Assuming each Subnetwork provides different quality of service at a cost, the Transport layer carries out functions necessary to meet the quality of service in a cost effective manner by choosing appropriate Subnetwork.

Session Layer:-

It provides the means necessary for the operating of Presentation entities to organize one synchronize their data exchange. It allows users on different machines to establish sessions between them. One of the services of session layer is to manage dialogue control. Session layer provides two way simultaneous, two way alternate and one way communication services. H traffic can only go one way at a time; the session layer can help keep track of whose turn it is.

A related session service is token management. For some protocols, it is essential that both sides do not attempt the same operation at the same time. To manage these activities, session layer provides tokens that can be exchanged. Only the side holding the token may perform the critical operation.

Another session service is synchronization. Consider the problems that might occur when trying to do a two hour file transfer between two machines on the network with a 1 hr. mean time between crashes. After each transfer was aborted the whole, transfer would have to start over- again and would probably fail again when the network next crashed. To eliminate this problem session layer provides a way to insert check points into the data stream, so that after the crash, only the data after the last check point have to be repeated.

Presentation Layer:-

Unlike the lower layers which arc just interested in moving bits reliably the presentation layer is concerned with the syntax and semantics of the information transmitted.

Different computers may have different codes (e.g. ASCII; EBCDIC etc.) for data representation. The presentation layer contains functions necessary to transform the transfer syntax to the required syntaxes used by the Application entities preserving the meaning.

Data compression can be used here to reduce the number of bits that have to be transmitted and cryptography is frequently required for privacy and authentication.

Application Layer:-

This layer is to allow communication's between the communicating entities. LOGIN, password checking, file transfer etc. are some of the functions of the Application Layer.

It is concerned with support of end user. The connection once established provides sequenced delivery of the data units. Parameters associated with this are:

E.g. In telephone system, each service can be characterized by a quality of service. Some services are reliable in the sense that they never lose data. Usually a reliable service is implemented by having the receiver acknowledge the receipt of each message so the sender is sure that it arrived. Acknowledgement process introduces overhead and delays which are often worth it, but arc sometimes undesirable.

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