s The Synchronous Digital Hierarchy (SDH) is a new standard for to re-examine the cost effectiveness of their existing transmission multiplexing together many low rate digital traffic channels into networks. In particular, they are looking to balance the quality of higher rate channels, in order that these low rate channels may be service delivered to their customers against the capital and operating more efficiently transported around a telecommunications network. costs of their networks. The objective of this balance is, of course, Viewed in this way, the SDH is merely a better alternative to the to maintain or increase profits. existing Plesiochronous Digital Hierarchy (PDH), which attempts The disturbing conclusion of most PTOs is that it is not possible to achieve more or less the same results. However, as we shall see to deliver the quality of service demanded by business customers, at later in this chapter, SDH has evolved to become much more than the right price, if they continue to operate transmission networks just the latest standard for combining and separating traffic chan- based on the current PDH. Even more disturbing is the fact that this nels. Due to the business pressures confronting the world's Public conclusion was reached with full awareness of the falling costs of Telecommunications Operators (PTOs), SDH has developed into a raw bandwidth i.e. the effects of reduced costs for optical fibres, comprehensive set of standards which address all aspects of a electro-optic devices and the Application Specific Integrated Cir- telecommunications transport network. The most important of these cuits (ASICs), which are the heart of any piece of transmission additional aspects concern the performance of traffic paths across hardware. the network, together with the automation of their management. The result of implementing the SDH standards is that a PTO now has a consistent mechanism for partitioning, monitoring and controlling the raw transport capacity of the whole network. 42.2 PDH deficiencies Nevert