er (step-by-step) selectors Dialled Routing digits digits Numerical digits The setting up of a call progresses through the exchange stage by Code selectors stage. Each selector connects the call to a free selector in the next stage in time for the train of pulses to set that selector. The wipers Heavy traffic step vertically to the required level under dial pulse control at 10 Medium pulses per second then rotate automatically at 33 steps per second Outgoing to select a free outlet by examining the potential on each contact. traffic junctions This is step-by-step operation and, when used with Strowger selec- Incoming Light traffic tors, it needs only simple control circuits which can be provided junctions economically at each selector. To tandem exchange Numerical selectors Final Uniselector Caller line circuits First selectors Figure 31.13 Strowger director exchange trunking From final selector To other junctions and services First digit 2 Outgoing junction (dialling code '3') allow the same list of codes to be used anywhere in the area, director Incoming equipment is provided, Figure 31.13, which translates the code junctions Second selectors dialled by the caller into the actual routing digits required. To other third selectors Even with the addition of the director, the step-by-step system Second digit 3 lacks flexibility. The convention of ten finger holes in a dial, means that the switching network branches in a decadic fashion. Each Third selectors switch requires a decadic ability and is mechanically designed To other final selectors accordingly. If, however, traffic is unevenly distributed throughout Third digit 5 the exchange numbering range, some parts of this capability will be under-used. The relative slowness of the switches also makes it To uniselector Final selectors impractical to attempt rerouting for call transfer or to by-pass Fourth and fifth congestion in the network. The mechanical complexity of the swit- digits 41 ches themselves presents reliability problems and requires expens- ive maint