THE OSI MODEL

(Open Systems Interconnection)

GORVAM SADDAR

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

The OSI 7-layer Model

All

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Data

Processing

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Pizza

Sausage

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Not

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Please

Defined in 1984 and become an international standard

Relationship of OSI layers

Physical

Communication

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 layerProvide 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.

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 sessionProvide dialog managementRegulate 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, compressionChanging 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 applicationsE.g. File transfer, emailGeneral capabilities:

Network access, flow control, Error recovery

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Example: File Transfer

FTP Server

FTP Client

Network

The most traditional network task Implemented by a simple Application Layer protocol called FTP

PictureTel

PictureTel