What is a Computer
Network?
• A computer network is a linked set of
computer systems capable of sharing
computer power and resources such as
printers, large disk drives, CD-ROM
and other databases.
• There are two types of network:


Local Area Network (LAN)
Wide Area Network (WAN)
Slide 1
Local Area Networks
• A local area network is a computer
network across one building or site.
Printer
Work Station
Fileserver
Network Cable
Slide 2
Peer to Peer Network
• This describes a very simple network
structure where shared resources such
as printers are available but where
there are very few other facilities.
• All computers on the network have
similar specification and status.
Slide 3
Client-Server Network
• This method of network organisation
requires one or more servers to which
a number of clients may obtain
services.
• The servers act as central resource
managers for the network.
• A star topology is often used for this
form of network.
Slide 4
Network Hardware
• Network hardware will include:


stations
servers
• file, print, web

passive components
• cables, connections, etc

active components
• repeaters, hubs, switches
Slide 5
Network Stations
• The network station (often called the
client) will normally comprise:


PC
Network interface card (NIC).
• The NIC allows the computer to
communicate with the network

Network Operating Systems
• eg. Windows 95/98, Windows NT,
Appleshare, etc
Slide 6
Network Servers
• A network server is a
computer which acts as a
central storage point for
files and applications.
• Servers also act as a
connection point to
shared peripherals such
as printers.
Slide 7
Active Components
• Active components are required for all
but the simplest of networks.
• These include:



repeaters
hubs
switches
Slide 8
Repeaters
• Due to loss of signal strength it is
often necessary to use a repeater to
boost the signal.
• A repeater is a hardware device to link
together two cable segments.
• The repeater amplifies the signal it
receives before passing it on.
Slide 9
Hubs
• A hub is a device which
allows the
interconnection of a
group of users. A hub
will forward any packet
of data it receives over
one port from one
station to all of the
remaining ports.
• Hubs are used in the
star topology.
Slide 10
Switches
• A switch is “smarter” than a hub and
offers more bandwidth. A switch
forwards data packets only to the
appropriate port for the intended
recipient.The switch establishes a
temporary connection between the
source and the destination.
Slide 11
Routers
• A router receives packets of data
transmitted over a network, and using
their IP address, forwards them to the
correct destinations over the most
efficient available path.
Slide 12
Network Topologies
• The network topology is the theoretical
arrangement of components on a
network.
• There are four network topologies
which you need to be aware of:




bus
star
Ring
mesh
Slide 13
Bus Topology
• Each device is connected to a main
communications line called a bus.
Slide 14
Star Topology
• In the star topology all stations are
connected to a central node, called a
hub.
Slide 15
Ring Topology
• Each device is
connected to a ring
around which signals
are sent. The devices
are connected to the
ring via spurs, or the
connection may pass
through the interface
card
Slide 16
Mesh Topology
• A mesh network is
the most expensive
type of network to
install
Slide 17
Protocols
• A protocol is a standard set of rules
used to ensure the proper transfer of
data between devices.
• In network design it is vital that a
recognised protocol is used.
Slide 18
TCP/IP
• TCP/IP is a protocol which allows
computer on different networks to
communicate.
• TCP/IP is a pair of protocols


TCP - transmission control protocol
IP Internet Protocol
Slide 19
IP Addressing
• Every computer station on the
internetwork must have an IP address.
• This IP address must be unique but is
not encoded in the network interface.
It is set by software in the computer.
Slide 20
Internetwork Hardware
• Connecting LANs together will require
extra hardware:



bridges
routers
gateways
Slide 21
Bridges
• A bridge acts as a link between LANs.
• This will allow two LANs to appear to
users as though they are a single LAN.
Slide 22
Routers
• A router is a device which receives
data packets and sends them on via an
appropriate route.
• It will have a number of network
interface ports to allow connection to
different segments to which data
packets may be directed.
Slide 23
Gateways
• A gateway is a link between two
dissimilar networks..
• The communications between the
networks are usually via public
telecommunications networks.
• A gateway will normally be used to
connect a LAN to the Internet.
Slide 24
Implications of Networks
• Computer networks have developed as
a result of a perceived need and as a
result of economic and technical
factors.
• Computer networks have social, legal
and ethical implications
Slide 25
Economic Factors
• The cost of communication technology

equipment costs are continually reducing and
improving features mean that more is available
for the same money.
• Access to expensive equipment

printers and mass storage are expensive so it
makes sense to share them.
• Geographic spread of organsiations

video-conferencing and email, etc allow
organisations to cope with geographic
Slide 26
isolation.
Technical Factors
• Adoption of Internet technology

TCP/IP
• Adoption of International Standards

ISO OSI
• Development of Communication
Technology

fast modems, ISDN, etc
Slide 27
Social Implications
• Individuals may become withdrawn
from society
• Personal privacy may be compromised
as a result of monitoring network use
• People may not have to move from
rural communities to find
• Home working may allow
strengthening of family life (?)
Slide 28
Ethical Implications
• Bogus doctors could easily offer
medical advice over the Internet.
• Availability of unacceptable and
inappropriate material within
education
• Antisocial, criminal, racist or other
prejudiced material may be distributed
freely.
Slide 29
Legal Implications
• Copyright law

British law makes it illegal to copy
someone elses copyright material. A lot
of material on web pages is copyright
Sometimes the breaking of this law is
unwitting but in others it is deliberate (eg
MP3 files of commercial recordings)
Slide 30