Unit 3

Lesson 3 - Communicating Across a Network

   Requesting Data from a Drive Across a Network

Assume your computer is connected to the network and your client software is set up and configured. Here is what happens when you tell your database application to display records stored on another computer:

1.

When you command the application to open the records, you select a target drive on a different network computer. This computer could be a file server or peer computer. However, you may only access files on a peer computer if the user of that computer has configured that drive or data to be shared by the rest of the network.

2.

The application asks the OS to retrieve the data. However, this time your computer's redirection software intercepts the request that would normally go straight to the OS.

3.

The redirector recognizes that the request is directed at a remote disk drive. The redirector passes the request to the network communication software.

4.

Each layer of the protocol stack handles a different part of the job of preparing the request to be transmitted. For example, one layer addresses the request to the specific destination computer, while another layer checks to see whether the network cable is ready to accept a new transmission. Each layer adds its own protocol header to the request.

5.

The lowest layer protocol passes the completed request to the NIC. This requires the help of the OS, but here the OS is providing a different service from the original "read data" request.

6.

The completed request is sent from the central processing unit (CPU) of the computer, across the local bus, to the NIC.

7.

The NIC creates a physical signal (electrical, optical, or radio) that represents the binary data in the request. It then transmits that signal onto the transmission medium of the network (cable or radio channel).

8.

The destination computer's NIC recognizes that the signal is addressed to itself. It copies the bits off the network, while other computers ignore the signal. It then passes the request to the local computer's network communication protocol stack.

9.

The request is passed up the destination protocol stack and is decapsulated as it moves from one layer to the next. When the message reaches the top of the stack, the highest layer passes the request to the database server application.

10.

The database server application then works with its local OS to read the data from its hard drive.

When the database server application transmits the data back to the client that requested it, this process is reversed. The data is passed down and encapsulated through the server's network protocol stack, transmitted over the physical network, received by the destination computer, decapsulated by the destination protocol stack, and delivered to the client application.

   Activities

See the Activities and Extended Activities section in Unit 3 Lesson 3 in your textbook Introduction to Networking to test what you have learned so far.

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