Osi Model

IT-5301-3Data Communications and Computer Networks

University of Education, Pakistan.

–Layered Architecture By: Abid Shahzad Aslam

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Lecture–Roadmap

• Need of a Layered Architecture• OSI Model

– 7 Layers of OSI Model– Functional Details of OSI Layers

• Physical Layer• Data Link Layer• Network Layer• Transport Layer• Session Layer• Presentation Layer• Application Layer

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Layered Architecture

• Design Philosophy of Layered Architecture– The complex task of communication is broken into

simpler sub-tasks or modules– Each layer performs a subset of the required

communication functions– Each layer relies on the next lower layer to perform

more primitive functions– Each layer provides services to the next higher layer– Changes in one layer should not require changes in

other layers– Helps in troubleshooting and identifying the problem

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An example Sending a letter

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OSI Model

• To standardize the design of communication system, the ISO created the OSI model.• Open Systems Interconnection• Developed by the International Organization for Standardization (ISO)• Contains Seven layers• It describes the functions to be performed at each layer

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OSI Model

• ISO Established in 1947ISO Established in 1947• ISO standard that covers all aspects of network ISO standard that covers all aspects of network

communications is the Open Systems Interconnection communications is the Open Systems Interconnection ((OSIOSI) model. ) model.

• First introduced in the late 1970s.First introduced in the late 1970s.• A layer model• Each layer performs a subset of the required communication functions• Each layer relies on the next lower layer to perform more primitive functions• Each layer provides services to the next higher layer• Changes in one layer should not require changes in other layers

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ISO is the organization.OSI is the model.

Note

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OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Peer-to-Peer Process

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An exchange using the OSI model

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OSI Model – Physical Layer

• Physical– Physical interface between devices– handles the transmission of bits over a

communications channel– Choice of Wired / wireless medium– Data is converted into signals– Includes voltage levels, connectors, media

choice– modulation techniques– EIA/TIA-232, RJ45, NRZ.

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Physical Layer

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The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

Note

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OSI Model – Data Link Layer

• Data Link– Transforms data into a frame– Means of activating, maintaining and deactivating a reliable link– Error detection and control– Flow Control– Higher layers may assume error free

transmission– Examples :- Ethernet, Frame Relay, FDDI.

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Data Link Layer

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The data link layer is responsible for moving frames from one hop (node) to the next.

Note

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OSI Model – Network Layer

• Network– Transport of information– Higher layers do not need to know

about underlying technology– Responsible for creating, maintaining

and ending network connections– Transfers a data packet from node to

node within the network.– Routing– Not needed on direct links– Examples :- IP, IPX, AppleTalk.

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Network Layer

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The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Note

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Transport Layer

• Transport– Exchange of data between end

systems (end to end flow control) – Error free – In sequence – Quality of service• Layer 4 protocols include TCP

(Transmission Control Protocol) and UDP (User Datagram Protocol).

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Transport Layer

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The transport layer is responsible for the delivery of a message from one process to another.

Note

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OSI Model – Session Layer

• Session– Control of dialogues between applications

• Half Duplex• Full Duplex

– Synchronization Points (backup points)– Grouping– Examples :- SQL, ASP(AppleTalk Session

Protocol).

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The session layer is responsible for dialog control and synchronization.

Note

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OSI Model – Presentation Layer

• Presentation– Data formats and coding– Data compression– Encryption– Examples :- JPEG, MPEG, ASCII,

EBCDIC, HTML.

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The presentation layer is responsible for translation, compression, and encryption.

Note

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OSI Model – Application Layer

• Application– Layer where the application using the

network resides.– Common network applications include

• remote login• file transfer• e-mail• web page browsing etc.

– Means for applications to access OSI environment

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The application layer is responsible for providing services to the user.

Note

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Summary of layers

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OSI Layers

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OSI Layers

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The OSI Environment

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The OSI Environment