width respectively; they have been in existence since the early 1960s and occupy a central position in the typical network. SONET, the Synchro- nous Optical Network, and SDH, the Synchronous Digital Hierarchy, provide standards-based optical transmission at rates above those pro- vided by the traditional carrier hierarchy. Dense Wavelength Division Multiplexing, or DWDM, is a frequency-division multiplexing technique that allows multiple wavelengths of light to be transmitted across a sin- gle fiber, providing massive bandwidth multiplication across the strand. It will be discussed in detail later in the chapter on optical networking. And DSL extends the useful life of the standard copper wire pair by 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 expanding the bandwidth it is capable of delivering as well as the dis- tance over which that bandwidth can be delivered. OSI Summary We have now discussed the functions carried out at each layer of the OSI Model. Layers six and seven ensure application integrity; layer five ensures security; layer four guarantees the integrity of the transmitted message; layer three ensures network integrity; layer two, data integrity; and layer one, the integrity of the bits themselves. Thus transmission is guaranteed on an end-to-end basis through a series of protocols that are interdependent upon each other and that work closely to ensure integrity at every possible level of the transmission hierarchy. So let's now go back to our e-mail example and walk through the entire process. The Eudora e-mai application running on the PC creates a message at the behest of the human user4 and passes the message to the Application Layer. The Application Layer converts the message into a format that can be universally understood as an e-mail message, in this case X.400. It then adds a header that identif