Introduction to Networking

Unit 1

Lesson 6 - From Applications to the Network

Application-to-Application Communication

Applications use the underlying operating system, such as Windows 98, to carry out the needed tasks of the application. This includes accepting keystrokes from a keyboard and displaying the typed information on a computer screen. If you are using a word processor and want to store a file on a hard drive, such as a local hard drive, the application would rely on the operating system to store the information. The operating system stores the information on the hard drive by communicating with the appropriate hard drive device driver to physically place the word processor's information on the drive.

Perhaps you want to store the information on a hard drive located on the other side of the building, in other words, over the network. What must happen in this case? The following three items must be installed on the local machine by the local operating system to provide for communication across a network:

	NIC and NIC device driver
	Client software
	Communication software

The appropriate accessory card, such as a NIC, must first be installed in the computer, along with the corresponding device driver. If you install a 3Com Ethernet NIC, Ethernet device driver software must also be installed. Client software is also needed to provide an interface between the local operating system and communication software. Some client software provides file and print sharing capabilities for computers, while others provide the capability to connect to shared resources on other computers. You must also have communication software loaded on the machine, such as a TCP/IP protocol stack.

Client software requests are placed inside protocol headers (discussed later) for delivery across a network. The following steps are typical (as illustrated in the Communicating Across a Network Diagram):

1.	The user of the application requests that a file be stored on a drive other than a local drive (a network drive).
2.	Computer software on the client machine (also known as a redirector) determines the file is not destined for a local disk drive, but is destined for a remote disk drive.
3.	The redirector takes the "store file" request from the application and requests the services of the communication software.
4.	The communication software adds the appropriate communication information on the "store file" request.
5.	The request is sent from the main CPU of the computer, across the local bus to the NIC.
6.	The NIC transmits the information across the networking cables to the final destination, such as a file server on the network.

Communicating Across a Network

From the user's perspective, we have the ability to store files at multiple locations, including a hard drive located in our computer, or other hard drives on the network. It is up to the local operating system to make sure the file gets properly stored on the local computer hardware. When we want to store information across a network, the request must be redirected out of the computer via a NIC to the appropriate machine located on the network. Specific details of this operation are the subject of a future lesson.

Activities

See the Activities and Extended Activities section in Unit 1 Lesson 6 in your textbook Introduction to Networking to test what you have learned so far.

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