Overview

Starting in 1988 up through 1996, Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) commanded all the investments from manufacturers and carriers alike. Both were touted as the backbone standards for future telecommunications. Therefore, there was no lack of manufacturers, research and development, or implementation schedules. As always, networks mature. Standards undergo changes, and many fall by the side of the road. SONET and SDH are no different. Recently, one manufacturer made a comment that SONET was dead.

In 1997, wave-division multiplexing (WDM) began its stellar rise in popularity. Manufacturers and technical wizards saw the benefit of using multiple wavelengths to dramatically increase the capacity of existing fibers without the problems created by time-division multiplexing (TDM) systems at these speeds. Both SONET and SDH standards were designed to carry TDM digital signals in the middle to late 1980s. Using TDM, a higher-speed signal is created by multiplexing many lower-speed channels. Problems surface when we plan to upgrade to OC-768 and above because TDM has trouble operating at these speeds. TDM on SONET or SDH still needs electrical signals and electronic switching systems. Conversion of these TDM signals may be too expensive at these higher speeds.

On the other hand, WDM carries multiple data speeds on a single fiber. Dense WDM (DWDM) is a fiberoptic transmission technique that employs light wavelengths to transmit data parallel-by-bit or serial-by-character. The all-optical networks using WDM with add-drop multiplexers and cross-connects permit this. DWDM systems multiplex up to...