modems-faq

使用efax的fax工具程序

                      What You Need to Know about Modems


May 31, 1992
Version 1.1
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Copyright (c) 1991-92 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 book/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 to G.

This article should be useful to anyone interested in high-speed modems (or 
2400-bps modems with MNP5 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 current 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 descriptive terms by following the capitalization 
style used by the manufacturer.

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CONTENTS

I.      Introduction

II.     Modulation Protocols

        1.      2400-bps modems

        2.      High-speed modems
                a.      V.32
                b.      V.32bis
                c.      US Robotics HST
                d.      Telebit PEP & Turbo PEP
                e.      Hayes Express 96
                f.      CompuCom CSP
                g.      Other proprietary protocols

        3.      Things to come
                a.      V.fast
                b.      ISDN

III.    Error Control Protocols

        1.      V.42 and MNP4
        2.      V.42 and MNP4 can provide error-free connections
        3.      V.42 and MNP4 can improve throughput
        4.      Are MNP 4 and V.42 useful?

IV.     Data Compression Protocols

        1.      MNP5 and V.42bis
        2.      Are MNP5 and V.42bis useful?
        3.      Compression by software vs. MNP5/V.42bis
        4.      Local flow control and data buffering
        5.      Macintosh and high-speed modems
        6.      PC and UART
        7.      Hayes ESP

V.      About Fax Modems

        1.      Protocols
                a.      Group 3
                b.      V.27ter
                c.      V.29
                d.      V.17
                e.      CAS
                f.      Class 1 & Class 2

        2.      How useful is a fax modem?

VI.     Profiles of High-speed modems

        1.      ATI 9600etc/e
        2.      CompuCom SpeedModems
        3.      Hayes modems
        4.      Intel modems
        5.      Microcom modem
        6.      Practical Peripherals modems
        7.      Prometheus modems
        8.      Supra modem
        9.      Telebit modems
        10.     US Robotics modems
        11.     Zoom Modems
        12.     Things to come

VII.    Buying a High-speed Modem

        1.      Should you pay extra for a V.32bis modem?
        2.      Should you buy a modem with a proprietary modulation protocol?
        3.      Should you buy a 2400-bps modem with V.42bis?
        4.      Beware of the ads

VIII.   Setting Up Software to Work with High-speed Modems

        1.      The proper software setup
        2.      Does your software initialize the modem properly?
        3.      Does your software configure itself to match the modem settings?
        4.      Why you may need to change the initialization string
        5.      Editing the initialization string
        6.      Match software settings to the modem settings
                a.      Speed setting
                b.      Hardware flow control
                c.      Dialing time-out value

IX.     Configuring Popular Communications Software to Work with High-speed 
        Modems

        1.      Procomm 2.0
        2.      Telix
        3.      Qmodem
        4.      HyperAccess 5
        5.      Crosstalk for Windows
        6.      MicroPhone II (for Macintosh)
        7.      White Knight (for Macintosh)
        8.      ZTerm (for Macintosh)

X.      Other Settings for Your Communications Software

        1.      Telephone number
        2.      Dial string:  ATDT
        3.      8-N-1 or 7-E-1 (data bits - parity - stop bits)
        4.      Half vs. full duplex:   local echo
        5.      Terminal emulation
        6.      Communications port
        7.      File transfer protocols
                a.      ASCII
                b.      Xmodem
                c.      Xmodem-1K
                d.      Ymodem
                e.      Ymodem-g
                f.      Zmodem
                g.      Kermit
                h.      Which file transfer protocol should you use?

Appendix A:  Resources
Appendix B:  How to reach the author
Appendix C:  What's "The Joy of Telecomputing"
Appendix D:  What's in "Life beyond CompuServe"
Appendix E:  What's in "Is AT&T the right choice?"
Appendix F:  Updates, bulletins, and tutorial articles
Appendix G:  How to order "The Joy of Telecomputing"

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I.      Introduction

Buying and using a modem used to be relatively easy. Not so long ago, almost 
all modems were 1200- or 2400-bps units and they were all compatible with the 
Hayes Smartmodems (although some were more Hayes-compatible than others). How 
time have 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, MNP5, 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.


II.     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.

II.1.   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 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 outside the U.S.)
        CCITT V.23              (1200/75 and 75/1200 bps, used in Europe)

In the past, most 2400-bps modems did 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.

II.2.   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.

II.2.a. 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.

II.2.b. 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 bps. 
V.32bis is downwardly compatible with V.32.

Unlike 2400-bps modems where a single modulation protocol (V.22bis) is 
supported by all modem makers, there are several proprietary modulation 
protocols used by modems from different manufacturers. Two notable examples 
are the U.S.Robotics HST and the Telebit PEP.

In the past three months, U.S. Robotics, Telebit and Motorola Codex have 
introduced new modems using proprietary protocols that yield better throughput 
than V.32bis modems.

II.2.c. U.S. Robotics HST

Until the recent surge of V.32 modems, the U.S. Robotics HST (High Speed 
Technology) was the de facto standard in the PC-based BBS community. U.S. 
Robotics introduced the Courier HST modem in 1986 and pioneered the market for 
high-speed modems in the IBM PC environment. The immense popularity of the HST 
modems was partly due to the generous discount program U.S. Robotics offered 
to the BBS Sysops (SYStem OPerators). Many modem manufacturers have 
implemented similar Sysop discount programs, but most BBS sysops remain loyal 
to the U.S. Robotics modems.

The original Courier HST modem ran at 9600 bps. U.S. Robotics later (in 1989) 
improved the speed of the Courier HST to 14400 bps. In March 1992, U.S. 
Robotics further enhanced HST to 16800 bps (which yields an effective 
throughput around 2000 cps with compressed files). Don't be surprised to see a