basics of networks

8/22/2019 basics of networks

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UNIT I


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What is the Computer Network? A communication network is a set of nodes connected by

links and able to communicate with one another.

A computer network is a communication network in which

nodes are computers.


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Benefits of Computer Networks

Resource Sharing

Hardware (computing resources, disks, printers)

Software (application software)

Information Sharing

Easy accessibility from anywhere (files, databases)

Search Capability (WWW)

Communication Email

Message broadcast


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Problems with Computer Networks

Relaying of message over multiple

communication links

Sharing of communication links between

many users

Distributed control of the system


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Networks Types Classification Based on Area

and Ownership

Local Area Networks

A LAN is a group of

computers connected in

small geographical area (a

few kilometers) i.e.connect computers in an

office building

Privately owned

LAN topologies-Bus, Star,

Ring and Mesh Speed-100Mbps to 1Gbps

Examples-Ethernet, Token

Ring


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Metropolitan Area Networks

A MAN connects an arealarger than a LAN (50km-80km), such as a city, withdedicated or high-performance hardware i.e.coaxial cable, optic fibres

Speed-1Gbps

Privately owned or serviceprovided by a telephonecompany


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Wide Area Networks

A WAN is a group ofcomputers connected inlarge geographical areasuch as country

connect distantcomputers usingmicrowaves, radio wavesand fiber optic.

Costly hardware, routers,dedicated leased lines and

complicated implementationprocedures.

Example: Internet - Userusing a telephone line toconnect to an ISP (InternetService Provider)


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Network Topologies

Topology refers to the shape of a network, or

the network's layout.

How, different nodes in a network areconnected to each other and how they

communicate are determined by the network's

topology.

The choice of topology is dependent upon

Type and number of equipments being used

Cost


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Network Topologies

Computer networks topologies

Bus

Star

Ring

Mesh


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Bus Topology

All devices are connected to a central cable,called the bus or backbone.

Systems connect to this backbone using Tconnectors or taps


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Bus Topology

Both ends of the network must beterminated with a terminator.


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Bus Topology

The backbone functions as a shared

communication medium

Device wanting to communicate with another

device on the network sends a message onto

the backbone

The message is heard by all stations, but only

the intended recipient actually accepts and

processes the message.

Terminator absorbs frames at end of medium


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Frame Transmission - Bus LAN

Station C want to

transmit a frame to

station A.


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Bus Advantages and

Disadvantages

Advantages

Simple, easy to use and construct

Requires least amount of cable (less expensive)

Disadvantages

A faulty cable will take the entire LAN down

Difficult to troubleshoot

Heavy network traffic can slow bus considerably

Network disruption when computers are

added or removed


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Star Topology


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Star Types

All computers/devices connect to a central device

called hub or switch.

Nodes communicate across the network by passing

data through the central device

Each device requires a single cable

point-to-point connection between the device and

hub.

Most widely implemented Hub/Switch is the single point of failure


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Star Advantages

Single computer failure doesnt bring down

whole network

Adding new devices to a Star network is very

simple compared to any of the othertopologies

No disruptions to the network when connecting

or removing devices.

Centralized control i.e. Centralized

network/hub monitoring

Easy to troubleshoot


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Star Disadvantages

If central device fails, the whole network fails Compared to the bus topology, a star network

generally requires more cable

More expensive than linear bus topologiesbecause of the cost of the central device


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Ring Topology

All devices are connected to one another in the shape ofa closed loop, so that each device is connected directly

to two other devices, one on either side of it.

Data is passed one way from device to device.


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Ring Topology

A short message (called a token) is circulated aroundthe ring, being passed from station to station.

The token originates from a controller or master

station which inserts it onto the ring.

A station which wants to transmit waits for the token to

arrive.

When the token arrives, the station changes it from atoken to a connector message, and appends its

message. This new message is then placed on the

outgoing side of the ring.


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Ring Topology

Each station passes on received tokens if they havenothing to transmit.

They monitor connector messages to see if the message isaddressed to them.

If connector messages are addressed to them, they copythe message, modify it to signify its receipt, then send it onaround the ring.

