2. 2
Intro
• A network is a group of computers that are
connected to share hardware and software.
• The group can be as small and simple as two
computers and a printer set up in a house or as
large and complex as thousands of computers
and hundreds of printers and other devices.
• In order for the computers to communicate with
each other, they must share three elements: a
network media and topology, a protocol, and
a software client or service.
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The Functions of a Network
• If the purpose of a network is to share resources
among computer systems, what types of
information and services are shared on a
network and why?
– Communication - videoconferencing, real-time chats,
email.
– Sharing hardware – printer, scanners, CD-ROM
drives, tape drives
– Data sharing – security is a major task
– Application sharing - application can be run from a
central location.
– Data backup and retrieval – managed central
backup
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Peer-to-Peer Versus Client/Server
• Peer-to-Peer
– also known as a workgroup, is a low-cost, easily
implemented network solution
– generally used in small network environments that
need to share a few files and maybe some hardware,
such as printers.
– all systems are equal, or peers. Each system can
share hardware or files and access the same things
on other systems.
– is often referred to as decentralized networking
because the network files, data, and administration
are not handled from a central location.
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Pros and Cons
• Advantages of Peer-to-Peer Networks
– Cost - Because peer-to-peer networking does not
require a dedicated server, such networks are very
cost-effective.
– Ease of installation
• Disadvantages of Peer-to-Peer Networks
– Security - security needs to be applied to each
computer and resource individually.
– Data backup - each system must have its data
backed up individually.
– Resource access - difficult to locate resources on the
network.
– Limited numbers of computers - Peer-to-peer
networking is effective only on small networks (fewer
than 10 computers).
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Client/Server Networking Model
• Scalable, allowing additional computers or
other networked devices to be added with
little difficulty.
• Centralized management of all network
services, security, and streamlined backup
procedures.
• two different types of computers are
required: the client and the server.
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Servers
• workhorses of the network.
• require additional resources and computing
power.
• servers also require a network operating system.
– Microsoft Windows 2000/2003, Unix, Mac OS X
servers, and Novell NetWare.
• network may have a single server that offers
more than one network service or hundreds of
servers, each performing a dedicated task.
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Client Computers
• connect to the network and access the
resources of the server.
• Software is needed to allow the client to
connect to the network, which is mostly
now built in to desktop operating systems.
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Pros and Cons
• Advantages of Client/Server Networking
– Centralized management and security
– Scalability
– Simplified backups
• Disadvantages of Client/Server Networking
– High cost - additional hardware and software costs
– Administration requirements - needs technically
skilled people to manage and maintain the network
and the servers.
– Single points of failure - If the server fails, the clients
can't access the services that reside on the server.
Great effort and expense are needed to ensure the
high availability of network servers.
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Distributed and Centralized
Computing
• The terms distributed and centralized computing
describe the location on a network where the
processing takes place.
• In an environment such as a mainframe, the
processing is performed on a centralized system
that also stores all the data. In such a model, no
data processing or data storage occurs on the
client terminal.
• In contrast, in a distributed processing
environment, processing is performed in more
than one place. If a network has servers and
workstations, processing can take place on the
server or on the client.
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Topology
• Basically, a topology is a shape, so a network
topology is the shape of a network.
• There is, however, a big difference between a
physical network topology and a logical network
topology. The physical network topology
represents how the network looks to the naked
eye—in other words, the way the components
are arranged.
• The logical network topology represents how the
flow of information works its way through the
network.
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Bus
• The bus topology was commonly used in
earlier networks but is not seen much
today. In a bus topology, a single cable
connects all the computers.
• A coaxial cable is used with special
connectors called BNC and T connectors.
• The bus only works if both ends of the
cable have a special resistor installed
called a terminator.
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Pros and Cons
• Pros
– The bus topology is very simple and inexpensive to
implement due to its low cost requirements for cable
installation (there's only one main trunk).
• Cons
– Without the proper terminations in place, expect a
very slow or inoperable network.
– The cable itself is a single point of failure. One break,
cut, or poor connection negatively impacts the entire
LAN.
– Because all workstations or devices share a common
cable, troubleshooting can be difficult when problems
occur. You must disrupt LAN service for
troubleshooting.
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Star
• A star topology is a group of computers that are
connected to a central location such as a hub or a
switch.
• This is the most common topology in use today.
• The computers may be physically located next to each
other or spread throughout an entire building, but the
flow of information from each computer to the other
computers must go through the central location.
• Since each computer has its own connection, the failure
of one computer will not affect the other computers in the
network; however, if the hub or switch should fail, then
all of the computers on that hub or switch will be
affected.
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Pros and Cons
• Advantages
– Management of the network is centralized around the
hub and switch components.
– Wiring installation is less obtrusive and therefore does
not disrupt LAN service with the addition or deletion of
nodes.
– A cable cut or bad connector does not take down the
entire LAN segment.
• Disadvantage
– The network component (hub or switch) is a single
point of failure, and
– a great deal of wiring is involved to implement the
star.
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Ring Topology
• Ring topologies are more complex than the bus
and star topologies, but they offer some
attractive features.
• Nodes logically communicate in a ring formation,
with each node communicating only directly with
its upstream and downstream neighbors.
• You can probably picture what a mess the wiring
would be in an office with hundreds of computer
systems, if a network like this were implemented
as a physical ring. So ring topologies are
typically implemented in a physical star topology.
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Pros and Cons
• Pros
– Token-controlled access provides greater overall
bandwidth use.
– Data packet transmission happens within a
determinable time interval. This quality of ring
topologies has made it a staple in situations in which
timing is essential.
– Because each node knows its upstream and
downstream neighbors, this information can be used to
determine where problems have occurred on the ring.
• Cons
– The firmware required to manage the ring is somewhat
complicated and must be on every network card that
participates in the ring which makes Token-Ring
network adapters more expensive devices.
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Mesh Topology
• A mesh topology is an interlacing of multiple
connections among several nodes. Typically, a
mesh is done for one purpose: redundancy.
• There are two types of mesh: full and partial.
• Except in the smallest network, a full mesh is not
very practical due to high cost.
• Full mesh means that every node contained in a
network has a connection to every other node
contained in the network.
• Partial meshes are designed to provide
redundancy where it is needed.
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Hybrid Topologies
• Three popular hybrids
are tree, hierarchical
star, and star-wireless.
• The figure below shows
a tree hybrid
combination that groups
workstations in a star
and joins the stars
along a linear bus. The
majority of the problems
of the bus are
eliminated because a
single workstation
cannot bring the entire
LAN to a halt.
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Hierarchical Star
• Another method that can be used to connect hubs is a
hierarchical star. This method uses a central hub to link
several hubs that have workstations attached.
• This method can be used to build very large LANs;
however, there are restrictions on the size of the LAN.
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Star-Wireless
• The most recent hybrid topology has arrived with the advent of
wireless technology.
• Wireless LAN technology in its current implementation requires a
user to be in the vicinity of an access point attached to the wired
data network. A configuration such as this gives you an amorphous
hybrid topology of star combined with wireless.
