Computer Terms

Computer Terms Exchanging files The networks provide a superb transport medium for sending files to other people. They offer fast and efficient transmission over any distance, a choice of systems catering for different circumstances, and a range of application software to facilitate the transmission or distribution of files. Application software may also automatically take care of any associated processing, for instance converting binary files to text format for email transmission and converting them back to binary. When sending files, select a method and a format which suits your recipient as well as yourself. A cornerstone of Internet file exchange is the MIME (Multipurpose Internet Mail Extensions) standard. By providing a common method of content description, MIME serves as a foundation for negotiating the exchange of files on the Web and on other Internet client-server systems. The content description indicates to clients such as Web browsers how the file should be displayed. MIME also handles international character sets.Email Using modern email packages, sending files by email is usually just a matter of selecting the menu option to attach a document to a mail message. The mailer will probably take care of the process of converting the document to plain text (ASCII) format, a necessary preliminary to sending documents via Internet email because not all mail servers through which the mail might go can handle binary formats. So, word-processed documents, spreadsheets, images and other binary files, need to be converted to plain text (ASCII) before sending. There are a number of ways in which this may be achieved: MIME originated because of a need to send non-ASCII documents through Internet email which, at the time, could not handle binary files. MIME is incorporated into much email software. With a MIME-compliant mailer, information is added to the header of message specifying the type of content included in the message and also specifying how the content is encoded. Various types of content can be specified, including binary data, images, audio and video files. For example, if message is a text file in Croatian language using ISO-8859-2 character set, the mailer inserts a line in the header of the message:Content-type: text/plain; char set=iso-8859-2 where iso-8859-2 is a registered MIME name for a character set. Content-Type Defaults are Content-type: text/plain; char set=us-asciiIf a GIF file is included, the mailer inserts a line like this in the header of the message: Content-Type: Image/Gif MIME usually uses Base64 for encoding binary files. This may be indicated in the header with a line like this: Content-Transfer-Encoding: base64 Quoted Printable is another encoding technique most appropriate for ASCII files with a small section of non-ASCII characters. If the receiving mailer is MIME compliant, it will correctly interpret the lines in the header and de code and display or save the file appropriately.Uuencode is a method of encoding binary files to plain text which originated on Unix systems, though software for other platforms is also available. Uuencode (and Uudecode) is incorporated into a number of mail packages. Exchanges between different systems using uuencode may not be entirely reliable as it has not been standardized. BinHex originated with Apple Macs and is primarily and seamlessly handled by many Mac applications, and also many PC applications such as Microsoft Internet Explorer and Netscape Navigator. Dedicated conversion software is available for a number of platforms. When a message with an attached encoded document is received, the recipients email program steps in, taking care of decoding, saving, and so on. Commonly, the message will include an icon representing the file. From there the user can select and display the file with a simple mouse click. If the email program does not have the required decoding facility it will probably save the file to some default directory leaving the user to locate and open the appropriate decoding application. This is cumbersome at the best of times, but doubly so if this is all an unfamiliar process. If you send files to people you should first establish that their mail program can handle the format you intend to use. A simple question like “Is your mail program MIME compliant?” can avoid headaches. The same considerations apply to files which are compressed as well as encoded. Some mail systems can handle large transmissions but it is not uncommon for large files or messages (e.g. above 500K) sent by email to be broken down into smaller chunks. While sophisticated mail programs may handle reconstituting these adequately, a safer option for large files is to reduce the size by first compressing them, or to make them available through ftp rather than email. Transmission of virusesExchanging files by email is one of the great conveniences of the Internet. However it is not without its down side, specifically, the transmission of viruses, which may reside in seemingly innocuous sources such as word-processed files. For instance, Word macro viruses can be transmitted by innocent parties exchanging files by email. The Melissa virus extends the insidious effects of the macro virus genre by propagating itself through further multiple email transmissions of an infected document, using the Microsoft Outlook address book to generate a list of recipients. Macro viruses will reproduce only if the automatic execution of macros is enabled in the targeted application, so the precaution is to disable macro execution by default. FTP If you have access to an online ftp archive where you can place files, ftp is a good option for large numbers of files or for very large files. They can be retrieved from the archive by the recipient using ftp software (or via a Web browser) but note that ftp requires a login and password. The easiest way to get around this restriction is to place the files in an anonymous ftp archive from where they can be retrieved with the login anonymous and with the users email address instead of a password. WWW If you want to distribute a document to a number of people to read, converting it to HTML and putting it on a Web page may be an appropriate option. Your correspondents need nothing more than their Web browsers to access it. This solution avoids problems associated with access from different computing platforms; sidesteps the issue of limitations of mailer