Connector messages which are not addressed to them arepassed directly on to the next station in the ring.

When the connector message travels full circle and arrivesat the original sending station, it checks the message tosee if it's been received. It then discards the message andreplaces it with a token.


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Frame Transmission - Ring LAN


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Ri Ad t d


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Ring Advantages and

Disadvantages

Advantages Equal access for all users

Perform well under heavy traffic

Disadvantages If one device/cable fails then the whole network goes

down.

Difficult to troubleshoot

Adding/Removing computers disrupts the whole

network


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Mesh Topology

In a mesh topology each device/PC isconnected to every other device/PC in the

network by its own cable

M h Ad t d


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Mesh Advantages and

Disadvantages

Advantages Mesh topology boasts the highest fault tolerance of all

of the network topologies

The network can be expanded implementation without

disruption to current uses

Disadvantages

Because each connection needs its own cable aMesh topology can get very expensive.

Complicated implementation


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Communication modes

Simplex Data in a simplex channel is

always one way. Simplexchannels are not often usedbecause it is not possible tosend back error or controlsignals to the transmit end. Anexample of a simplex channelin a computer system is theinterface between thekeyboard and the computer,in that key codes need only

be sent one way from thekeyboard to the computersystem.

Example Keyboard, Monitor


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Half Duplex A half duplex channel can

send and receive, but not

at the same time. Its like a

one-lane bridge where twoway traffic must give way in

order to cross. Only one

end transmits at a time, the

other end receives.

Example Walkie-Talkie


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Full Duplex Data can travel in both

directions simultaneously.

There is no need to switchfrom transmit to receive

mode like in half duplex.

Its like a two lane bridge

on a two-lane highway.

Example Telephone

network


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OSI Layer Model


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Network Protocols

Define format and order of messages sent and

received among network entities, and actions taken on

message transmission and receipt

Govern all communication activities in a network

control sending and receiving of messages

Examples - TCP, IP, HTTP, FTP, MPEG

ISO Organization for


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ISO - Organization for

Standardization.

International standards organisation is

responsible for a wide range of

standards, including many that are

relevant to computer networking.

In 1984 , the Open Systems

Interconnection (OSI) Reference Model

was approved as an internationalstandard for communications

architecture.


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OSI - Open Systems Interconnection

Developed by the International

Organization for Standardization (ISO)

Seven layers- Application - Data link

- Presentation - Physical

- Session- Transport

- Network


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OSI - The Model

Layered Architecture Peer to Peer Process

Interface between Layers

Organization of the Layers-----------------------------------------------------------

Each layer performs a subset of the required

communication functions

Layer 1,2,3-Network support layer

Layer 5,6,7-User Support Layer

Layer 4-Links the two subgroups.


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Layered Architecture

Layer architecture simplifies the network

design.

It is easy to debug network applications

in a layered architecture network.

The network management is easier due

to the layered architecture.


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The OSI model describes how information ordata makes its way from application (such asspreadsheets) through a network medium(such as wire) to another application located

on another network. The OSI reference model divides the problem

of moving information between computersover a network medium into SEVEN smallerand more manageable problems .

This separation into smaller more manageablefunctions is known as layering.


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Layering Characteristics

Each layer relies on services from layer

below and exports services to layer

above

Interface defines interaction

Hides implementation - layers can

change without disturbing other layers

(black box)


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Peer-to-Peer Communication


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Network Architecture of OSI Model


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OSI Model: 7 Protocol Layers

Physical: how to transmit bits

Data link: how to transmit frames

Network: how to route packets Transport: how to send packets end2end

Session: how to tie flows together

Presentation: byte ordering, securityApplication: everything else


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OSI Layers and Locations

Switch RouterHost Host

Application

Transport

Network

Data Link

Presentation

Session

Physical


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Application Layer

Application The top layer of the OSI model

Provides a set of interfaces for sending and receivingapplications to gain access to and use network services, suchas: networked file transfer, message handling and database

query processing

Examples of application layer protocols:

Telnet

SMTP

FTP

SNMP


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Application Layer.


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Presentation Layer.

