20902881 the seven layers of the osi model

8/9/2019 20902881 the Seven Layers of the OSI Model

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OSI Layers


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ISO-OSI ISO-International Standards Organisation

Multinational body dedicated to worldwide agreementon international standards

An ISO standard that covers all aspects of network

communication is the OSI OSI-Open System Interconnection

OSI is a model and not a protocol

Vendor specific protocols close off communication

between unrelated systems OSI model is to open communication between different

systems without requiring changes to the logic of theunderlying hardware and software


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Layered Communication

Person-A

(English)

Person-B

(French)

I like rabbits

Jaime les lapins

Ik hou van konijnen

Translator-B(French-Dutch)

Translator-A(English-Dutch)

Ik hou van konijnen

Secretary-A

(FAX)

Ik hou van konijnen Secretary-A

(FAX)Ik hou van konijnen

FAX


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Layered Communication Each layer has a distinct identity and a specific set

of functions so that any change in theimplementation does not affect other layers.

Each layer has an active element to carry out thelayer functions

Each layer provides certain services to the nexthigher layer which uses these services to carry out

its assigned functions. Layers must follow some meaningful sequence

Number of layers should be minimum as the eachadded layer adds to the cost and delay


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OSI Layers

The OSI model is built of seven ordered

layers:

Layer-1: Physical

Layer-2: Data Link

Layer-3: Network

Layer-4: Transport

Layer-5: Session

Layer-6: Presentation

Layer-7: Application

ApplicationPresentation

Session

Transport

Network

Data Link

Physical


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OSI Layers The seven layers can be thought of as

belonging to three sub groups

Network Support Layers (Layers 1-3) Deal with the physical aspects of moving data from

one device to another

User Support Layers (Layers 5-7)

Allow interoperability among unrelated softwaresystems

Layer-4 ensures end to end reliable data

transmission


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OSI LayersEnd System

Network Network Network Network

Data Link Data Link Data Link Data Link

Application

Presentation

SessionTransport

Physical PhysicalPhysical Physical

PSPDN PSTN Dedicated CSPDN

User Support Layers

Network Support Layers


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Peer-to-Peer communication Between machines, layer-n on one machine

communicates with layer-n on another machine

This communication is governed by an agreed-upon

series of rules and conventions called protocols The processes on each machine that communicates

at a given layer are called peer-to-peer processes

At the physical layer, communication is direct

At higher layers, communication moves down

through the layers on the transmitting machine and

back up through the layers at receiving machine


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Hierarchical & Peer-to-Peer Communications

Peer-to-Peer

PCI

Interface Control Information (ICI)

Protocol Control Information (PCI)

Interface

N+1-Layer

N-Layer

Interface

N+1-Layer

N-Layer

Hierarchical

ICI


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(N+1)-Layer

(N)-Layer

Data Units in the OSI Model

(N+1)-SDU(N+1)-PCI

(N+1)-PDU

(N+1)-SDU(N+1)-PCI (N)-ICI

(N)-IDU

(N)-PCI

(N)-PDU

(N+1)-SDU(N+1)-PCI

(N)-PCI

(N)-ICI

(N+1)-SDU(N+1)-PCI(N)-SDU

(N)-ICI


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Data Units Protocol Control Information (PCI)

(N)-PCI is the protocol control informationexchanged between the (N)-entities tocoordinate their functions

Service Data Units (SDU) (N)-SDU is the data unit transferred between

the ends of a (N)-connection whose identity ispreserved during the transfer

Protocol Data Unit (PDU) (N)-PDU is the combination of (N)-PCI and

(N)-SDU


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Data Units Interface Control Information (ICI)

(N)-ICI is the information exchanged between

(N+1)-entity and (N)-entity to coordinate their

functions Interface Data Unit (IDU)

