Download A Computer Network

IT2101: Communication
Technology
3. Computer Networking
Fundamentals
Program: BSCS I (January Semester – 2014)
Lecturer: Rebecca Asiimwe
Email: [email protected]
Learning Outcomes
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Define a computer network
Understand how networks operate
Understand types of networks
Identify network medium e.g. Coaxial, UTP
Identify common hardware network
components.
• Introduce common network protocols
2
A Computer Network
• Two or more computers connected together
either (wirelessly or using cables) usually for
the purpose of sharing information or other
resources like printers.
• A typical network consists of computers
(nodes), a connecting medium, and
specialised
equipment
to
enable
communication. Computer hardware &
computer software.
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Background
• Motivated by the need for better
communication,
new
networking
technologies emerged and had a more
immediate impact on the average person.
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Background Cont’d
• It became obvious that each organization
with many computers needed to transfer /
move data among them to facilitate
information sharing.
• The first data transfers from one computer
to another involved removable media
storage devices, usually magnetic tapes or
disks but this was a time consuming and
tiresome process.
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Background Cont’d
• In linking up computers, Circuit boards
were built on computers to help link a
computer to an external device for
instance a printer or a video camera etc.
• Once communication circuits have been
plugged into the computers and connected
by a cable, the computers use them to
transfer data electronically
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Communications Model
• As earlier mentioned, the fundamental
purpose of a communication’s system is
the exchange of data between two parties
– this is something that computer networks
enable us achieve.
• In this section we shall look at the basic
model of communication as illustrated by
the block diagram in the following slide.
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Simplified Communications Model – the
General Block Diagram
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Key Elements of the Communications Model
• Source
– Generates data to be transmitted for example telephones
and personal computers
• Transmitter
– Converts data into transmittable signals- Usually the data
generated by a source system are not transmitted directly
in the form in which they were generated, rather a
transmitter transforms and encodes the information in such
a way as to produce electromagnetic signals that can be
transmitted across a transmission system. For example a
modem takes a digital bit stream from an attached device
such as a PC and transforms that bit stream into an analog
signal that can be handled by the telephone network.
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• Transmission System
– Carries data - this can be a single transmission line or
a complex network connecting source and destination
• Receiver
– Accepts the signal from the transmission system and
coverts it into a form that can be handled by the
destination device. For example a modem will accept
an analog signal coming from a network or
transmission line and convert it into a digital bit stream
that a computer can understand and work with.
• Destination
– Takes incoming data from the receiver
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Data Communications Model
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• The figure below presents one particular
example, which is communication between a
workstation and a sever over a public telephone
network.
• Another example is the exchange of voice
signals between two telephones over the same
network.
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Technical Illustration
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Elements of a Telecommunications System
[Figure 6.3]
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Modem Linked Communications
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Advantages of Computer Networks
(Why Networks?)
• Resource sharing e.g software and hardware like
printers, scanners and other peripheral devices
• Enable central management of resources for
example, an anti virus solution can be easily
deployed and managed over a network, ie security
updates are installed only on one Machine
(server), then all the client machines receive the
updates and get protected from viruses and other
types of malware.
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Advantages of Computer Networks Cont’d
• Allow many users to access programs and
data concurrently.
• Cost saving- e.g. Expensive Laser Jet printers
can be shared.
• Easier backup especially in centralized
databases.
• Make personal communication easier.
• Security can be assured e.g. restricting user
file access.
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Disadvantages of Networking
• Initial cost may be high e.g. Implementation of
Large networks.
• Depending on the Topology, a fault in one
section on the Network may cause the entire
network to be inaccessible.
• If not properly secured, a network can be target
for security threats like Hackers, Viruses etc.
• In case the Network grows, there’s Performance
degradation due to high traffic
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Categories of Networks
Two main categories:
a) Client-Server Networks.
b) Peer-to-Peer networks.
• Note: These categories are based on the
network design background rather than
the geographical area it spans
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a) Client-Server Networks
Consists of two categories of Computers i.e.
i. A client &
ii. A server
A client is usually the User’s workstation
which accesses a network through a more
powerful machine called a Server
 A client requests for resources from a
server and the server responds to the client
by providing the resources requested for.
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Client-Server Networks Illustrated
Image from:
http://en.wikipedia.org/wiki/File:Clientserver-model.svg
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b) Peer-to-Peer Networks
• Involve the sharing of Information or
resources between two or more workstations
connected to each other without the use of a
Server.
• Are relatively easy to setup but do not take
full advantage of networks since central mgt
of the network is not possible.
• Ideal for home use and small offices
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Types of Computer Networks
• Networks can be categorized based on the
following grounds:
oThe Network design (Discussed in the
previous slides)
oThe geographic area it spans.
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Based on the Geographical Area Spaned
• In general, Most Computer networks are classified
according to the distance between individual
computers that are attached to the network.
