A Primer on Asynchronous Modem Communication

A Primer on Asynchronous Modem Communication - A Primer on...

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A Primer on Asynchronous Modem Communication by Winfield Stanton and Thomas Spencer Last Updated: May 1998 This paper covers the basic RS-232 signaling, error correction, compression, and modulation standards. In the near future, the asynchronous modem will continue to be an important means of transferring information to and from the personal computer. By some estimates, more than 60 percent of people who access the Internet do so with a conventional dial-up modem and a connection to an analog telephone line (www.boardwatch.com). You may be connecting to a bulletin board system (BBS) through a piece of communications software or you may be using the Windows NT® Remote Access Service (RAS) to dial out to your Internet Service Provider (ISP). Whether you use a simple file transfer protocol such as XMODEM or another protocol such as TCP/IP over the RAS Point-to-Point (PPP) protocol, the underlying asynchronous communications issues are the same. Most asynchronous modem communication implemented today conforms to the RS-232 signaling standard (RS stands for recommended standard), and the most important standard layered onto the RS- 232 signaling is the modulation standard. Other standards such as error correction and data compression also come into play. Asynchronous communication is a method of serial data transfer that is common to a number of protocols, such as XMODEM, YMODEM, and Kermit. For the purpose of this paper, the set of rules common to asynchronous communication will occasionally be referred to as the asynchronous protocol . One of the main things to learn about the asynchronous protocol is how the modem and the computer use the different leads in the serial cable to communicate according to the RS-232 standard. 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 port, and application software
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A Primer on Asynchronous Modem Communication - A Primer on...

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