Lecture 2.ppt

7/17/2019 Lecture 2.ppt

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The OSI ModelMITM 205 Advance Computer Networks

Ralph !ncent "# $adon


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%hat !s the OSI model &

Open Systems Interconnection model isfundamental to all communicationsbetween network devices.

Developed in 1974 by ISO after te

!merican Department of Defence be"anusin" te #$%&I% suite of protocols.

'inally adopted in 1977. It is now teteoretical model for ow communication

takes place between network devices.


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%hat are the seven la'ers &

Session

%ysical

#ransport

(etwork

Data )ink

%resentation

!pplication)ayer 7

)ayer *

)ayer +

)ayer 4

)ayer ,

)ayer -

)ayer 1


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(a'ers

In te sense of purpose and responsibilityeac layer is separate and independent

/ac as its own function but also providesa service to tose layers above and below

itself #e model sould be considered an aid tounderstandin" te nature of communicationon te network 0 and useful in sortin" out

troubles tat mi"t occur on a network y providin" it allows bot softwareen"ineers and ardware manufacturersensure teir products work to"eter.


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The (a'ers at %ork

!s te ne2t slide sows3 en communicatin" eac OSI layer talkswit te same layer in te oter device

/.". te !pplication )ayer of Device !

communicates wit te !pplication )ayer ofDevice by passin" te data trou" teoter layers

#e !pplication )ayer of eac device is not

concerned wit ow te oter layers arefunctionin" but it does rely on tem to doteir 5ob


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The la'ers at %ork

Supports the communication between

applications over the network

Presents data to the receiver

in a form it recognises

Establishes a connection andterminates it when no longer required

Acknowledges the flow of data

including re-transmission where required

Adds the appropriate network

addresses to packets

Adds the MAC addresses to packets

Transmits the data on the medium

NODE A

Sending Device

NODE B

Receiving Device

Layer 7

Application a!er

Layer 6

Presentation a!er

Layer 5

Session a!er

Layer 4

Transport a!er

Layer 3

"etwork a!er

Layer 2

#ata ink a!er

Layer 1

Ph!sical a!er

Layer 7

Application a!er

Layer 6

Presentation a!er

Layer 5

Session a!er

Layer 4

Transport a!er

Layer 3

"etwork a!er

Layer 2

#ata ink a!er

Layer 1

Ph!sical a!er


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Remem)er!n* the (a'ers

!pplication !ll

%resentation %eople

Session Seem#ransport #o

(etwork (eed

Data )ink Data

%ysical %rocessin"


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"ow does data +low &

en data is sent from te application onte source computer te followin"appens

Data in te form of a packet moves down

trou" te layers en it reaces te %ysical )ayer it isready to be sent alon" te cable

!t te %ysical )ayer te bits may be

analo"ue or di"ital in te form ofelectrical li"t or radio waves


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#e data is transmitted to te destinationdevice

It travels up trou" te layers of te OSImodel reacin" te user.

!s data moves down trou" te layers it isencapsulated 0 ie additional information isadded as eaders or trailers

#e data in te packet does not can"e

See te followin" dia"ram

"ow does data +low & cont###


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,ncapsulat!on

Application

Presentation

Session

Transport

Network

Data Link

Physical

DATA

DATA

DATA

DATA

DATA

DATA

P resentation Header

Session Header

Transport Header

Network Header

Data Link Header Data Link Trailer

PH

SH

TH

NH

DLH DLT

PH

PH

PH

PH

SH

SH

SH

TH

THNH

Bits

To Destination Device


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Appl!cat!on (a'er

6$losest layer to te user

orks wit te applications you use tocommunicate over te network

/.".. Services include S8#% ##% and '#%

$lickin" on a link on a web pa"e issues acommand for te browser to retrieve terelevant information from te Internet

In tis e2ample your computer is te

source and te ost of te web siteinformation is te destination

#e application completes your re:uest anddelivers te information to your computer


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Appl!cat!on Serv!ces

'ile Services

/lectronic;mail Services

(etwork;printin" Services

!pplication Services

Database Services


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-resentat!on (a'er

#is layer as tree fundamental functions


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Sess!on (a'er

%rimarily responsible for andlin" tesession between devices e"ulates te flow of data

It takes responsibility for te followin" ?.


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Sess!on Serv!ces

#e followin" services are provided3

/stablisin" a $onnection

8aintainin" te Session

/ndin" te $onnection

Dialo"ue $ontrol

Dialo"ue Separation


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Sess!on Serv!ces cont###

andsakin" 0 S@( and !$A packets

6Aeep alive messa"es

Session must be terminated


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Transport (a'er

/nsures reliable transport of packets fromsource to destination

!lso mana"es te speed of transmission 0flow control

#ere are two types of transmission


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Connect!on.Or!ented Transm!ss!ons

!lso known as 6 >eliable #ransport 8etod0 uses acknowled"ement


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Connect!onless Transm!ss!ons

In tis mode te transmittin" device doesnot re:uire acknowled"ements from tereceiver and continues to transmit on teassumption tat te data was received

'eatures are3

)ittle or (o >eliability

'aster #ransmission

%ackets are not >e;transmitted


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/low Control

/stablises te ma2imum speed at wicbot sender and receiver can communicateat

#ransport )ayer determines lar"est packetsiBe wic can be sent

%ackets are numbered 0 to allow re;assembly in te correct order

$$M%& '()M%&

Transmission occurs at the

agreed speed of $$M%&


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Network (a'er

>esponsible for te correct addressin" anddelivery of packets of data

#ese are known as data"rams

Cses te network address < tis is a lo"ical

address 0 and does not depend upon anyardware in te device or te devicespysical location=


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#e (etwork )ayer does te followin"3

!dds te address to te packet


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"ow does !t work &

/ncapsulation at te #ransport )ayer involves

addin" te address of te sender to te data"ram #e destination address is now added. ot

addresses are lo"ical.

ot addresses are necessary for packets to move

between end systems. If a packet must move to anoter network a

routin" protocol is re:uired

If different packet len"ts are used on te differentnetworks te (etwork )ayer formats te dataaccordin"ly

#e primary piece of ardware wic works on tislayer is te router.


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ata (!nk (a'er

as two sub layers of its own3

)o"ical )ink $ontrol


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MAC 1Med!a Access Control

/ac (I$ as a uni:ue number ard codedin to te card 0 its pysical address

#e first * di"its denote te manufacturerte ne2t si2 are uni:ue= 0 type winipcf"Eon your %$

en te 8!$ address is added to tepacket it is now known as a frame

It now as all te information re:uired to

travel from te source to te destination


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-h's!cal (a'er

#e lowest bottom layer 0 responsible forte pysical connection between devices

#e (I$ converts te data


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Summar'


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OSI versus TC-3I- Model

OSI model is an important concept

%rotocol most in use on modern networksis #$%&I%

#$%&I% does not map its layers precisely to

OSI model OSI G 7 layers #$%&I% G 4 layers


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OSI vs TC-3I-

Session

Physical

Transport

Network

Data Link

Presentation

Application

Application or

Process Layer

Transport

Data Link

Internet


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TC-3I- Model Summar'

!pplication or %rocess )ayer 0 concernedwit ow data at bot ends is andled.

#ransport )ayer 0 mana"es flow of data

Internet )ayer 0 consists of several

protocols primary protocol is I%


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An' 4uest!ons&

/nd of )ecture.

8I#8 -H+ !dvance $omputer (etworks

Lecture 2.ppt

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