tire transport, switching and routing occurs in optical mode. With the inclusion of new 3R (reshape, retime, retransmit) devices, internal to the DWDM system, circuits utilizing only DWDM equipment are now able to be built that can span the country. New performance monitoring capabilities have been built into these devices SO that maintenance and service of the links is possible. With DWDM as the transmission method, the bandwidth of the existing fiber plant is maximized. In designing a hybrid SONET and DWDM network (Figure 7.11), the most common architecture is SONET/SDH over DWDM, where the underlying or core transport is DWDM while the SONET routing is achieved by the overlaid or integrated MSPPs (defined later). The need of SONET granularity is due to the existence of low-speed service demands such as DS1/DS3 and the lack of sub-wavelength grooming in the DWDM platforms. DWDM has a goal to carry the same traffic in as few topological structures (e.g., rings) as possible. DWDM does this at the cost of allowing some traffic to traverse longer distances than it does in SONET. For a simple example, the best SONET design may generate a bunch of topologically different OC-48 and OC-192 rings, while the most economic DWDM transport network may consist of just a single DWDM ring. Therefore, there are two options in the topological optimization, both of which need to be decided at the SONET design stage. Traditionally Internet traffic runs on IP which rides on ATM and SONET/SDH or IP over SDH then to an optical layer (Figure 7.12). The misconception that IP rides on ATM/SDH is that IP traffic is small and has to be combined with other services for cost-effective delivery. Contrary to this delusion the concept of IP over DWDM supports voice, video and data traffic. IP over DWDM networks work by deploying for example a STM-16 circuit directly over DWDM channel. IP over DWDM based networks will allow the carriers to use few wavelengths for carrying legacy traffic and dedicate the rest to high speed data traffic. Eliminat