FTP传输的两种模式：binary vs ascii

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What is ASCII?

ASCII: the American Standard Code for Information Interchange. It's the code by which computers have stored and exchanged text for the last 40 or more years. 　　ASCII has been the mainstay of information interchange for four decades, but it has limitations. Designed during a time when computer memory (RAM) was very expensive, it used only the low 7 of the 8 bits in a standard byte. This was adequate, and allowed the leftover bit to be used (occasionally) for other purposes.

What is Binary?

http://courses.wccnet.edu/computer/mod/na36c_f01.gifBinary is the number system based on 2. In this context, though, it refers to numeric, as opposed to text, information. While ASCII text uses only 7 of the 8 bits in a byte, numbers use all 8. 　　Not only do numbers themselves use all 8 bits, but so do most other types of computerized information (all of which use numbers to represent information), including: programs images databases music video word processing documents compressed files 　　It comes as a surprise to many people that word processing documents, such as Microsoft Word and Word Perfect files, are binary rather than ASCII. That's because they store a great deal of information besides just the letters: they contain font and formatting information, images, user preferences, and many other "bells and whistles".

　　
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ASCII and Binary in FTP
Why Does FTP Have Both?
　　FTP was developed at a time when typical modem speeds were 110 to 300 bits per second (as compared with 28,000 to 56,000 today). Since ASCII only used 7 bits, long files could be transmitted more quickly by not sending all the unused bits.
　　The big drawback is that if a file that uses all 8 bits in each byte is accidentally sent using ASCII transfer, it will lose 1/8 of its information content. In most files, even the loss of one bit is enough to make it invalid, and losing 1/8 makes them totally unreadable. So ASCII transfer can be fatal to a file's health!
　　With today's higher speeds, the time lost by sending all 8 bits of an ASCII file is practically unnoticeable. But FPT has incorporated features into ASCII transfer that make it useful for other reasons, so the two modes remain.
　　The fact that they both remain means that FTP users need to exercise great caution in transferring files!
What Extra Services Does ASCII Transfer Have?
　　What features are so useful that FTP retains ASCII transfer in spite of the risk of ruining files in transmission? There are two: end-of-line translation, and end-of-file translation.
　　Although ASCII has standardized almost all aspects of text files shared between computers, there are two areas where operating systems have "agreed to disagree". One is how you symbolize the end of a line, the other is how you mark the end of a file.
　　When files are transferred using ASCII mode, FTP clients and servers will translate the different symbols into the native "language" of the receiving computer. This eliminates files that appear to have "garbage" in them, or that appear to be all one long line stretching from here to the horizon. There are some programs used in making the Web interactive, that depend on having the right line-break characters.
　　In case you're interested or need to know the details, here they are:
 ASCII character numbers
followed by
[Character Names]SystemEnd-of-LineEnd-of-FileIBM, DOS, Windows, VAX-VMS13+10
[CR]+[LF]26
[^Z] Linux, Unix10
[LF]04
[^D] Macintosh OS 1-9 13
[CR]04
[^D]Macintosh OS X Flexible: either
[LF] or [CR] 04
[^D]　　
　　
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How to Set Transfer Mode
Setting Transfer Mode in Character-Based FTP Clients
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　　If you use a command-line FTP client: The default mode is ASCII. Every time you intend to transfer a binary file, you need to type the command,
binary
before typing in the getor putcommand.
　　FTP will stay in binary mode until you change it, so if you then transfer ASCII files, you need to type the command
ascii
before typing the getor putcommand.

