d become, let's assume that communications carrier operating on a OC-12 ring had a significant number of requests from businesses to provide LAN-to-LAN connectivity Fiber in the WAN over the ring. In this situation the OC-12 ring, which operates at approxi- mately 622 Mbits/s, could support twelve 10-Mbit/s connections, in effect wasting 80 percent of the optical bandwidth. If we consider emerging broadband service, such as the different fla- vors of digital subscriber lines (DSLs) and Cable Modem as well as the need of businesses to interconnect geographically separated locations and provide access to Web services, we can also note a growing problem. The frame structure and static mapping of SONET and SDH is ineffi- cient for the transport of certain types of high-speed data services that cannot be easily multiplexed onto T1, T3, E1, and E3 circuits. Thus, the demand for bandwidth has increased not only because of an increase in data services but, in addition, because of the poor fit of certain types of data services into SONET and SDH frame structures. This growth in bandwidth demand is being satisfied in two ways. The most common method is the primary focus of this section, which is an increase in the capacity of fiber through wavelength division multiplexing (WDM) and dense WDM (DWDM). A second method, which is the focus of the end of this section, is the use of an alternative protocol stack. Earlier in this chapter we briefly examined the rationale for WDM and three of the key technologies that made DWDM possible. In this section we expand our knowledge of both areas and familiarize ourselves with some specifics concerning WDM and its evolution into dense WDM. RATIONALE The growth in the use of the Internet and graphical images by both commercial and residential telephone subscribers result- ed in the demand behind the development of modern WDM systems. While just about every person on the planet is aware of the growth in the use of the Internet, the effect of graphical images on the infrastruc- ture of comm