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Summary
In this unit, we learned one of the most basic principles of data communication: that computers use multiple processes and protocols to transmit information across a network. These protocols work together like an assembly line, forming a "stack" of logical layers. Each layer uses the services of the layer below, and provides a service to the layer above. Upper layer protocols direct data from one application to another, while lower protocols direct data to a particular destination machine. The lowest protocols are concerned only with transmitting and receiving signals on the network's physical transmission medium. On a logical layer above the network protocol stack, a client application called a "redirector" plays a vital role. When client applications request resources, such as data or storage services, the redirector intercepts these requests that would otherwise go straight to the OS. The redirector then determines whether each requested resource is available on the local computer, or on a computer across the network. If the resource is local, the redirector passes the request to the local OS. If the resource is remote, the redirector passes the request to the network protocol stack for transmission across the network. 
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