ions first sight, as it has often been said that SDH does not really make (TTFs) surrounding a traffic path connectivity function (see Figure economic sense without a large measure of software control over all 42.21). The TTFs are essentially the conversion functions from the the network element functionality and, in particular, that of routing VC level to either the STM or plesiochronous signal levels, while flexibility. However, the complexity of even relatively simple SDH the connectivity function allows VCs to be routed between the network elements leads to a situation where control of complete various TTFs. More than anything else, it is the size and flexibility networks rapidly escalates unless some restrictions are put on the 42/18 The Synchronous Digital Hierarchy (SDH) TRANSPORT TERMINATION FUNCTION SECTION MULTIPLEXER MULTIPLEXER REGENERATOR ORDER ORDER ADAPTION SECTION SECTION SECTION PHYSICAL PROTECTION TERMINATION TERMINATION INTERFACE TERMINATION CONNECTION attach cross load VC4 attach attach convert connection into STM logical to C4 of VC4s and attach STM to pointer optical signal Figure 42.22 Fragment of an SDH network element showing how it is broken down into atomic functions acording to CCITT G.783 traffic routing complexity. In short, every additional piece of traffic reported at set intervals, etc. At the lowest level, some of this routing flexibility is a potential network control headache. information may look rather different to that specified in G.783/4 The general problem of control of a whole SDH network, notwith- etc, however, the internal network element control system takes standing the traffic routing problem, is almost impossible if it is these and from them, synthesises the information that is required by composed of network elements from different manufacturers, which the next level in the management hierarchy i.e. the element man- do not have some form of common control interface. Before we can ager. arrive at such a universal interface, we must first standardise, to a The eleme