ng scheme. Every system must have a field for the following: Sender (From) Recipient (To) Subject Attachment Message body Signature (optional) Priority Various other miscellaneous fields The point is that the number of defined fields is relatively small, and as long as each mail system knows what the fields are and where they exist in the coding scheme of the canonical standard, it will be able to map its own content to and from X.400 or SMTP. Problem solved. Once the message has been encoded properly as an e-mail message, the Application Layer passes the now slightly larger message down to the Presentation Layer. It does this across a layer-to-layer interface using a simple set of commands called service primitives. The Presentation Layer provides a more general set of services than does the Application Layer, which have to do with the structural form or syntax of the data. These services include code conversion, such as seven- bit ASCII to eight-bit EBCDIC translation; compression, using such ser- vices as PKZIP, British Telecom Lempel-Ziv, the various releases of the Moving Picture Experts Group (MPEG), the Joint Photographic Experts Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Chapter 2 Group (JPEG), and a variety of others; and encryption, including Pretty Good Privacy (PGP), Public Key Cryptography (PKC), and so on. Note that these services can be used on any form of data: Spreadsheets, word processing documents, and rock music can all be compressed and encrypted. Compression is typically used to reduce the number of bits required to represent a file through a complex manipulative mathemat- ical process that identifies redundant information in the image, removes it, and sends the resulting smaller file off to be transmitted or archived. To explain how compression works, let's examine JPEG. JPEG was developed jointly by ISO