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OSI Model

OSI MODEL

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Layer ArchitectureLayer architecture simplifies the network design.It is easy to debug network applications in a layeredarchitecture network.

The network management is easier due to thelayered architecture.

Network layers follow a set of rules, called protocol.

The protocol defines the format of the data beingexchanged, and the control and timing for thehandshake between layers.

OSI Model

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OSI Reference ModelThe OSI model is now considered the primaryArchitectural model for inter-computer communications.

The OSI model describes how information or datamakes its way from application programmes (suchas spreadsheets) through a network medium (suchas wire) to another application programme locatedon another network.

The OSI reference model divides the problem of moving information between computers over anetwork medium into SEVEN smaller and moremanageable problems .

This separation into smaller more manageablefunctions is known as layering.

OSI Model

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OSI Reference Model: 7 Layers

OSI Model

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OSI: A Layered Network ModelThe process of breaking up the functions or tasks of networking into layers reduces complexity.Each layer provides a service to the layer above it inthe protocol specification.Each layer communicates with the same layers

software or hardware on other computers.The lower 4 layers (transport, network, data link andphysical Layers 4, 3, 2, and 1) are concerned withthe flow of data from end to end through the network.The upper four layers of the OSI model (application,presentation and sessionLayers 7, 6 and 5) areorientated more toward services to the applications.Data is Encapsulated with the necessary protocol

information as it moves down the layers beforenetwork transit.

OSI Model

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Physical Layer P rovides physical interface for transmission of

information.

Defines rules by which bits are passed from onesystem to another on a physical communicationmedium.

Covers all - mechanical, electrical, functional andprocedural - aspects for physical communication.

Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximumtransmission distances, physical connectors, andother similar attributes are defined by physical layer specifications.

OSI Model

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Data Link Layer Data link layer attempts to provide reliablecommunication over the physical layer interface.

Breaks the outgoing data into frames andreassemble the received frames.Create and detect frame boundaries.Handle errors by implementing an acknowledgementand retransmission scheme.Implement flow control.Supports points-to-point as well as broadcastcommunication.Supports simplex, half-duplex or full-duplexcommunication.

OSI Model

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Network Layer Implements routing of frames (packets) through thenetwork.

Defines the most optimum path the packet shouldtake from the source to the destinationDefines logical addressing so that any endpoint canbe identified.Handles congestion in the network.

Facilitates interconnection between heterogeneousnetworks (Internetworking).The network layer also defines how to fragment apacket into smaller packets to accommodatedifferent media.

OSI Model

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Transport Layer P urpose of this layer is to provide a reliablemechanism for the exchange of data between twoprocesses in different computers.

Ensures that the data units are delivered error free.

Ensures that data units are delivered in sequence.

Ensures that there is no loss or duplication of dataunits.P rovides connectionless or connection orientedservice.P rovides for the connection management.

Multiplex multiple connection over a single channel.

OSI Model

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Session Layer Session layer provides mechanism for controlling thedialogue between the two end systems. It defines how tostart, control and end conversations (called sessions)between applications.

This layer requests for a logical connection to beestablished on an end-users request.Any necessary log-on or password validation is alsohandled by this layer.Session layer is also responsible for terminating theconnection.This layer provides services like dialogue disciplinewhich can be full duplex or half duplex.Session layer can also provide check-pointingmechanism such that if a failure of some sort occursbetween checkpoints, all data can be retransmitted fromthe last check oint.

OSI Model

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Presentation Layer P resentation layer defines the format in which the

data is to be exchanged between the twocommunicating entities.

Also handles data compression and data encryption(cryptography).

OSI Model

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Application Layer Application layer interacts with applicationprograms and is the highest level of OSI model.

Application layer contains management functions tosupport distributed applications.

Examples of application layer are applications suchas file transfer, electronic mail, remote login etc.

OSI Model

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TC P/ IP Model

TC P/ IP MODEL

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OSI & TCP/

IP

Models

TC P/ IP Model

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TCP/

IP

Model

TC P/ IP Model

Application Layer Application programs using the network

Transport Layer (TC P/U DP )

Management of end-to-end message transmission,error detection and error correction

Network Layer (I P )Handling of datagrams : routing and congestion

Data Link Layer Management of cost effective and reliable data delivery,

access to physical networks

P hysical Layer

Physical Media