 Imagine a shopping mall that has a wide, open central area that all the shops open onto. An announcement comes over the PA system informing people that "The mall is now closed; 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. Transport Technologies Chapter 7 please make your way to the exit." Shoppers begin to make their way to the doors, but some wander from store to store, window-shopping along the way, while others take a relatively straight route to the exit. The result is that some shoppers take longer than others to exit the mall because there are different modes. Now consider a mall that has a single, very narrow corridor that is only as wide as a person's shoulders. Now when the announcement comes, everyone heads for the exit, but they must form a single file line and head out in an orderly fashion. If you understand the difference between these two examples, you understand multimode VS. single- mode fiber. The first example represents multimode; the second repre- sents single mode. Multimode Fiber The first of these examples was multimode fiber, which arrived in a vari- ety of different forms. Multimode fiber bears this name because it allows more than a single mode or ray of light to be carried through the fiber simultaneously because of the relatively wide core diameter that char- acterizes the fiber. And while the dispersion that potentially results from this phenomenon can be a problem, there are advantages to the use of multimode fiber. For one thing, it is far easier to couple the relatively wide and forgiving end of a multimode fiber to a light source than that of the much narrower single-mode fiber. It is also significantly less expen- sive to manufacture (and purchase) and relies on LEDs and inexpensive receivers rather than the more expensive laser diodes and ultrasensitive receiver devices. However, advancements in techno