livering and to pinpoint those network ele- ments responsible for any poor performance. Beyond that, the SDH network model greatly facilitates management of the all-important traffic flexibility points; that is, those subnet- works that consist of an electronically controllable connectivity function. SDH Network Topologies Once again, traffic-routing flexibility, its control and physical distribution within an SDH network, is one of the most important influences on the topologies in which SDH equipment is deployed. The most obvious manifestation of this is the drop-and-insert ring topology that is finding favor in the PTO access networks (see Figure 10.31). The idea behind a drop-and-insert ring is that the ring structure can give a high degree of protection against cable cuts, and SO on, because of its potential for routing traffic either way around the ring. In fact, one of the biggest advantages of this topology, in control terms, is the limitation on the rerouting possibilities for any traffic affected by a cable break. Any- thing more complicated than a simple clockwise/counterclockwise routing decision requires up-to-date knowledge of a rather more extensive portion of an SDH network than just a simple ring. The nodes of such a ring are populated by drop-and-insert multiplexers that have a restricted traffic-routing flexibility that is tailored to the requirements of a ring. This gives a relatively low-cost ring implementation, which nevertheless, when viewed as a single entity, appears as a restricted form of cross-connect. The versatility of drop-and-insert rings also extends to drop-and-insert chains, which can be considered as a "flattened" version of a conventional ring (see Figure 10.32). Beyond this, a ring can also be made from a chain of drop-and-insert multiplexers, whose ends have been joined by an SDH line system. The main problem with implementing this type of ring, at least in the access area, is that the existing layout of cables and ducts usually takes the form of a star, which formerly link