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The classification includes the following:
Personal Area Networks (PAN)
Local Area Networks (LAN)
Campus Area Networks
Metropolitan Area Networks (MAN)
Wide Area Networks (WAN)
Global Area Networks (GAN)
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1) Personal Area Network (PAN)
• A computer network used for communication
among computer devices that are close to a
person. Devices include printers, fax
machines, PDAs, scanners etc.
• Note: These device examples are only relevant
here if in close distance to a person’s body.
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2) Local Area Network (LAN)
• Connects network devices over a relatively
short distance ie a small geographic area,
like a home, office, or building. Current
LANs are based on the Ethernet
technology.
• For example, a library will have a LAN for
users to connect to the internet.
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Characteristics of LANs
• Local i.e. computers are in close proximity
• Higher rate of data transmission
• Privately owned by the organisation in which
they are set up- network devices are privately
owned by the organisation.
• Characteristic of fewer devices than in CANs,
MANs or WANs
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LANs and - the Ethernet
• In 1985, the Institute of Electrical and Electronic
Engineers (IEEE) – www.ieee.org , developed
a set of standards for LANs called IEEE 802
standards.
• Originally developed by Xerox in the 1970s, the
Ethernet defined in the IEEE 802.3 is the most
implemented LAN in modern day Networking.
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3) Campus Area Networks
• A network that connects two or more
LANs but is limited to a specific and
contiguous geographical area such as a
college campus, industrial complex, or
a military base etc.
• It spans multiple LANs but smaller than
a MAN
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4) Metropolitan Area Network (MAN)
• A Metropolitan Area Network is a network that
connects two or more Local Area Networks or
Campus Area Networks together but does not
extend beyond the boundaries of an immediate
town, city, or metropolitan area.
• Involves connection of multiple
switches & hubs to create the MAN.
routers,
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5) Wide Area Network (WAN)
• A WAN is a network that covers a relatively
broad geographic area (i.e. one city to
another and one country to another) and that
often uses transmission facilities provided by
common carriers, such as telephone
companies.
• Are not owned by any one organization but
exist under collective or distributed
ownership & management.
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Characteristics of a WAN
• Data usually transmitted over leased
telephone lines at lesser transmission
rate than LANs.
• Some WANs use circuit switching or
Packet switching methods to transmit
data.
• WANs span over large geographic
areas
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WAN Illustrated
Images from Google
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WANs:- Leased Lines
• As discussed, WANs communicate over long
distances and therefore need appropriate
Technology to achieve this.
• A Leased line at times called a dedicated line is
a private line permanently connecting two
locations together. Unlike traditional Telephone
lines (PSTN) , these lines don’t have telephone
numbers.
• In Uganda, Companies like MTN, Uganda
Telecom, Warid, etc provide these services
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WANs: - Circuit Switching
• A circuit switched network is one that requires
dedicated connection between the sender and
the receiver before transmission can begin.
Traditional phone lines use this method.
• The exclusivity of the connection during
communication can be time consuming which
led to the development of packed
switching Networks.
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WANs: - Circuit Switching
Note:
– With Circuit switching, connection set up has
to first be made, then information is
transmitted between sender and receiver, and
terminated by either party when the
conversation is over. No one can use that
particular line when it is busy.
– Connection setup takes time
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WANs: - Packet Switching
• Packet switching involves the splitting of data into
packets which are then transmitted to the receiving end
(destination) over multiple channels.
• At the receiving end, the data is then re-assembled to
the original message and delivered. This is made
possible through routing Algorithms (We shall look at
routing in Computer Networks).
• The robust nature of this technology has made it
dominant in modern telecommunication Technology
including the Internet.
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6) Global Area Network (GAN)
• Global area network (GAN)refers to
any network that is composed of different
interconnected computer networks (WANs)
and also covers an unlimited geographical
area.
• It is generally a model for supporting mobile
communications across an arbitrary number of
wireless LANs, satellite coverage areas, etc.
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Network Topology
• In networking, the term "topology" refers to the
layout of connected devices on a network.
• The physical layout & configuration of computers,
cables, nodes, and other peripherals on a Network is
generally referred to as the physical Network
topology.
• Network topology may also be logical and this
generally refers to the protocols used on the
Network.
Note: Protocols will be discussed later
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Network Topology Cont’d
• So, Network topology is the arrangement or
mapping of the elements (links, nodes, etc.) of a
network, especially the physical (real) and logical
(virtual) interconnections between nodes.
• Topologies are the most important part of the
network design theory.
• A better network can be built if you have the
knowledge of these topologies and if you know the
difference between them.
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Common Network Topologies
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Point-to-Point
Star
Bus
Mesh
Ring
Tree
etc
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1) Point-to-point
• The simplest topology is a permanent
link between two endpoints.
• Like
– A microphone to a single public address
speaker.
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2) Star Topology
• The Star topology involves each node on
the computer network being directly
connected to the central Hub or
concentrator as shown in the next slide.
• Each of the nodes of the network is
connected to a central node with a pointto-point link.
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The Star Topology
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Star Topology
• Data on a star network passes through the
hub or switch before continuing to its
destination.
