over the be created by a small extension of the existing cable pattern. This cross connects, and here there are some potential problems. can be supplemented by creating a logical ring when existing cables The main problem is that of database integrity. With a network of are laid in a star arrangement, although this obviously affords less meshed cross connects, when a transmission link fails, the path protection against cable breaks (See Figure 42.34.) Finally, it is restoration action will usually involve re-routeing all the affected sometimes possible to produce a physical ring by linking some of paths through several alternative cross connects. In order to do this the ring nodes with microwave radio rather than optical fibre. efficiently the network control system must rapidly command sim- Outside of the former junction areas, i.e. within the transmission ultaneous switching actions in all of these crossconnects, which core of the average PTO network, the normal topology advocated is usually implies that the control system has a pre-determined plan of that of a mesh of cross connects, interconnected by point to point action which is based on the spare bandwidth that is thought to be transmission systems. (See Figure 42.35.) For this application, the available on the relevant transmission links and cross connects. SDH transmission systems, like their PDH counterparts, require Unfortunately the integrity of the control systems database may SDH DROP & INSERT IMPLEMENTATION NEW LINKS PRODUCE PLESIOCHRONOUS PHYSICAL STAR - LOGICAL RING PHYSICAL RING 1111 IIII TTT TITT Figure 42.34 Comparison of topologies possible with PDH and SDH multiplexers SDH based networks 42/25 STM-4 or STM-16 STM-1 Multiplexers Transmission or STM-4 links PERIPHERY Figure 42.35 Example of core transmission network topology that relies on cross connects for flexibility have been compromised because of other recent reconfiguration. 42.6.2 Deployment strategies This problem escalates rapidly as the number of cross connects in a network increase