The presentation layer ensures that the information that theapplication layer of one system sends out is readable by theapplication layer of another system.

The major duties of the presentation layer are:

Format conversion:

Convert message from one format into another format .

Compression. Compress the message to take less bandwidth on the

transmission media and less time for transmission.

Encryption:

Convert the message into a form that will not be readable byothers.

Provides security to the message. Protocols of the presentation layer are JPEG, MPEG, ASCII etc.


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Presentation Layer.


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Session Layer. Enables two networked resources to hold ongoing communications (called a

session) across a network Applications on either end of the session are able to exchange data for the

duration of the session

This layer is:

- Responsible for initiating, maintaining and terminating sessions

- Responsible for security and access control to session information (via sessionparticipant identification)

- Responsible for synchronization services, and for data checkpoint services

- Responsible for Dialog control

Session layer determines that which role is to be played at any giventime by a host.

Duplex: Two-way simultaneous.

Half-Duplex: Two-way alternate.

Simplex: One-way.

Session layer protocols are SQL, ASP (AppleTalk Session Protocol), RemoteProcedure Call (RPC), SSH


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Session Layer.

T t L (E d t E d)


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Transport Layer (End-to- End)

The transport layer is responsible for process-to-process delivery of theentire message.

The major duties performed by the transport layer are:

Port address 16 bits

- Computers often running several processes (running programs)

at the same time

Segmentation and reassembly

- A message received form the upper layers is divided into

transmittable segments, each segment contains a sequence

number.

- These number enables the transport layer to reassemble the

message correctly upon arrival at the destination and to identify

and replace packets that were lost in the transmission.


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Transport Layer

Connection Control- The transport layer can be either connectionless or connection

oriented.

- Connection Oriented - Provides acknowledgements of

successful transmissions and requests resends for packets

which arrive with errors- Connectionless service no guarantee on the order of the

messages (possibly with some errors)

Flow-control

Error control

Multiplexes several low rate transmissions with different ServiceAccess Points (SAPs) onto one virtual circuit

The PDU of network layer is segment Transport layer protocols TCP, UDP


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Transport Layer.

Reliable Process-to-Process


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Reliable Process to Process

Delivery.


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Network Layer.

The network layer is responsible for the source-to-destination delivery of a packet possiblyacross multiple networks.

If two systems are connected to the samelink, there is usually no need for a networklayer.

However, if the two systems are attached todifferent networks with connecting devicesbetween the networks, there is need for thenetwork layer to accomplish the delivery.


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Network Layer.

The major duties performed by the network layer are: Logical addressing

Logical address is also called IP address which is of 32-

bits and represented in decimal format.

192.168.32.97 Routing

To route the packets from the source to destination in an

internetwork, the router uses network layer information.

The PDU of network layer is packet.

Network layer protocols are IP, IPX, AppleTalk.


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Network Layer.


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Network Layer.


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Data Link Layer.

The data link layer is responsible for moving frames from one hop

(node) to the next.

The major duties of the data link layer are:

Framing

Physical addressing

Physical address is the MAC address, which is hard coded

into NIC and is of 48-bit represented by Hexadecimal format. 00.27.AB.CC.EE.23

00.27.AB-Vendor

CC.EE.23-User

Flow control

Error Control Access Control

Data Link layer protocols are CSMA/CD, CSMA/CA,HDLC


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Data Link Layer.

Data Link divided into two sub layers- Medium Access Control- MAC address will beadded- Logical Link Control - CRC will be added


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Data Link Layer.

D t Li k L


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Data Link Layer


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OSI Layers (1)

Physical Bottommost layer

Uses synchronization bits to synchronize the receiver

Specify the modulation scheme

The physical layer defines the transmission rate, the number of bitssent each second.


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Physical Layer


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Encapsulation

As the data flows down through the layers in thehierarchy, each layer adds some extra information tothe data in the form of headers or tailors. This processof wrapping data with headers and tailors is calledencapsulation.

Path of a Webpage Request and Encapsulation


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Path of a Webpage Request and Encapsulation

Summary of the Layers


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Summary of the Layers.

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