Multimedia Mail

Multimedia mail

Electronic mail is a speedy way to transmit a message across a computer network. Messages which simply contain text may be passed from one computer system to another without much difficulty but when a message is to contain an image or sound or video clip than this attachment must be encoded in one way or another.


At one time computer screens were able to display only a very limited range of characters and symbols, roughly those on a typewriter keyboard. When connected to a network in order to be able to send and receive electronic mail messages the same limitations would apply; the messages could only contain text. Now computer screens are much more sophisticated and are able to display images and simultaneously electronic mail has obtained a new importance because of increased availability of access. These developments have combined to lead to increased sophistication in electronic mail use and messages which contain image or sound or video files as attachments are now common.

In order to send an attachment of this kind, the sender must encode it before transmission and the recipient will decode it upon receipt. The purpose of the encoding and decoding is not to keep the contents of the message secret [it is not encryption] rather the encoding is essential in order to allow the message to be transmitted at all. This method of transmission is analogous to the way in which a digital signal is sent over an analogue transmission line by having the sender modulate the signal and having the receiver de-modulate it.

On the Internet, multimedia mail is sent and received using MIME standards, one of which is Base64 encoding. According to the MIME Frequently Asked Questions list,

MIME, the Multi-purpose Internet Mail Extensions, is a freely available specification that offers a way to interchange text in languages with different character sets, and multi-media e-mail among many different computer systems that use Internet mail standards.

The Base64 encoding scheme samples a message to be sent, extracting groups of twenty-four input bits at a time. These are considered as four groups of six bits and the six bits can be represented as shown below; there are sixty-four [two to the power six] possible combinations of six bits.

               Values  Encoding
                0-25    [A-Z]
               26-51    [a-z]
               52-61    [0-9]
                  62    +
                  63    /

Thus the groups of six bits are represented using only the uppercase letters, the lowercase letters, the digits and the symbols plus, division sign and the equals sign [used as a padding character]. These characters can be easily transmitted anywhere on the Internet. The characters are made into lines of sixty-four characters long and the message is sent.

For a user of a modern mail program, the conversion to and from Base64 is completely automatic but of course, it can be laborious to be asked to learn how to use a new program every time that a development of this kind takes place and so some electronic mail users prefer to use an old-fashioned mail program which they are familiar with [but which does not know about MIME encoding] together with a simple Base64 encoder/decoder such as Mpack, available for Unix, DOS, Apple Macintosh and other systems.

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