Page 207 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks Ka Profess

Notes Unit 13: Session Layer and Presentation Layer

CONTENTS
Objectives
Introduction

13.1 Session Layer – Design Issues
13.2 Session Layer – Synchronization
13.3 Presentation Layer

13.4 Presentation Layer – The Design Issues
13.5 Summary
13.6 Keywords
13.7 Review Questions
13.8 Further Readings

Objectives

After studying this unit, you will be able to:
Explain the concepts of session layer and related design issues

Conceptualize the synchronization points as an effective mechanism for handling flow
control and errors
Describe the services offered by the presentation layer and related design issues

Introduction

The upper three layers namely session, presentation and application layers are considered as
user or application layers of the OSI models. They are implemented in software. In most of the
protocols, the functions of these layers are converged into a single layer called the application
layer. TCP is one of the examples of such types of protocols.
The session layer is located above the transport layer and intends to provide value added
services to the underlying transport layer services. The session layer creates, manages and
releases communication sessions between presentation layer entities at source and destination
machines. Session layer’s protocols manage the communication sessions including creating a
communication session service requests and service responses, security and authentication.
They take place between applications located at source and destination machines in network.
Protocol Data Unit (PDU) is the data at this layer. This layer responds to service requests from
the presentation layer and issues service requests to the transport layer.
The presentation layer preserves and maintains the meaning of information transmitted across
a network. It encodes the data in various ways, for example, data compression or encryption.
Similarly, the receiving machine will convert the encoding back into its original form. The
application layer, the highest layer of OSI model interacts with software applications which
enable source and destination machines to communicate properly.

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