How To Set Transfer Mode in a GUI-based FTP Client 　　http://courses.wccnet.edu/computer/mod/na36c_f03.gif 　　http://courses.wccnet.edu/computer/mod/na36c_f04.gif	GUI-based FTP clients are each different, but since we use WS-FTP at WCC, we'll focus on that. 　　WS-FTP has three controls for mode: ASCII, Binary, and Auto. Auto is a check box, and does not affect the status of the other two, while ASCII and Binary are "radio buttons", meaning you have to choose one or the other. Here's how they work: If Auto is OFF(unchecked), and: ASCII is ON: All transfers will be in ASCII mode. Binary is ON: All transfers will be in binary mode. IfAutomatic is ON(checked), files are transferred in binary mode unless the extension is listed in a table set up in WS-FTP. Details follow...
"Automatic" Mode http://courses.wccnet.edu/computer/mod/na36c_f05.gif	The behavior of WS-FTP in Auto mode depends on a list of file extensions it keeps.You can access that list by clicking the Options button at the bottom of the WS-FTP LE window (or the top of the WS-FTP Pro window).
http://courses.wccnet.edu/computer/mod/na36c_f06.gif	In the Options dialog, click the Extensions button. This brings you to the list of extensions. The illustration at left shows the default list of ASCII extensions - only .TXT. (This is notcase-sensitive.) 　　If you're transferring Web files, HTML files could also be in this list.In addition, there are a number of other file type used on the Web that are text files and should be transferred using ASCII mode. These are listed in the table in the next section.

　　
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Summary: Practical Advice
WS-FTP Help offers this advice:

• A file that is text, HTML, postscript, other ASCII format (.txt, .html, .htm, .cgi), or EBCDIC should be transferred in ASCII mode. Note: Transferring a binary file in ASCII format can damage the file.
  
• A file that is a bitmap, an executable, or compressed (.bmp, .jpg, .exe, .zip) should be transferred in binary mode. (Binary mode should also be used if there is no file extension or the file extension is not registered.)
  

　　Here's a quick overview of files and how they should be transferred:
ASCIIBinary.txtASCII text.txtUnicode text - see notebelow.htm
.html HTML
Web page.jpg
.jpeg JPEG image.xmlExtensible Markup Language.gifCompuServe GIF image.cgiCommon Gateway Interface script.pngPortable Network Graphics.plPerl scrips must be transferred in ASCII.bmpWindows bitmap image.phpPHP program script.docMS Word or Word Perfect document.cfCold Fusion script.xlsMS Excel spreadsheet.svgScalable Vector Graphics image.mdbMicrosoft database file.aspMS Active Server Pages script.exe
.com
.dll Executable programs.rtfRich Text Format.zipCompressed files.psPostscript files.pdfAdobe Acrobat Portable Document Format filesNote about Unicode
　　Unicode is the system which ultimately will replace ASCII, because it has the potential to include all the characters of all human languages. It can do this because instead of 7 bits it can use 8, 16, or 32. It is still evolving, but more and more programs are coming to use it.
　　Since it uses all the bits in each byte, it must be transferred in binary mode. Unfortunately, the extension .txt is routinely used for Unicode text files, making it impossible to tell from the extension whether to transfer it in ASCII or binary.
　　
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 About this document...
http://courses.wccnet.edu/computer/mod/review.gifClick herefor a review of this module. Audience:
This is for people who are familiar with the basics of FTP and need to distinguish between files appropriate for binary transfer, and those appropriate for ASCII transfer. Objectives:
　　When you successfully complete this lesson, you will be able to...

• Explain what is meant by "binary" files
  
• Explain what is meant by "ASCII" files
  
• Identify ASCII and Binary files by their extensions
  
• Discuss the reasons for maintaining a distinction between ASCII and binary transfer
  
• Recognize the commands for switching between ASCII and binary transfer in command-line FTP clients
  
• Recognize the commands for switching between ASCII and binary transfer a GUI FTP client such as WS-FTP
  
• Discuss the uses and proper setup of "automatic" mode in WS-FTP
  

Module NA36c:
This document is part of a modular instruction series in Computer Instruction. For more information, see the overviewor the list of modules in this series, NA: Network Applications. This document has been used in the following classes: INP 150, INP 152, INP 220. Author:
Laurence J. Krieg Institution:
Department of Computer Information Systems, Washtenaw Community College
http://courses.wccnet.edu/computer/mod/wcclogoc.gifHistory:Original: 16 Oct 2002
This version posted  Thursday, 01-Jun-2006 09:37:40 EDTCopyright:Copyright © 2002, Laurence J. Krieg, Washtenaw Community College
Instructors: You may point to this file in your Web-based materials.
Students: you may make a copy for your personal use.