发一本教材A Primer on Asynchronous Modem Communication

西楼昨夜风

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.

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