Unit 4

Lesson 8 - The Application Layer

   Introduction

The Application Layer, the highest layer in the OSI protocol stack, contains the programs that invoke the services of the network to get useful work done. Some of these applications are written specifically for one network, while others are widely used standard applications. When these applications need to communicate with peers over the network, they can use their own protocols, plus the services of the lower layers.

If the Application Layer contained only custom user programs with their own custom protocols, we would not need to say much about this layer. In fact, in the original OSI model, the Application Layer was not "feature-rich." However, over the years, requirements for services that needed to be provided at this layer, that is, services that did not fit logically into the lower layers, were identified, and a number of Application Layer facilities were specified for OSI.

Therefore, Application Layer programs fall into two classes:

User applications provide standard services directly to the user. Each of these applications has its own standard protocol at the Application Layer level. Chief among these applications is e-mail.

Application services provide services to other applications, but not directly to the user. The purpose of these facilities is to keep application programmers from having to reinvent the wheel when they write network applications. Chief among these services is virtual filestores.

   Objectives

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

1.

Explain the difference between a user application and an application service

2.

Name some of the most common Application Layer programs and describe
what they do

3.

Describe a typical interaction between a Web browser and Web server

Key Point
The Application Layer includes some programs that interact directly with users, and some that provide services to user applications.


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