The application software is the boss

Network designers divide software into layers where specific functions are performed. The layers form a stack in which programs use the services of the layer just below it, but ignore everything else. This is like the boss telling a subordinate to do something without spelling out how to get it done:

Send this package to Miami -- I don't care if you use Federal Express, UPS, or any other means. Also, let me know when it arrives or if it cannot be delivered.

We have discussed the application layer extensively, but have not looked at the functions of the lower layers. We have just assumed they somehow allowed for communication between the application program running on the client and the application program running on the server:

LayerFunction
Application Do useful work like Web browsing, email, and file transfer
Lower layers Handle communication between the client and server

Like the boss with a package to send, the application program tells the lower layer software something like:

Send this request to the server -- I don't care how you do it or whether it goes over phone lines, radio, or anything else about the details. Just send the message, and let me know if it cannot be deliverd for some reason.

It turns out there are several layers below the application layer: transport, network, data link, and physical. An application program uses the transport layer to send and receive messages. The first step for a message is usually within a LAN, and communication within the LAN is handled by the data link layer. When messages leave the LAN, network layer software routes them across the internetwork to the receiving LAN.

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