exibility potential for routing traffic either way round the ring. In fact, one of that is tailored to the requirements of a ring. This gives a relatively the biggest advantages, in control terms, of this topology is the low cost ring implementation, which nevertheless, when viewed as STM-1 STM-1 STM-1 (155 Mbit/s) STM-1 STM-1 STM-1 'SPUR' Remote Concentrator Unit 'SPUR' Private Circuits (2 Mbit/s) DLE = Digital Local Exchange SMX = SDH multiplexer Figure 42.32 SDH multiplexers deployed in a drop and insert ring 42/24 The Synchronous Digital Hierarchy (SDH) Node B Node C Node A Node D Node F Node E Figure 42.33 Drop and insert chain produced by 'flattening' a conventional drop and insert ring a single entity, appears as a restricted form of a cross connect. The almost no routing flexibility. This is more than compensated by the versatility of drop and insert rings also extends to drop and insert cross connects, which provide complete traffic routing flexibility at chains, which can be considered as a 'flattened' version of a con- the VC4 and VC12 (and possibly other VCs as well), although not ventional ring. (See Figure 42.33.) Beyond this, a ring can also be usually in the same network element. Used in this way, cross made from a chain of drop and insert multiplexers, whose ends have connects can be viewed as electronic replacements for the present been joined by an SDH line system. The main problem with imple- day digital distribution frames. The benefits of this deployment of menting this type of ring, at least in the junction area, is that the flexibility are alleged to be those of easier traffic path provisioning existing layout of cables and ducts usually takes the form of a star, and a fast, efficient scheme for restoring failed paths which requires which formerly linked the old analogue local exchanges to their the absolute minimum of standby transmission capacity. This last district switching centre. In many cases, a partial physical ring can benefit is heavily dependent on the control that is exercised 