(N)-IDU is the total data unit transferred across

the SAP between (N+1)-entity and (N)-entity


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OSI Layers

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

7-Application

6-Presentation

5-Session

4-Transport

3-Network

2-Data Link

1-Physical

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

7-Application

6-Presentation

5-Session

4-Transport

3-Network

2-Data Link

1-Physical

3-2 Interface

2-1 Interface

3-Network

2-Data Link

1-Physical

Intermediate

Node

Link Link


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Layer-1(Physical)

First of three network support layers Concerned with physical transmission of data

bits and ensures that a bit entering at one end of

the transmission media reaches the other end Deals with the mechanical and electrical

specifications of the interface and transmission

medium e.g. Optical, coax, RF, twisted pair etc.

Defines the type of encoding i.e. how 0s and 1sare changed to signals

Defines data rate / transmission rate i.e. defines

the duration of a bit

A

P

S

T

N

D

P


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Layer-1(Physical)

Responsible for synchronisation of sender

and the receiver clocks Concerned with the connection of the

devices to the medium Point-to-point configuration Multipoint configuration

Physical topology Mesh; Star; Ring; Bus

Transmission Mode Simplex; Half-Duplex; Full-Duplex

A

P

S

T

N

D

P


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Layer-1(Physical)

L2 Data

L1 -1010100000

L2 Data

L1 -1010100000

Transmission Medium

Physical Layer

From Data Link Layer To Data Link Layer


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Layer-2 (Data Link) Second of three network support layers Divides the bit stream received from network

layer into manageable data units called frames

Transforms the physical layer to a reliable link by adding mechanism to detect and retransmit

damaged frames Responsible for physical addressing of the

devices Responsible for link-by-link flow control and

error free delivery of data Responsible for Media Access Control

A

P

S

T

N

D

P


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Layer-2 (Data Link)

Data Link

LayerH2T2 H2T2

L3 Data L3 Data

L1- 00101111000001 L1- 00101111000001

From Network Layer To Network Layer

To Physical Layer From Physical Layer


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Layer-3 (Network) Last of the three network support layers Responsible for Source-to-Destination

delivery of individual packets across

multiple links If two systems are connected to the same

link there is usually no need for a network

layer Responsible for the unique logical

addressing of the sender and the receiver Responsible for routing of packets

A

P

S

T

N

D

P


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Layer-3(Network)

Network LayerH3 H3

L4 Data L4 Data

L3 Data L3 Data

From Transport Layer To Transport Layer

To Data Link Layer From Data Link Layer


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Layer-4 (Transport) Responsible for Source-to-Destination

delivery of the entire message Uses service-point address (port address)

for end-to-end delivery Network layer gets each packet to correctcomputer, transport layer gets the entiremessage to the correct process

Responsible for segmenting a messageinto transmittable segments

At the destination the message is correctlyreassembled

A

P

S

T

N

D

P


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Segmentation and Reassembly

Transport LayerH4

L3Data

L5 Data

H4

L3Data

H4

L3Data

L5 Data

H4

L3Data

From Session Layer To Session Layer

To Network Layer From Network Layer


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Layer-4 (Transport) Utilises network layer to ensure reliable,

sequenced data exchange Transport layer can be connectionless or

connection oriented A connectionless transport layer treats each

segment as an independent packet A connection oriented transport layer makes

a connection with the transport layer at thedestination machine before delivering the

packets After all the data is transmitted, the

connection is terminated

A

P

S

T

N

D

P


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Layer-4 (Transport) Responsible for end-to-end flow control

of data Responsible for end-to-end error control

of data Error correction is usually achieved through

retransmission

A

P

S

T

N

D

P


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Layer-5(Session) First of the three user support layers It is the network dialog controller It establishes, maintains, and synchronises

the interaction between communicatingsystems It allows the communication between two

processes to take place either in half-

duplex or full-duplex Allows a process to add checkpoints

(synchronisation points) into a stream ofdata

A

P

S

T

N

D

P


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Layer-5(Session)

