o allow a traffic reroute. On some occasions, networks based on this topology have been adopted by new opera- tors, who have had more freedom to lay their cables in rings. Usually the network elements themselves are a halfway house between add-drop multiplexers and cross-connects, what really makes them attractive is the bandwidth saving that can be achieved with the so-called Shared Protection Ring mechanism (MS-SPRing). This is usually a ring version of the familiar 1:1 N protection scheme previously used on point-to-point line systems. Com- pared to the alternative 1 + 1 protection scheme, it can easily give an improvement of two or three times, especially with the even traffic distributions found in the core of the network. Conclusion SDH and SONET became widely established in an era when voice was the dominant traffic type in telecommunications networks. As the demand for data transmission has grown, other protocols better suited to data traffic have emerged, such Next Generation SDH, Multi- protocol Label Switching, and Provider Backbone Bridging with Traffic Engineering. However, the deployment of SDH is SO widespread that it will continue to be used for many years to come, particularly in conjunction with Next Generation SDH, which enables much more efficient use of SDH capacity when carrying data. This page intentionally left blank 11 Next Generation Synchronous Digital Hierarchy Ken Guild University of Essex Introduction In the last 20 years, Synchronous Digital Hierarchy (SDH) has established itself as the pre- dominant optical fiber transmission technique in European operator networks. The SDH format is very similar to the North American SONET format but with some modifications to accommodate the different legacy Plesiochronous Digital Hierarchy (PDH) transmission schemes used in Europe. Although this chapter focuses primarily on SDH, the fundamental principles of Next Generation SDH (NG-SDH) are identical to those of its SONET counter- part, except for the different multiplexing hierarchies between S