4.0 Chapter Introduction

Data networks and the Internet support the human network by supplying seamless, reliable communication between people – both locally and around the globe. On a single device, people can use multiple services such as e-mail, the web, and instant messaging to send messages or retrieve information. Applications such as e-mail clients, web browsers, and instant messaging clients allow people to use computers and networks to send messages and find information.

Data from each of these applications is packaged, transported, and delivered to the appropriate server daemon or application on the destination device. The processes described in the OSI Transport layer accept data from the Application layer and prepare it for addressing at the Network layer. The Transport layer is responsible for the overall end-to-end transfer of application data.

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3.0 Chapter Introduction

3.0.1 Chapter Introduction

Most of us experience the Internet through the World Wide Web, e-mail services, and file-sharing programs. These applications, and many others, provide the human interface to the underlying network, enabling us to send and receive information with relative ease. Typically the applications that we use are intuitive, meaning we can access and use them without knowing how they work. However, for network professionals, it is important to know how an application is able to format, transmit and interpret messages that are sent and received across the network.

Visualizing the mechanisms that enable communication across the network is made easier if we use the layered framework of the Open System Interconnection (OSI) model. In this chapter, we will focus on the role of one layer, the Application layer and its components: applications, services, and protocols. We will explore how these three elements make the robust communication across the information network possible.

In this chapter, you will learn to:

• Describe how the functions of the three upper OSI model layers provide network services to end user applications.
• Describe how the TCP/IP Application Layer protocols provide the services specified by the upper layers of the OSI model.
• Define how people use the Application Layer to communicate across the information network.
• Describe the function of well-known TCP/IP applications, such as the World Wide Web and email, and their related services (HTTP, DNS, SMB, DHCP, SMTP/POP, and Telnet).
• Describe file-sharing processes that use peer-to-peer applications and the Gnutella protocol.
• Explain how protocols ensure services running on one kind of device can send to and receive data from many different network devices.
• Use network analysis tools to examine and explain how common user applications work.

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2.0 Chapter Introduction

2.0.1 Chapter Introduction

More and more, it is networks that connect us. People communicate online from everywhere. Efficient, dependable technology enables networks to be available whenever and wherever we need them. As our human network continues to expand, the platform that connects and supports it must also grow.

Rather than developing unique and separate systems for the delivery of each new service, the network industry as a whole has developed the means to both analyze the existing platform and enhance it incrementally. This ensures that existing communications are maintained while new services are introduced that are both cost effective and technologically sound.

In this course, we focus on these aspects of the information network:

• Devices that make up the network
• Media that connect the devices
• Messages that are carried across the network
• Rules and processes that govern network communications
• Tools and commands for constructing and maintaining networks

Central to the study of networks is the use of generally-accepted models that describe network functions. These models provide a framework for understanding current networks and for facilitating the development of new technologies to support future communications needs.

Within this course, we use these models, as well as tools designed to analyze and simulate network functionality. Two of the tools that will enable you to build and interact with simulated networks are Packet Tracer 4.1 software and Wireshark network protocol analyzer.

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