A network connects two or more computers together to share resources. Computers on a network can be connected through cables, phone lines, wireless connections or other methods. A local area network (LAN) connects computers within a small area like a building through cables or wireless connections. Each computer needs a network interface card to connect to the LAN. Most LANs use a powerful server computer that stores shared files and software and controls the network, while other computers that access these resources are called clients.

2. What is a Network?
0 A network consists of two or more computers that are
linked together in order to share resources (such as
printers), exchange files, or allow electronic
communications.
0 The computers on a network may be linked through
cables, telephone lines, radio waves, satellites, or
infrared light beams.
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3. Local Area Network (LAN)
0 Local Area Networks (LANs) are networks that are
confined to a relatively small area. It is generally limited
to an area such as a school, office or building.
0 The computers in a LAN are linked together by cable or
wireless connections.
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4. Local Area Network (LAN)
To enable a computer to connect to a LAN
myou require a device called a
network interface card (NIC)
0 On most LANs, cables are used to connect the network
interface cards in each computer to the network.
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5. Local Area Network (LAN)
0 A Network Interface Card (NIC) is fitted to each device
connected to the network.
0 An NIC stores data being transferred to and from a
computer (workstation) and the network.
0 If required the NIC converts the data being transferred to
make sure that it follows the communications rules that
run on the network.
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6. Local Area Network (LAN)
Client/Server Network
0 In most LANs a powerful computer is used as the
server.
0 The server stores all of the software that controls the
network, as well as the software and files that can be
shared by the computers connected to the network.
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7. Client/Server Network
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8. Local Area Network (LAN)
Client/Server Network
0 Most LANs will have more than one server. For
example:
0 a printer server;
0 a web server;
0 an applications server.
0 Computers connected to the server on a network are
called workstations. These workstations are the
clients.
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9. Local Area Network (LAN)
Client/Server Network Advantages:
0 Centralised - resources and data security are
controlled through the server;
0 Resource Sharing – expensive peripheral
devices such as laser printers and scanners can
be shared by users
0 Reliability – ability to continue operating when
problems occur;
0 Scalability - elements can be added or replaced
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10. Local Area Network (LAN)
Client/Server Network
Advantages Continued:
0 Flexibility - new technology can be easily
integrated into the system;
0 Interoperability - all components should work
together;
0 Accessibility - resources can be accessed
remotely.
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11. Local Area Network (LAN)
Client/Server Network Disadvantages:
0 Expensive – requires initial investment in network
hardware;
0 Maintenance – large networks will need staff to ensure
efficient operation for example, a network
manager, network technicians;
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12. Local Area Network (LAN)
Client/Server Network
Disadvantages continued:
0 Dependence – when a server goes down, operations
will cease across the network.
0 Interoperability - all components may not work
together;
0 Security/Privacy – as data is shared there is a greater
need for security
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13. Wide Area Network
(WAN)
0 Wide Area Networks (WANs) connect computers across large
distances, such as Scotland, Europe, or the world.
0 Telecommunications links are used to connect this type of
network.
For example: connections (links) via telephone lines and
satellites.
0 A computer must have a modem to enable connection.
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14. Transmission Media
0 Networks use different types of media to transmit data.
0 The main types of media used are:
0Copper cabling
0Fibre-optic cabling
0Wireless
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15. Transmission Media
Copper cabling
0 Twisted-pair cabling:
0 twisted pair cabling is the most popular media used in
LANs;
0 It can support data transmission rates of 100 Mbps.
0 Advantage: It is quite cheap to buy.
0 Disadvantage: twisted-pair cabling may be susceptible to
radio and electrical frequency interference.
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16. Transmission Media
Copper cabling
0 Coaxial cabling:
0is highly resistant to signal
interference;
0it can support greater cable lengths
between network devices than twisted
pair cable.
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17. Transmission Media
Fibre-optic cabling
0Fibre optic cabling transmits light
rather than electronic signals
eliminating the problem of electrical
interference.
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18. Transmission Media
Fibre-optic cabling
0 Advantages:
0It has the ability to transmit signals over
much longer distances than coaxial and
twisted pair;
0It has the capability to carry data at much
greater speeds than coaxial and twisted
pair;
0It is a secure medium.
0 Disadvantage: expensive to install.
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19. Transmission Media
Wireless networking
0 Not all networks are connected by cabling; some
networks are wireless.
0 Wireless LANs use high frequency radio
signals, infrared light beams, or lasers to
communicate between workstations and the
server(s).
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20. Transmission Media
Wireless networking
0 Advantages:
0 no need for cabling;
0 portability – network workstations can be moved and
used in different locations.
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21. Transmission Media
Wireless networking
0 Disadvantages:
0 provide poor security;
0 slower than LANs using cabling;
0 limited range (approximately 100m)
0 signals absorbed by walls;
0 susceptible to interference from lights and electronic devices;
0 performance decreases as more computers used.
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22. Multi Access
0 Multi Access allows many users to share(access) the
processor of a large computer(Mainframe).
0 Processor time is divided between the users by
allocating them a time slice.
0 Each time slice lasts only milliseconds and is carried out
on a rotational basis making the users unaware that the
processor is being shared.
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23. Multi Access
Workstations 23
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24. Security Measures
0Files stored on the network
must be protected against
loss, corruption, hacking and
viruses.
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25. Security Measures
0 Ways of securing this are to use:
0 user IDs and Passwords to gain access to the network;
0 different access rights for different users;
0 data encryption – data is changed into a code. To read the
data a user must have the key to the code;
0 advanced ID measures – the use of smart cards to store
fingerprint or voiceprint information
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26. Security Measures
continued
Physical Security measures:
0 To prevent unauthorised access:
0 Lock the door of the computer room(s);
0 Put lockable covers over power switches, etc…
and give only authorised users the key or door code.
0 Disable floppy disc drives to prevent:
0 files being saved to floppy disc and taken away;
0 possible loading of viruses from floppy discs.
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28. Mesh
Topology
0 Advantages 0 Disadvantages
 Fault in 1 channel  Lots of wiring,
doesn’t affect whole
network  Expensive
 Many transmissions can
take place at any one
time
 Excellent Performance
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29. Star Topology
0 Advantages 0 Disadvantages
 Shorter paths  loss of network if central
node fails
 simpler to monitor and
 many cables needed
audit
 costly to expand
 easier fault finding
 greater initial costs
 simple access protocols
 central node slows
 easy expansion communication between
nodes
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30. Bus
Topology
0 Advantages 0 Disadvantages
 only 1 main channel  fault diagnosis is difficult
required  a fault in one section of main
 easy to extend channel causes a fault in the entire
channel
 cheap
 more sophisticated protocols &
 Station faults have no
hardware required
effect on rest of
network  monitoring and auditing becomes
complicated
 High rate of data crashes
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31. Ring
Topology
0 Advantages 0 Disadvantages
 suitable for fibre optic  difficult to diagnose
cables faults
 signal is boosted at  extension of the network
each station is impossible without
 node failure will not bringing the rest of the
cause network network down
failure, just rerouting  Control software and
equipment expensive
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32. Tree
Topology
0 Advantages 0 Disadvantages
 As the star, and the bus  As the star and bus.
 If link between branches
goes down, branch can
work on it’s own.
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