The Session Layer

   Introduction

In this lesson, we begin to discuss the top three layers of the OSI model. Important differences exist between the four lower layers we have studied so far, and the remaining three layers.

As we have seen, the job of the lower layers is to provide reliable communication of data across a network. The job of the upper layers, taken collectively, is to provide user-oriented services through a set of widely available standard applications, and specialized applications written for the users by programmers. The Session Layer, and the Presentation Layer above it, provide reusable services for the applications that reside in the Application Layer.

The Session Layer is so named because it facilitates a step-by-step interaction or session between two entities. For example, an interactive user session begins with a user logging on to the computer, and ends with the user logging off.

   Objectives

At the end of this lesson you should be able to:

1.

Describe the services of the Session Layer

2.

Explain the relationship between sessions and connections

3.

Describe the process of token passing

Key Point
The Session Layer is where a conversation starts and stops, and the rules of the conversation are agreed upon.