ed the old analogue local exchanges to their district switching center. In many cases, a partial physical ring can be created by a small extension of the existing cable pattern. This can be supplemented by creating a logical SDH-Based Networks STM-1 STM-1 STM-1 (155 Mbit/s) STM-1 STM-1 STM-1 "Spur" "Spur" RCU = PSTN Switch Remote Concentrator Unit PC = Private Circuits (2 Mbit/s) DLE = Digital Local Exchange SMX = SDH Multiplexer Fig. 10.31 SDH multiplexers deployed in a drop-and-insert ring. Node B Node C Node A Node D Node F Node E Fig. 10.32 Drop-and-insert chain produced by "flattening" a conventional drop-and-insert ring. ring when existing cables are laid in a star arrangement, although this obviously affords less protection against cable breaks (see Figure 10.33). Finally, it is sometimes possible to produce a physical ring by linking some of the ring nodes with microwave radio rather than optical fiber. Outside of the access areas (i.e., within the transmission core of the average PTO network), the normal topology advocated is that of a mesh of cross-connects interconnected by point-to-point transmission systems (see Figure 10.34). For this application, the SDH line transmission systems, like their PDH counterparts, require almost no routing flexibility. This is more than compensated for by the cross- connects, which provide complete traffic-routing flexibility at a variety of VC rates. Used in this way, cross-connects can be viewed as electronic replacements for the present-day digital distribution frames. The benefits of this deployment of flexibility are alleged to be those of easier traffic path provisioning and a fast, efficient scheme for restoring failed paths that requires the absolute minimum of standby transmission capacity. This last benefit is Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy SDH Drop-and-Insert Implementation New Links Produce Plesiochronous Physical Star-Logical Ring Physical Ring IIII 1111 Fig. 10.33 Comparison of topologies possible with PDH and SDH