A Primer on Asynchronous Modem Communication
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
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
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