A Primer on Asynchronous Modem Communication
It is necessary to first clarify some of the basic terms important to this topic. For example, what are DTE and DCE? What are the differences between the baud rate and the speed of data transfer measured in bits per second (bps)? What does a modem actually do, and where did it get its name? And finally, what exactly is meant by a modulation standard, and what are the key elements that make up such a standard? These are the questions that will be addressed in the following sections. Most discussions of networking protocols are framed against the protocol layers defined by the Open Systems Interconnect (OSI) model. Although a detailed discussion of that model is beyond the scope of this paper, it is important to know that the asynchronous protocol consists of just the application, data link, and physical layers of the OSI model. Also, the functions of the data link and physical layer overlap, because they are performing functions that are supposed to be performed by the other layer according to the OSI model. For example, the data-link layer works by changing electrical voltage levels of circuits (wires and pins) in the physical layer, which is a function of the physical layer in the OSI model. Data terminal equipment (DTE) is the combination of a computer, a serial
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