From: "kkirksey@eng.auburn.edu"@ducvax.auburn.edu Subject: What You Need To Know About Modems Date: Thu Jan 23 17:57:03 CST 1992 Organization: Auburn University Engineering Lines: 565 Status: RO This file was posted on America Online this week. I've found it to be the best source for information about modems, protocols, transfer compression, etc. that I have yet seen. This file is one chapter from a three chapter book entitled "The Joy of Telecomputing" Ordering information is at the end of the file. I hope ya'll find this as informative as I did. ----------------------------------------------------------------------------- What You Need To Know About Modems December 25, 1991 Version 1.0 ------------------------------------------------------------------------------ Copyright (c) 1991 Patrick Chen. All rights reserved. Distribution Notice: This document may be distributed by electronic bulletin boards and commercial on-line services. This document may not be edited or changed in any way for redistribution. This article is Part One of a three-part report entitled "The Joy of Telecomputing." As a plain ASCII file, it cannot contain any of the illustrations and graphic elements provided in the printed version. For further information about "The Joy of Telecomputing," see Appendix C. This article should be useful to anyone interested in high-speed modems (or 2400 bps modems with MNP-5 or V.42bis). Most modems mentioned in this article are external units. These modems can be used on any microcomputer system, be it a PC, a Macintosh or an Amiga. Although only certain communication programs are used as examples, the discussions about setting up software apply to packages not covered herein. It is assumed that the reader knows the basics about going online. For example, we would not discuss how to connect an external modem to your computer, nor would we explain what 8-N-1 means. This file is in the IBM ASCII format. Each line ends with a linefeed and a carriage return. If you use a Macintosh, open the file with a text editor and get rid of the linefeeds. (I recommend McSink, a shareware text editor widely available. Just launch McSink and open the file, then choose the Convert menu and select Strip Linefeeds.) To print this document, use a 10-pitch (12 characters per inch) mono-spaced font such as Courier. Every effort has been made to supply complete and accurate information. However, information contained herein is subject to change without notice and should not be construed as a commitment by the author who assumes no responsibilities for any errors that may appear. Trademarks: The author has attempted throughout this document to distinguish proprietary trademarks from discriptive terms by following the capitalization style used by the manufacturer. ----------------------------------------------------------------------------- CONTENTS Introduction Modulation Protocols 2400 bps modems High-speed modems V.32 V.32bis U.S. Robotics HST Telebit PEP Hayes Express 96 CompuCom CSP Things to come V.fast ISDN Error Control Protocols V.42 and MNP-4 V.42 & MNP-4 can provide error-free connections V.42 and MNP-4 can improve throughput Are MNP 4/V.42 useful? Data Compression Protocols MNP-5 & V.42bis Are MNP-5 & V.42bis useful? Compression by Software vs. MNP-5/V.42bis Local Flow Control and Data Buffering Macintosh and high-speed modems PC and UART Profiles of High-speed modems ATI 9600etc/e CompuCom SpeedModem Champ/Star/Storm Hayes modems Image Communications: Twincom 96/42 Intel 9600EX & 14.4EX Practical peripherals PM9600SA & PM9600 Prometheus modems Telebit modems U.S. Robotics modems Zoom V.32 Turbo Modems Things to come Buying a High-speed Modem Should you pay the extra for a V.32bis modem? Should you buy a modem with a proprietary modulation protocol? Should you buy a 2400 bps modem with V.42bis? Beware of the ads Setting Up Software to Work with High-speed Modems The Proper Software Setup Does Your Software Initialize the Modem Properly? Does Your Software Configure Itself to Match the Modem Settings? Why You May Need to Change the Initialization String Editing the Initialization String Match Software Settings To the Modem Settings Configuring Popular Communications Software to Work with High-speed Modems Procomm 2.0 Telix Qmodem HyperAccess 5 Crosstalk for Windows MicroPhone II (for Macintosh) ZTerm (for Macintosh) Other Settings for Your Communications Software Telephone Number Dial String: ATDT 8-N-1 or 7-E-1 (data bits-parity-stop bits) Half vs. Full Duplex: Local Echo Terminal Emulation Comm Port File Transfer Protocols ASCII Xmodem Xmodem-1K Ymodem Ymodem-g Zmodem Kermit Sealink Which file transfer protocol should you use? Appendix A: Resources Appendix B: How to reach the author Appendix C: About "The Joy of Telecomputing" ------------------------------------------------------------------------- Introduction Buying and using a modem used to be relatively easy. Not so long ago, almost all modems are 1200 or 2400 bps units and they are all compatible with the Hayes Smartmodems (although some are more Hayes-compatible than others). How time has changed. Today, modems not only run faster, they are also loaded with features like error control and data compression. Suddenly, you are confronted with all the buzzwords: V.32, V.32bis, V.42, V.42bis, MNP-5, LAP-M, etc. What do they mean? And what do they mean to you? To make the most of a high-speed modem, you need to understand three different kinds of protocols and the relationships among them. They are the modulation protocols, error control protocols and data compression protocols. Modulation Protocols Modem stands for MOdulator/DEModulator. A modem converts digital signals generated by the computer into analog signals which can be transmitted over a telephone line and transforms incoming analog signals into their digital equivalents. The specific techniques used to encode the digital bits into analog signals are called modulation protocols. The various modulation protocols define the exact methods of encoding and the data transfer speed. In fact, you cannot have a modem without modulation protocols. A modem typically supports more than one modulation protocols. The raw speed (the speed without data compression) of a modem is determined by the modulation protocols. High-speed modems are modems that feature modulation protocols at 9600 bps or higher. A 2400 bps modem with data compression that can theoretically yield a 9600 bps throughput is not a high-speed modem. "CCITT" is a French acronym for the International Telegraph and Telephone Consultative Committee. CCITT, a United Nations agency, is an international telecommunications standards committee that makes recommendations on a broad range of subjects concerning data communications. 2400 bps Modems A 2400 bps Hayes-compatible modem typically supports the following modulation protocols: Bell 103 (300 bps U.S. Standard) Bell 212A (1200 bps U.S. Standard) CCITT V.22 (1200 bps standard used outside the U.S.) CCITT V.22bis (2400 bps International Standard) Some 2400 bps modems also support the following protocols: CCITT V.21 (300 bps standard used outside the U.S.) CCITT V.23 (1200/75 and 75/1200 bps, used in Europe) In the past, most 2400 bps modems do not support any error correction or data compression protocols. Recently, however, many modem manufacturers have introduced 2400 bps modems with extra features like data compression, error correction and fax capability. High-speed Modems There are two standard modulation protocols for high-speed modems: V.32 and V.32bis. Both are standards established by the CCITT. V.32 This is the standard for 9600 (and 4800) bps modems. CCITT V.32 is adopted by the CCITT in 1984. But the market has not taken off until recently. V.32 modems used to cost more than modems using proprietary modulation protocols (Hayes introduced the Smartmodem 9600, a V.32 modem, in 1988 with a $1999 price tag). But it is no longer true. At present, street prices for most V.32 modem are below $500. Every modem manufacturer is making V.32 modems now. Packet-switching networks like Sprintnet (Telenet) and CompuServe are also starting to support V.32 modems. Companies that make modems with proprietary modulation protocols are making modems with "dual standard." U.S. Robotics, Telebit, Hayes and CompuCom all have modems that support V.32 and their own proprietary protocols. V.32bis V.32bis, established in early 1991, is the CCITT standard for 14400 bps modems. A V.32bis modem also can fall back to 12000, 9600, 7200 and 4800 b...
Amiga7878