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This is the first slide of my ppt. Before I proceed to Transport Layer let me introduce you to OSI Model. What is OSI Model.
The OSI Model is a Model that illustrate How data communication should take place. It devides the process into seven groups called Layers. A layer is a collection of conceptually similar functions that provide services to the layer above it and receive services from the layer below it. The OSI Model affirms what protocols and standards should be used at each layer.
The lower three layers are called Media Layer because they follow virtual circuit approach. and the upper four layers are called Host layers.
Function of OSI Layers
Physical Layer:- It defines the standards required for physical interconnections e.g. The Wire. It handles the Bits.
Data Link Layer:- It defines the protocols for exchanging data frames over the Network.
Network Layer:- It handles routing between networks.
Transport Layer:- This layer provide end to end connection between machines. Conceptually it is outside the network.
Session Layer:- This layer provides for dialogue between processes.
Presentation Layer:- It provides for translation of data structure between different architectures.
Application Layer:- It provides for application level access to the network, file transfer, remote terminals etc..
In the above picture it is shown that If data is transmitted the OSI Model follows Top-Down approach means first Application Layer work than Presentation Layer and so on. When data is passes through Physical Link the OSI Model follows Bottom-Up approach means first Physical layer work than Data Link Layer and so on. To understand it more lets look at the following image which shows the working of OSI Model.
In the above a business correspondence is taking place. First the Manager of left side write or dictates the Message to be send to other business Manager. the same work is done by the Application of OSI Layer when data is send. In next lavel the assistant corrects the format, checks errors and prepare final version. Which is the work of Presentation layer.
In the next lavel The secretary needed address and packs the latter which is In OSI done by Session Layer. Than the latter drives the latter to the Post Office. Which is done by Transport Layer in OSI Model.
Than in Post Office a person Intakes the latter and sort them according to their compartment. Which is the work of Network Layer of OSI Model.
In the next level Packaging is take place which is in OSI is done by Data Link Layer.Finally the worker loades the latters into transmission which is actually a physical medium of transmission. Till now the OSI model was following the Top-Down approach but when it is passes through the Physical link it will follow Bottom-Up approach.
Now When latters reach to other post office a worker will take out all the packets which is done by Physical Layer when data is received. than the packets are unpacked which is the work of Data Link Layer on receiving side. When packets are unpacked the latters are sorts for individual city which is done by network layer of OSI Model. Than through Post Man the latters reaches at the business corporation which is the work of Transport Layer of OSI. Then the secretary opens the latter and makes a copy of it which is being done by Session Layer. Than the assistant alerts the Manager of incoming of message and transmits the latter which is the work of Presentation Layer of OSI Model. Then finally the Manager reads the message.
Same as the OSI Model Works. Now move to the Transport Layer and understand it deeply.
Transport Layer manages end to end message delivery in a network. Transport Layer accepts data from the Session Layer breaks it into packets and deliver these packets to the Network layer. It is the responsibility of Transport Layer to guaranteed successful arrival of data at the destination device.
It also provides the error checking and hence guarantees that no duplication or errors are occurring in the data transfers accross the network.
Transport Layer also provides the acknowledgement of the successful data transmission and retransmits the data if no error free data was transferred. If the Network Layer do not provide adequate services for the data transmission. Data loss due to poor network management is handled by using Transport layer. It checks for the packets that are lost or damaged along the way.
In the above image in blue colored frames is the information carried by the protocols of Transport Layer , Network Layer and Data Link Layer . The box between Transport Layer defines the information carries by Transport Layer ant box between Network Layer shown the information carried by protocols of Network Layer and box between Data Link Layer shows the information carried by the protocols of Data Link Layer.
Functions of Transport Layer
(i) Process Level Addressing:- Addressing performed at the Transport Layer is used to differentiate between software program's. This is part of what enables many different software programs to use a network layer protocol simultaneously.
(ii) Multiplexing and De multiplexing:- Using the address, Transport Layer protocols on a sending device multiplex the data received from many application programs for transport, combining them into single stream of data to be sent.
The same protocol receive data and then demultiplex it from the incoming stream of data gram.
(iii) Segmentation,Packaging and reassembly:- The Transport Layer segments the large amount of data it sends over the network into smaller pieces on the source machine and then reassemble them on the destination device.
(iv) Connection Establishment, Management and Termination:- Transport Layer connection oriented protocols are responsible for series of communication required to establish a connection, maintain it as data is sent over it, and then terminate the connection when it is no longer required.
(v) Acknowledgement and Retransmission:- When data is sent a timer is started ; if the receiver received the data it will sends back an acknowledgement to the transmitter to indicate successful transmission. If no acknowledgement comes back before the timer expires, the data is retransmitted.
