ower end of the SDH range. The reduction in cost that accompanies a network element with restricted connectivity results not only from simpler hardware but also from simpler control software. This is somewhat surprising at first sight because it has often been said that SDH does not really make economic sense without a large measure of software control over all network element functionality and, in particular, routing flexibility. However, the complexity of even rela- tively simple SDH network elements leads to a situation where control of complete networks rapidly becomes unmanageable unless some restrictions are put on the traffic-routing com- plexity. In short, every additional piece of traffic-routing flexibility is a potential network control headache. The general problem of control of an entire SDH network, notwithstanding the traffic- routing problem, is almost impossible if it is composed of network elements from different manufacturers that do not have some form of common control interface. The network element functional standard ITU-T G.783 standardizes the functions that each of the network elements actually performs. This recommendation describes how the functionality of a network element can be decomposed into the basic "atomic" elements of functionality, together with how such atomic functions can be combined and thereafter exchange both traffic and control information (see Figure 10.21). The rules for combination of these atomic functions enable a variety of network ele- ments to be synthesized, with differing traffic capacities and routing capabilities. Indeed, one of the problems is the potentially large number of different ways a conformant network element can be constructed. It is rather difficult for software-based control systems to cope with this variety, and hence a more restricted range of network element functionalities has been stardardized. G.783 was created primarily to describe different types of drop-and- insert multiplexers; however, the general principles and, indeed, much of the d