heless, without some sound business drivers, most PTOs would view a move from the current, well proven, PDH to the more In order to fully appreciate SDH, it is useful to briefly examine some advanced SDH as, at best, an interesting academic exercise, and at of the limitations of the PDH. These limitations fall under three worst, a scandalous extravagance. To see why there is such frenetic main headings: activity to changeover to SDH it is necessary to briefly examine the business environment in which the world's more advanced PTOs 1. Lack of flexibility. It is more difficult, using PDH based equip- are now operating. ment, to drop and insert a low rate channel from a high rate The source of the problem is the PTOs' business customers. These channel, without the use of a 'multiplexer mountain'. The are the customers from whom the PTOs derive a disproportionately normal example given is that of dropping out a 2Mbit/s channel large percentage of their profits, and who are becoming increasingly from a passing 140Mbit/s stream. (See Figure 42.1.) The cause dependent on telecomms services for their very survival. Not sur- of this particular problem lies in the amount of information prisingly, business customers are looking to their use of telecomms processing necessary to locate and extract the required traffic to give them a competitive edge and thus, in addition to a straight channel. The basic PDH multiplexing process involves a bit reduction in tariffs, they are now demanding other things such as interleaving operation which obscures knowledge of the indi- lower error rates and higher availability on their existing services, vidual byte boundaries, and thus leads to an inflexible system. together with the bandwidth flexibility (i.e. channel capacity and This inflexibility extends beyond drop and insert functionality, routing capability) that enables the introduction of completely new to crossconnects. These become rather large and cumbersome services. In the past most PTOs operated as monopoly suppliers, and becaus