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To: ftth who wrote (2736)	1/14/1999 9:00:00 AM
From: Hiram Walker	Respond to of 12823

Dave, good article about DWDM. telecoms-mag.com A new network architecture based on IP over DWDM promises to lower costs and reduce management complexity by eliminating the ATM and SONET layers. Is this a viable model? Andrew K. Bjerring and Bill St. Arnaud There is considerable debate in the industry about the best technology for transporting IP services, with the basic choices depending on whether or not ATM and SONET technologies are layered between IP and dense wave division multiplexing (DWDM). In the short run, the top consideration is the extensive deployment of networks using those middle layers and of services based on them. Given the dramatic growth of internet traffic and the development of new internet applications, however, just about every kind of traffic will ride, in time, over IP. A network optimized to carry the unique properties and capabilities of IP traffic, such as an optical internet, will clearly result in dramatic savings. A new network architecture based on IP over DWDM promises to lower costs and reduce management complexity by eliminating the ATM and SONET layers. Is this a viable model? An optical internet can easily be configured to take advantage of these asymmetric data flows. By coupling routers to individual wavelengths, the direction and number of wavelengths can be configured to closely match the traffic load on any given link. The typical asymmetry ratios on major internet links vary from as low as 3:2 to as high as 16:1 at some large internet exchange points. Optimizing the allocation and direction of DWDM wavelengths to match this asymmetric traffic flow can result in a cost saving over traditional balanced and symmetric SONET links. Basically, traditional networks have been designed for two-way conversations, and hence all circuits have been automatically configured as bi-directional paths. The Internet, on the other hand, was designed on the assumption that Tx and Rx data flows are independent. Currently, these independent, uni-directional paths tend to be configured on bi-directional trunks through interface cards on the routers, but this is not required and generally doesn't make sense. Not only could the router be configured with distinct Tx and Rx paths, but it could also be configured to support multiple parallel channels along the same physical link. By optimizing the direction and number of wavelengths to match the offered traffic load, up to a 50-percent savings could be realized. Goodbye, SONET One obvious source of savings in an optical internet comes from the elimination of the SONET layer. This step raises more fundamental challenges for the network engineer, including survivability. One of the major advantages of SONET ring networks is that they have been able to do restoral in as little as 50 milliseconds. In an internet, the timers on router interfaces can be configured so that they will switch or provide path failure notification just as fast. In theory, a router could be placed at the end of a fiber link instead of at the end of a SONET terminal to provide automatic protection switching. However, a new protocol called Multi-Protocol Label Switching (MPLS) promises to eliminate the need to propagate routing updates in the event of a local network outage. Once the standard is approved, MPLS will allow optical internets to carry out restoral and path protection switching at Layer 3 (the IP layer), rather than Layer 1. Hiram