 these are the principal techniques in use today. The next step is to convert the preform into optical fiber. 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 Transport Technologies Drawing the Fiber To make fiber from a preform, the preform is mounted in a furnace at the top of a tall building called a drawing tower. The bottom of the preform is heated until it has the consistency of taffy, at which time the soft glass is drawn down to form a thin fiber. When it strikes the cooler air outside the furnace, the fiber solidifies. Needless to say, the process is carefully managed to ensure that the thickness of the fiber is precise; microscopes are used to verify the geometry of the fiber. Other stages in the manufacturing process include monitoring processes to check the integrity of the product, a coating process that applies a protective layer, and a take-up stage where the fiber is wound onto reels for later assembly into cables of various types. Optical Fibers There are dozens of different types of fiber. Some of them are holdovers from previous generations of optical technology that are still in use and represented the best efforts of technology available at the time; others represent improvements on the general theme or specialized solutions to specific optical transmission challenges. Generally speaking, there are two major types of fiber: multimode, which is the earliest form of optical fiber and is characterized by a large diameter central core, short distance capability, and low bandwidth; and single mode, which has a narrow core and is capable of greater distance and higher bandwidth. There are varieties of each that will be discussed in detail, later in the book. To understand the reason for and philosophy behind the various forms of fiber, it is first necessary to understand the issues that confront trans- mi