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Introduction to Computing
Lecture # 11
Outline
• Network Definition
• Benefits of Networks
• Types of Networks
• Types of LANs
• Components of a LAN
• Topology of LANs
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Network Definition
• Network (or communications network) - a system of
interconnected computers, telephones, or other
communications devices that can communicate with
one another and share applications and data. (Williams
and Sawyer)
• A network is a set of technologies – including
hardware, software, and media (such as wires) – that
can be used to connect computers together, enabling
them to communicate, exchange information, and
share resources in real time. (Peter Norton)
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Benefits of Networks
• Sharing of peripheral devices
– Expensive devices such as printers, disk drives, and
scanners can be shared.
• Sharing of programs and data
– People in an organization can use the same software and
have access to the same files.
• Better communications
– Networks make e-mail possible, keeping everyone posted
about important information.
• Security of information
– Information on a network is apt to be backed up (duplicated)
somewhere. Thus, a fire or flood is less apt to destroy
essential information.
• Access to databases
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– Networks enable users to tap into all kinds of databases
(whether private or public/online).
Types of Networks
• Networks may be classified into three
categories according to their geographical
range:
– Wide Area Network (WAN)
– Metropolitan Area Network (MAN)
– Local Area Network (LAN)
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Types of Networks
• Wide area network (WAN) - a communications
network that covers a wide geographical area, such
as a country or the world
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• May use a
combination of
satellites, fibertopic cable,
microwave, and
copper wire
connections
• WANs are used to
connect LANs
together
• Examples: internet,
most telephone
companies
Types of Networks
• Metropolitan area network (MAN) - a communications
network covering a city or a district
– The purpose of a MAN is often to bypass local telephone
companies when accessing long-distance service.
– Example: many cell-phone systems are MANs
• Local area network (LAN) - connects computers and
devices in a limited geographical area, such as an
office, a building, or a group of buildings closed
together.
– Small LANs are sometimes called PANs, for Personal Area
Networks.
– Some terms used in LANs: host computer, node, backbone
– Example: a university campus
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Types of LANs
• LANs consists of two principal types:
– Client/Server LANs
– Peer-to-peer LANS
• Many LANs mix elements from both
client/server and peer-to-peer models.
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Types of LANs
• Client/Server LAN:
– A client/server LAN consists of requesting
microcomputers, called clients, and supplying
devices that provide a service, called servers.
– Clients: microcomputers that request data
– Server: a powerful microcomputer that manages
shared devices
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Common Types of Servers
• File server – a computer that acts like a disk drive,
storing the programs and data files shared by users
on a LAN.
• Database server – a computer in a LAN that stores
data but doesn’t store programs.
• Print server – controls one or more printers and stores
the print-image output from all the microcomputers on
the system.
• Web server – contains web pages that can be viewed
using a browser.
• Mail server – manages e-mail.
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Types of LANs
• Peer-to-peer LAN:
– The word peer denotes one who is equal in standing with
another
– All microcomputers on the network communicate directly with
one another without relying on a server
– Peer-to-peer networks are less expensive than client/server
networks and work effectively for up to 25 computers.
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Components of a LAN
• Local area networks are made up of several
standard components.
– Connection or cabling system
– Microcomputers
with network
interface cards
– Network OS
– Other shared
devices
– Routers,
bridges,
gateways, and
hubs
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Components of a LAN
• Connection or cabling system
– LANs may use wired (twisted-pair wiring, coaxial cable, or
fiber-optic cable) or wireless (infrared, radio-wave
transmission, bluetooth, or WiFi) connection system.
• Microcomputers with network interface cards
– Two or more microcomputers are required, along with
network interface cards.
– A network interface card (inserted into an expansion slot in a
PC) enables the computer to send and receive messages
over a cable network.
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Components of a LAN
• Network OS
– The network operating system (NOS), such as Novell
NetWare, Microsoft Windows NT/2000, Unix, and Linux, is
the system software that manages the activity of a network.
• Other shared devices
– Printers, scanners, storage devices, and other peripherals
may be added to the network as necessary and shared by all
users.
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Components of a LAN
• Routers, bridges, gateways, and hubs
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– A LAN normally connects to the Internet. Routers, bridges,
gateways, and hubs are used (as necessary) as interfaces to
make these connections.
– A router is a special device that directs communicating
messages when several networks are connected together.
– A bridge is an interface used to connect the same types of
networks.
– A gateway is an interface permitting communication between
dissimilar networks—for example, between a LAN and a
WAN or between two LANs based on different OS or different
layouts.
– A hub is a common connection point for devices in a network
– a place of convergence where data arrives from one or
more directions and is forwarded out in one or more other
directions.
Topology of LANs
• Topology – the logical layout, or shape, of a
network.
• The three basic topologies are bus, ring, and
star.
Bus
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Topology of LANs
• Bus network – all nodes are connected to a single
wire or cable, the bus, which has two endpoints.
• Each communication device on the network transmits
electronic messages to other devices.
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– If some of those messages collide,
the sending device waits and tries
to transmit again.
– The advantage of the bus network
is that it may be organized as
either a client/server or peer-topeer network.
– The disadvantage is that extra
circuitry and software are needed
to avoid collisions between data;
and
– Entire network may stop working if
a connection in the bus is broken.
Topology of LANs
• Ring network – one in which all microcomputers and
other communications devices are connected in a
continuous loop.
– There are no end points.
– Messages are passed
around the ring until they
reach the right destination.
– There is no central server.
– The advantage of a ring
network is that messages
flow in only one direction (no
danger of collisions).
– The disadvantage is that if a
connection is broken, the
entire network stops working.
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Topology of LANs
• Star network – one in which all microcomputers and
other communications devices are connected to a
central server.
– Messages are routed
through the central hub
– Hub prevents collisions
between messages
– If a connection is broken
between hub and a
device, the rest of the
devices will continue
operating.
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– If the hub goes down, the
entire network will stop.
– END
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