SYN

H5

L5 Data

L6 Data

H5

L5 Data

L6 Data

Session

Layer

To Transport Layer From Transport Layer

From Presentation Layer To Presentation Layer


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Layer-6 (Presentation) Second of the three user support layers Concerned with the syntax and semantics

of the information exchanged between

two systems At sender end, changes the information

from sender dependent format into acommon format

At the receiving end, changes theinformation from common format into itsreceiver dependent format

A

P

S

T

N

D

P


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Layer-6 (Presentation) Responsible for encryption and decryption

of sensitive information Responsible for data compression of the

data to be transmitted

A

P

S

T

N

D

P


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Layer-6(Presentation)

Presentation

LayerH6 H6

L7 Data L7 Data

L6 Data L6 Data

Encoded, Encrypted

and Compressed data

Decoded, Decrypted and

Decompressed data

To Session Layer From Session Layer

From Application Layer To Application Layer


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Layer-7(Application) Top of the three user support layers Enables the user, human or software, to

access the network

It provides user interfaces and support forservices e.g. electronic mail, remote file

access and transfer, shared database

management and other types of distributedinformation services

No headers or trailers are added by this

layer

A

P

S

T

N

D

P


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MTP:MESSAGE TRANSFER PART: RESPONSIBLE FOR TRANFERING MESSAGE TAKING MESSAGE FROM ONE NETWORK TO ANOTHER NETWORK CONSIST OF THREE LAYERS

TELEPHONE USER PART

RESPONSIBLE FOR CALL STTING, CALL SUPERVISING,CLEARING

ITU ALLOWED THE VARIATION IN ONE MESSAGES DEPENDENT UPONCOUNTRY TO COUNTRY THAT IS KNOWN AS NATIONAL USERPART NUP

ON ACCOUNT OF INTRODUCTION OF ISDN SOME EXTRA SET OFMESSAGES ARE REQUIRED THAT IS KNOWN AS A ISUP THAT ISHEPING FOR SETTING OF CALL


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SCCP

SIGNALLING CONNECTION AND CINTROL PART

AS THE SIGNALLING REACH THROUGH DIFFERENT PART.

DUE TO THIS SEQUENCE OF ORDER OF MESSAGE ATRECEIVING END MAY DIFFER HENCE THERE IS A NEED

FOR ESTABLISHING A VIRTUAL CONNECTION THAT IS A

CONNECTION ORIENTED PROTOCOL WHICH ENABLE A

CORRECT SEQUENCING.

ANOTHER ISSUE THAT MTP IS CAPABLE OF ROUTING AMESSAGE WITH IN ONE NETWORK ONLY. IN CASE OF

ABSENCE OF A CALL ACROSS MULIPLE NETWORK MTP

CAN NOT SEND A SIGNALLING ACROSS MULTIPLE

NETWORK


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EXTRA SIGNALLING REQUIREMENT IN GSM DUE TODIFFERENT ARCHITECTURE BECAUSE IN THIS CASE

GREATER PART IS REQUIRED FOR NON CALL RELATED

DUE TO MOBILITY. FOR THAT ADDITIONAL PROTOCOL

LAYERS ARE.

o BASE STATION SUBSYSTEM APPLICATION PART: THIS

LAYER IS USED WHEN MSC COMMUNICATES WITH BSC

AND MOBIKE STATION.o AUTHENTICATION VERIFICATION AND ALLOCATION OF

TMSI WILL TAKE PLACE THROUGH BSSAP. IT SERVE THE

o MSC----- BSC SIGNALLINGo MSC----- MS SIGNALLING


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MOBILE APPLICATION PART : IN CASE OF MOBILETERMINATED CALL DURING INTERNATIONAL ROAMING

MSRN IS REQUESTED FROM HLR WITHOUT ROUTING A

CALL, FOR THAT ANOTHER LAYER IS REQUIRED THAT IS

A MOBILE APPLICATION PART FOR SIGNLLING BETWEEN

NSS ELEMENTS

TRANSACTION CAPABILITIES APPLICATON PART: WHEN HLR

SENDS SOME RESULTS TO MSC. IT MIGHT BE IN PARTS;

