application of 'common control' where one control circuit (e.g. a marker or a register) is shared by several switches. In the Strowger system, control elements are associated with each element of the switch SO that control is distributed with no centrali- Greater centralisation of the control, however, offers some further sation. Communication between control elements is minimal, in- operational advantages. When numbering changes are made during the life of the exchange, for example, it may be possible to avoid the structions being received directly from the subscriber at the need to change a large number of circuits such as registers. It is also relatively slow rate of ten pulses per second. possible to employ more effective search patterns to find free paths, Although centralisation of the control brought many benefits it thus making economies possible in switch provision. For example, also has its problems. Exchanges grow to meet traffic demand in register systems, the register chosen initially for the call does not during their 'life' and their processing power requirements grow necessarily have an unrestricted choice of switch paths for the with them. This is not easy to achieve economically if all the forward connection of the call. It may have access only to some of processing power resides in one central processor. Also, it is usual the switches at the next stage and it cannot govern the choice of free to provide at least two central processors to guard against failure yet switches beyond that. Although suitable free paths may exist the doubling the capacity in this way is wasteful. Various architectures have, therefore, been devised which retain exchange may be unable to allocate them and the customer may fail to get his call. the essential functional centralisation of processing while allowing decentralisation of the processors themselves. This facilitates In any block of switches under one control, it is only possible to set one call at a time. To extend the control across a whole exchange growth and p