• The hub/ switch manages and controls all
functions of the network. It also acts as a
repeater for the data flow.
• This configuration is most common with
twisted pair cable.
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Characteristics of the Star Topology
• Note: These can also explain the
advantages and disadvantages of the
Star Topology as given in the next
slides
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Advantages of the Star Topology
• Easy to install, implement and extend (in the
event of Network expansion).
• The failure of one node does not have a major
effect on the functionality of the others.
• No problems with collisions of data since
each node has its own cable to the switch /
hub.
• Security can be implemented in the
concentrator for all.
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Advantages of the Star Topology Cont’d
• It is easy to modify and add new
computers to the star network without
disrupting the rest of the nodes.
• Network faults can easily be identified
centrally and parts are easy to remove.
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Disadvantages of a Star Topology
• Requires more cable length than a linear
topology.
• If the hub or concentrator fails, the entire
network will fail.
• More expensive than linear bus
topologies since it involves purchase of
additional hardware – the concentrator.
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3) The Bus Topology
• The Bus Topology involves the use of one
Linear Backbone cable with a
terminator at each end, to which all the
nodes / computers are connected.
• This can be
– Linear Bus
– Distributed bus
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The Bus Topology Cont’d
a) Linear bus:
• All the nodes of the network are
connected to a common transmission
medium which has exactly two
endpoints.
• The two endpoints are normally
terminated with a device called a
terminator.
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The Bus Topology Cont’d
b) Distributed bus:
• The type of network topology in which
all of the nodes of the network are
connected to a common transmission
medium which has more than two
endpoints
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The Bus Topology Cont’d
Linear Bus topology
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Characteristics of the Bus Topology
These define the advantages and
disadvantages of the Bus topology.
Advantages.
• Cheap (probably the cheapest topology to
install).
• Easy to implement.
• Less cable length required.
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Disadvantages of the Bus Topology
• Difficult to identify the source of a problem in case of
network failure.
• Limited cable length and number of stations.
• A break of the main cable means a break of the entire
network.
• Performance may degrade as the network expands
• Low security (all computers on the bus can see all
data transmissions).
• Terminators are required at both ends of the backbone
cable. Proper termination is required.
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4) Ring Topology
• A ring Topology is one where each node
is connected to two other nodes with the
first and last being connected to each
other to form a ring-like structure.
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Ring Topology Cont’d
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Ring Topology Cont’d
• It uses a token to carry data from the
source to the destination: Data is sent
from one machine to the next around the
ring until it reaches its destination then
back to the origin.
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Characteristics of the Ring Topology
Advantages and disadvantages define them.
Advantages:
• Data transmission is relatively simple since data
moves in one direction.
• Logically, a token passes from one computer to
another, so equal attention is given to all nodes.
• Reliable and easier to manage.
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Disadvantages of the Ring Topology
• Difficult to troubleshoot.
• Difficult to add or remove a computer.
• A failure on one computer / node may cause
disruption in the entire Network since the
Token can’t be passed on.
• Before the data reaches the destination, it must
travel through all the nodes before it –What is
the problem in this?
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5) Mesh Topology
• A Mesh Topology involves all the nodes
on the network being interconnected to
one another.
• It is a network topology that resembles a
wire mesh and involves the concept of
routes.
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a) Full Mesh Topology
A mesh network in which every device
connects to every other is called a full mesh.
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• The main advantage of this continuous
connection between the nodes (Full Mesh
Topology) is that when one node goes
down, the others remain unaffected.
• However, implementing such a network
is relatively expensive and difficult.
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b) Partial Mesh Topology
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6) Tree Topology
• Tree topologies integrate multiple star
topologies together onto a bus. The
topology combines characteristics of
linear bus and star topologies.
• In its simplest form, only hub devices
connect directly to the tree bus and each
hub functions as the "root" of a tree of
devices.
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Tree Topology
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Advantages of a Tree Topology
• Point-to-point wiring for individual segments. (a
point to point connection is also more secure that a
multipoint connection where many machines are
connected to one cable)
• Tree topologies allow for the expansion of an
existing network
• Supported by several hardware and software
venders.
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Disadvantages of a Tree Topology
• Overall length of each segment is limited by
the type of cabling used.
• If the backbone line breaks, the entire segment
goes down.
• More difficult to configure and wire than other
topologies.
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7) Hybrid Network Topologies
• The hybrid topology is a type of network
topology that is composed of one or more
interconnections of two or more networks
that are based upon different physical
topologies
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Hybrid Network Topologies
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Considerations when Choosing a Topology
• Funds. A linear bus network may be the least
expensive way to install a network i.e no
concentrators are required and not so many
cables (and cable length) are required as it is in
other topologies like star and mesh.
• Length of cable needed. The linear bus
network uses shorter lengths of cable.
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Considerations when Choosing a Topology
• Future growth. With a star topology,
expanding a network is easily done by adding
another concentrator.
• Cable type. The most common cable in use is
unshielded twisted pair, which is most often
used with star topologies.
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Q&A