FOR THAT MAP REQUIRES CERTAIN ASSISTANCE TO

TRANSACT THAT MESSAGES BETWEEN MULTIPLEELEMENTS; THIS IS CALLED A TCAP


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EXTRA SIGNALLING REQUIREMENT IN GSM DUE TODIFFERENT ARCHITECTURE BECAUSE IN THIS CASE

GREATER PART IS REQUIRED FOR NON CALL RELATED

DUE TO MOBILITY. FOR THAT ADDITIONAL PROTOCOL

LAYERS ARE.

o BASE STATION SUBSYSTEM APPLICATION PART: THIS

LAYER IS USED WHEN MSC COMMUNICATES WITH BSC

AND MOBIKE STATION.o AUTHENTICATION VERIFICATION AND ALLOCATION OF

TMSI WILL TAKE PLACE THROUGH BSSAP. IT SERVE THE

o MSC----- BSC SIGNALLINGo MSC----- MS SIGNALLING


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User plan : A+P+S+T

TRANSPORT PLANE: P+D+N

CONTROL PLAN: A+P+S+TWhen the bearer passes through some

reference plan than the defination of thethese reference points

when the defined bearer is used for

traffic , the control of this traffic in

the bearer in the network is user

plane signalling.


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TRANSPORT PLAN PROVIDE THE MEANS HOW

THE PHYSICAL CONNECTION IS ESTABLISHED

BETWEEN THE MOBILE TERMINAL AND THE

NETWORK.

IN UMTS WE CAN USE DIFFERENT PHYSICAL

CONNECTIONS. HOWEVER THE SPECIFICATION

IS MAINLY BASED ON THE ATM.


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PACKETS CONTAINING THE TRANSFORMED

INFORMATION IS KNOWN AS ATM CELLATM CELL CONSIST OF TWO PARTS

PAYLOAD INFORMATION 48 BYTES

HEADER INFORMATION 5 BYTES

TWO TYPES OF ATM CELLS AREUNI USER NETWORK INTERFACE: ATM SW-

ATM END POINTS

NNI NETWORK NODE INTERFACE : ATM SW-

ATMSW

ATM TRANSMISSION PATHS : VIRTUAL PATHS

VIRTUAL CCTS

ATM LAYERS: PHYSICAL LAYER, ATM

LAYER,ATM ADAPTION LAYER


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ATM ADAPTION LAYERS:

AAL1: SYNCHRONOUS MODE------ CONNECTION

ORIENTED CONSTANT BIT RATE

AAL2: SYNCHRONOUS MODE ----CONNECTION

ORIENTED VARIABLE CONSTANT BIT RATEAAL3/4: CONNECTION LESS CONSTANT BIT RATE

AAL5: CONNECTION LESS VARIABLE CONSTANT

BIT RATE


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An exchange using OSI model

L6-Data

L5-Data

L4-Data

L3-Data

L2-Data

0100011110001001000111111000011101010

H6

H5

H4

H3

H2T4

A-7

P-6

S-5

T-4

N-3

D-2

P-1

L7-Data


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An exchange using OSI model

L7-Data

L6-Data H6

L5-Data H5

L4-Data H4

L3-Data H3

L2-Data H2T4

A-7

P-6

S-5

T-4

N-3

D-2

P-10100011110001001000111111000011101010


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Summary of OSI Layers Functions

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

Application

Presentation

Session

Transport

Network

Data Link

Physical

Allow access to network

resources

Translate, encrypt and

compress data

Establish, manage and

terminate sessions

Reliable end to end delivery

& error recovery

Movement of packets;

Provide internetworking

Transmit bits; Mechanical

and electrical specifications

Organise bits into streams;Node to node delivery

20902881 the seven layers of the osi model

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