead of analogue signals.



More stable and less likely to be affected by surrounding interference.
Greatest network coverage among all broadband connections
Gap of transmission speed of ISDN is slower than other popular
broadband connections
16.5 Methods of Internet Access

Broadband connections

Very popular form of Internet connection

It provides:

A much higher data transmission speed than the standard 56 Kbps
modem

Constant availability without dialing required

Simultaneous use of data and voice connections

Data transmission speed: 1.5 Mbps to 1,000 Mbps

Can be used for most online multimedia presentations.

Enables a continuous transmission of video, audio and 3-D
animation.
16.5 Methods of Internet Access

Broadband connections

Equipments required:

Digital modem



Can send and receive digital data from the ISP through a
dedicated transmission cable.
Network interface card
Main broadband systems for home users:

Asymmetric digital subscriber line (ADSL)

Symmetric digital subscriber line (SDSL)

Cable modem
16.5 Methods of Internet Access

Broadband system
Broadband
system
Description

ADSL



SDSL


It uses standard telephone lines to connect with an ADSL
modem.
The system is ‘asymmetric’ because the download speed (from
1.5 to 9 Mbps) is higher than the upload speed (from 16 to 640
Kbps).
Example: Netvigator
It has identical upstream and downstream data transmission
speed.
The maximum data transmission speed is 24 Mbps.
They use the whole bandwidth of the communication cable for
the broadband service and therefore the cable cannot be used for
the transmission of vocal signals.
Main broadband systems
16.5 Methods of Internet Access

Broadband system
Broadband
System
Description

Cable
modem



It connects a computer to a cable TV network.
It has a designated capacity of 30 Mbps, but this has to be
shared among users at the same location.
The data transmission speed is normally much lower.
Example: i-Cable
Main broadband systems
16.5 Methods of Internet Access

T-carrier




Mainly used by large companies and schools that need a
continuous Internet connection.
Provides a permanent connection between the network server
and the ISP’s server.
Allows network users to access the Internet simultaneously.
Various types of carriers support different data transmission
speeds:


Maximum data transmission speed of T-1: 1.54 Mbps
Maximum data transmission speed of T-3: 44 Mbps
16.5 Methods of Internet Access

Wireless Internet access


Can access the Internet or connect to a computer network
instantly almost anywhere with our notebook computer or other
mobile devices.
Popular wireless connection methods:

Wi-Fi hot spots

Wireless Application Protocol (WAP)

General Packet Radio Services (GPRS)
16.5 Methods of Internet Access

Wireless Internet access

Wi-Fi hot spots



Large number of Wi-Fi hot spots have
been built in public areas.
Users may need to subscribe to an ISP
for connection to the Wi-Fi hot spots.
Free Wi-Fi hot spots

Airport

Government properties

Public facilities
Wi-Fi hot spot at the
telephone post
16.5 Methods of Internet Access

Wireless Internet access

WAP


A communication protocol designed for handheld wireless devices
Maximum data transmission speed of WAP ranges from 10 Kbps to
153 Kbps
16.5 Methods of Internet Access

Wireless Internet access

GPRS

A technology designed for mobile phones

Maximum data transmission speed: 115.2 Kbps



Provides a constant connection that allows users to surf the WWW
and use e-mail services.
A notebook computer with a GPRS network card or a communication
medium with a GPRS-enabled mobile phone can use GPRS to
connect to the Internet.
Most mobile phone companies provide WAP, GPRS and HSDPA
services for their customers to obtain instant information.
16.5 Methods of Internet Access

Comparison of Internet access methods
Dial-up
ISDN
connection
Broadband
connection
Wi-Fi
hot
spot
WAP
Data
transmission
speed
Maximum
56 Kbps
Maximum
128 Kbps
1.5 Mbps –
1,000 Mbps
11
Mbps–
600
Mbps
10 Kbps – Maximum
153 Kbps 115.2
Kbps
Cost
Low
Average
High
Very
high
Very high
Characteristics of different Internet access methods
GPRS
Very high
16.5 Methods of Internet Access

Comparison of Internet access methods
Dial-up
connection
ISDN
Area
Area covered
covered by
by the
Availability
the
telephone
telephone
network
network
Security
High
Very high
Broadband
connection
Wi-Fi hot
spot
WAP
GPRS
ADSL: Area
covered by the
telephone
network
Cable: Area
covered by the
cable TV
network
Area
Covered by
Wi-Fi access
points
Area
covered by
the mobile
phone
network
Area
covered by
the mobile
phone
network
High
Low
Average
Average
Characteristics of different Internet access methods
16.5 Methods of Internet Access

Comparison of Internet access methods
Dial-up
connection
Wi-Fi hot
spot
WAP
• Remote
households
Application
• Travellers
• Schools
• Households
• Travellers
• Companies
• Schools
• PDA users
• Large
• Large
organizations
organizations
Internet
access
by:
• Smart
phone
• PDA
Internet
access by:
• Smart
phone
• PDA
• Notebook
computer
Popularity
Minority
Minority
Minority
Minority
ISDN
Broadband
connection
Majority
Characteristics of different Internet access methods
Moderate
GPRS
16.5 Methods of Internet Access

Comparison of Internet access methods
Dial-up
connection
Wi-Fi hot
spot
WAP
• Remote
households
Application
• Travellers
• Schools
• Households
• Travellers
• Companies
• Schools
• PDA users
• Large
• Large
organizations
organizations
Internet
access
by:
• Smart
phone
• PDA
Internet
access by:
• Smart
phone
• PDA
• Notebook
computer
Popularity
Minority
Minority
Minority
Minority
ISDN
Broadband
connection
Majority
Characteristics of different Internet access methods
Moderate
GPRS
16.6 Latest Development for Data
Communications

Latest development for data communications




A new wireless transmission protocol has been developed

Can transmit data at 2.5 Gbps

The fastest Wi-Fi connection available in the market today.
However, the security concerns about wireless transmission is still
an unsolved problem
No encryption method is proved to make a wireless network as
safe as a wired network.
It may take ten more years to complete the migration of all
existing 10/100 Mbit computer networks to Gigabit networks.
16.6 Latest Development for Data
Communications

Latest development for data communications




A new wireless transmission protocol has been developed

Can transmit data at 2.5 Gbps

The fastest Wi-Fi connection available in the market today.
However, the security concerns about wireless transmission is still
an unsolved problem
No encryption method is proved to make a wireless network as
safe as a wired network.
It may take ten more years to complete the migration of all
existing 10/100 Mbit computer networks to Gigabit networks.