OSI MODEL for vii sem

8/7/2019 OSI MODEL for vii sem

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OSI MODEL


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OSI Reference Model: 7 Layers


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Provides physical interface for transmission of information.

Defines rules by which bits are passed from one system to

another on a physical communication medium.

Covers all - mechanical, electrical, functional and procedural -

aspects for physical communication.

Such characteristics as voltage levels, timing of voltage

changes, physical data rates, maximum transmission

distances, physical connectors, and other similar attributes are

defined by physical layer specifications.

Physical Layer


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Data Link Layer

Data link layer attempts to provide reliable communication over

the physical layer interface.

Breaks the outgoing data into frames and reassemble the

received frames.

Create and detect frame boundaries.

Handle errors by implementing an acknowledgement and

retransmission scheme.

Implement flow control.

Supports points-to-point as well as broadcast communication.

Supports simplex, half-duplex or full-duplex communication.

OSI Model


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Implements routing of frames (packets) through the network.

Defines the most optimum path the packet should take from

the source to the destination

Defines logical addressing so that any endpoint can be

identified.

Handles congestion in the network.

Facilitates interconnection between heterogeneous networks

(Internetworking).

The network layer also defines how to fragment a packet into

smaller packets to accommodate different media.

Network Layer


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Transport Layer

Purpose of this layer is to provide a reliable mechanism for the

exchange of data between two processes 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 data units.

Provides connectionless or connection oriented service.

Provides for the connection management.

Multiplex multiple connection over a single channel.


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Session Layer

Session layer provides mechanism for controlling the dialogue between

the two end systems. It defines how to start, control and end

conversations (called sessions) between applications.

This layer requests for a logical connection to be established on an end-

users request.

Any necessary log-on or password validation is also handled by this layer.

Session layer is also responsible for terminating the connection.

This layer provides services like dialogue discipline which can be full

duplex or half duplex.

Session layer can also provide check-pointing mechanism such that if a

failure of some sort occurs between checkpoints, all data can be


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Presentation Layer


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Application Layer

Application layer interacts with application programs and is the

highest level of OSI model.

Application layer contains management functions to support

distributed applications.

Examples of application layer are applications such as file

transfer, electronic mail, remote login etc.


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OSI Model

Data unit Layer Function

Host

layers

Data

7. Application Network process to application

6. Presentation Data representation,encryption and decryption

5. Session Interhost communication

Segments 4. Transport End-to-end connections and reliability,Flow control

Media

layers

Packet 3.Network Path determination and logical addressing

Frame 2. Data Link Physical addressing

Bit 1. Physical Media, signal and binary transmission


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Shortcomings of OSI Model

Just because someone says it is a model/standard doesnot mean you have to follow

All layers do not have the same size and importance-session and presentation layers seldom present

-data link, network, and transport layers oftenvery full

Little agreement on where to place various featuresEncryption, network management

Large number of layers increases overheads


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UNIFIES VIEW OF LAYERS , PROTOCOLS,

SERVICES


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PDUs are relevant in relation to each of the first 4 layers of the

OSI model as follows:

The Layer 1 (Physical Layer) PDU is the bit

The Layer 2 (Data Link Layer) PDU is the frame

The Layer 3 (Network Layer) PDU is the packet

The Layer 4 (Transport Layer) PDU is the segment (e.g. TCP

segment)


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The PDU at any given layer, layer 'n', is the SDU of the layer

below, layer 'n-1'.

In effect the SDU is the 'payload' of a given PDU. That is, the

process of changing a SDU to a PDU, consists of an

encapsulation process, performed by the lower layer.

All the data contained in the SDU becomes enapsulated within

the PDU. The layer n-1 adds headers or footers, or both, to the

SDU, transforming it it into a PDU of layer n-1.

The added headers or footers are part of the process used to

make it possible to get data from a source to a destination.


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A layer may impose a limit on size of data block that can be

transferred due to implementation or due to other reasons.

The layer n SDUmay be too large to be handled as a single

unit by layer (n-1)

Sender side : SDU is segmented into multiple PDUs.

Receiver side : SDU is reassembled from sequence of PDUs


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Multiplexing: sharing of n service by a multiple layer n+1 users

Multiplexing a tag or ID required in each PDU to determine which

user an SDU belongs to

Eg port number in TCP/IP


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Application Layer

the services that user applications use to communicate over the

network, and it is the layer in which user-access network processes

reside

process user data, such as data encryption and decryption and

compression and decompression.

manages the sessions (connections) between cooperating

applications.

responsible for standardizing the presentation of data.


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Transport Layer

It is responsible for end-to-end data integrity

two most important protocols employed at this layer are the

Transmission Control Protocol(TCP) and User Datagram Protocol

(UDP).

TCP provides reliable, full-duplex connections and reliable service by

ensuring that data is resubmitted when transmission results in an

error (end-to-end error detection and correction). Also, TCP enables

hosts to maintain multiple, simultaneous connections.

When error correction is not required, UDP provides unreliable

datagram service (connectionless) that enhances network throughput

at the host-to-host transport layer


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Internetwork Layer

the basic packet delivery service for all TCP/IP networks

Define the datagram, which is the basic unit of transmission in

the Internet

Move data between the Network Access Layer and the Host-to-

Host Transport Layer

Fragment and reassemble datagram


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Network Access Layer

it is concerned with getting data across a specific type of

physical network

encapsulating the IP datagram intoframes that are

transmitted by the network

maps the IP addresses to the physical addresses used by the

network

responsible for routing and must add its routing informationto the data


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Documents

OSI MODEL for vii sem