The OSI Model in Action

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Let's say you want to retrieve a spreadsheet file called Budget.xls from a network file server. Here's how the layers would work to accomplish the retrieval.

First, the application layer on your computer detects that you're asking for information from a remote server and formulates a request that Budget.xls should be read from the file server's disk. The presentation layer then encrypts the request, if necessary, and passes the packet to the session layer. The session layer adds information about which application is requesting the information and which service is being requested from the file server. The packet is then handed off to the transport layer.

The transport layer verifies that a reliable connection exists to the file server and then breaks down the accumulated information into blocks. If more than one frame is necessary, sequence numbers are added and the blocks are passed to the network layer.

The network layer adds the network address for both the system that is the source of the request and the file server. This is added to each chunk of information before it is passed to the data link layer.

The data link layer packages the blocks into frames and adds a frame header and the source and destination MAC addresses. The MAC addresses plus the content of the frame are used to create a CRC trailer. The frames are then handed off to the physical layer. The physical layer knows no more about frames than a highway knows about cars. It is just the road by which the data is moved.

At the file server, the data link layer reads the transmitted frame and identifies the MAC address for the destination system as its own MAC address. The data link layer performs a CRC and if all is well, strips off the header and trailer and passes the frame to the network layer. (If the CRC isn't valid, the data link layer discards the frame and requests a replacement from the source computer.)

The network layer on the file server analyzes the information that was added by the network layer on the source computer, notes the network address of the source computer, and records it in a table. The frames are then passed up to the transport layer.

The transport layer reviews the information in the frame that was supplied by the transport layer on the source system. If sequencing was used, it queues the frames until all are received. The sequencing is used to identify any missing frames, and a request for the missing data is sent to the source computer. If all the data is present, the transport layer passes the request to the session layer.

The session layer verifies that the data is from a valid connection and that all security criteria have been met. The data is then passed to the presentation layer. The presentation layer performs any necessary decryption and the data is then passed on to the application layer. The application layer passes the request to the process responsible for access to the file system.

Then, incredibly enough, the process reads the requested file and passes the information back to the application layer and the whole process begins again. That a file request can be processed in less than a day is remarkable—that in reality, it takes only a few seconds, is quite astounding.

Although when this model is broken down into steps, it appears to be complex (and it is), it's also a good way to ensure flexibility and reliability. Because Ethernet confines its functionality to the data link layer, it can work with any number of protocols such as IP and IPX, which operate in the network layer. You should also bear in mind that the OSI model is a model and isn't exactly adhered to anywhere.



Microsoft Windows 2000 Server Administrator's Companion, Vol. 1
Microsoft Windows 2000 Server Administrators Companion (IT-Administrators Companion)
ISBN: 1572318198
EAN: 2147483647
Year: 2000
Pages: 366

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