es, networks, interconnection is achieved by means of a fourth stage so that all registers are fully available to all customers and a standard (D) of switching and groups of link circuits which provide local setting up procedure can be used. supervision where necessary. The adoption of a general purpose switching network, as de- scribed, avoids the need for auxiliary switches for the connection of 32.4.2.3 Interrogator markers and supervisory processors registers and other circuits. All such peripheral circuits are termi- nated on the general purpose network via junction A switches. Any Interrogators and markers are provided for each sub-unit of the peripheral may be associated with any termination by a path (or switching network, although each marker can control up to five paths) through the network. This principle leads to the concept of interrogators on the same plane. When path selection is required, the serial connection of switching paths known as serial trunking. main control unit signals to all interrogators through their control- ling markers and the free paths available are identified. The main 32.4.2 The control equipment control unit then selects the most appropriate route available and instructs the particular markers to operate the crosspoints within the The control equipment supervises the setting up of all calls through selected sub-units and also to allocate the required type of link the switching network. circuit for the connection. The supervisory processor controls both the bridge link used for supervising local and terminating calls and also the outgoing junc- 32.4.2.1 Cyclic data store and scanning tion circuit used for supervising outgoing calls. Each supervisory processor is triplicated for security and can control over 6000 The data stores provide common central storage for the exchange connections at a time, serving 16 switching units. Data is obtained data relating to customers, junctions, code translations and routing by monitoring the highways between the main control unit and the inf