the technology itself, focusing on the three key components within an optical network: the light emitter, the transport medium, and the receiver. We also discussed the various forms of transmission impair- ment that can occur in optical systems and the steps that have been taken to overcome them. The result of all this is that optical fiber, once heralded as a near- technological miracle because it only lost 99 percent of its signal strength when transmitted over an entire kilometer, has become the standard medium for transmission of high-bandwidth signals over great distances. Optical amplification now serves as an augmentation to traditional regenerated systems, allowing for the elimination of the optical-to- electrical conversion that must take place in copper systems. The result of all this is an extremely efficient transmission system that has the abil- ity to play a role in virtually any network design in existence today. Dense Wavelength Division Multiplexing (DWDM) When high-speed transport systems such as SONET and SDH were first introduced, the bandwidth that they made possible was unheard of. The early systems that operated at OC-3/STM-1 levels (155.52 Mbps) pro- vided volumes of bandwidth that were almost unimaginable. As the tech- nology advanced to higher levels, the market followed Say's Law, creating demand for the ever more available volumes of bandwidth. There were limits, however; today, OC-48/STM-16 (2.5 Gbps) is extremely popular, 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 but OC-192/STM-64 (10 Gbps) represents the practical upper limit of SONET's and SDH's transmission capabilities given the limitations of existing time-division multiplexing technology. The alternative is to sim- ply multiply the channel count-and that's where WDM comes into play. WDM is really nothing 