THE OSI MODEL

(Open Systems Interconnection)

GORVAM SADDAR

The Open Systems Interconnection The Open Systems Interconnection ((OSIOSI)) is the layered model for the network is the layered model for the network system, Designed by system, Designed by ISOISO ( ( International International Organization StandardizationOrganization Standardization ). OSI ). OSI enables devices in the network to enables devices in the network to communicate with each other.communicate with each other.

The OSI 7-layer Model

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• Defined in 1984 and become an international standard

Relationship of OSI layers

PhysicalCommunication

1. Physical Layer

• Responsible for transmitting bits from one computer to another

Example protocols: Ethernet, Token Ring (Physical part)

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Converts bits into electronic signals for outgoing messages.

Defines the way in which two or more devices can be connected physically.

2. Data Link Layer

• Provides for flow of data over a single link from one device to another.

• Packets placed into frames at this layer

• Provide error-free frame transfer by acknowledgment and retransmission

Example protocols: Ethernet, Token Ring

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3. Network Layer

• Logical address to physical address translation

For TCP/IP running on Ethernet

Logical address: IP address

158.132.148.99

Physical address: Ethernet address

00 00 E2 15 1A CA

• Determine the route from source to destination computer

Example protocols: IP, IPX

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4. Transport Layer

• Ensure packets are delivered error free, in sequence

• Translate between packets and message

• Control flow of data and acknowledges successful receipt of data.

1 2 3

Sending

Example protocols: TCP, SPX, NWLink

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5. Session Layer Example application program: Winsock, UNIX Socket

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• Establishment, maintaining and release of session

• Provide dialog management

• Regulate which side transmit, when, for how long (Sync.)

• Provide synchronization between user tasks

6. Presentation Layer Example application program: redirector (NT), SSL

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• Translate data from Application Layer to the format suitable for session layer (the network)

• Provide data encryption, compression

• Changing or converting character set, graphic, and file format

7. Application Layer Example protocols: FTP, Telnet, HTTP

• Entry point for application to access network

• Directly support user applications

• E.g. File transfer, email

• General capabilities:

Network access, flow control, Error recovery

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Example: File Transfer

PictureTelPictureTel

PictureTel

FTP ServerFTP Client

Network

• The most traditional network task

• Implemented by a simple Application Layer protocol called FTP