gaged tone may be varied to ensure optimum loading. The B and C switches are (EET) and is required to make another attempt later. interconnected to obtain full accessibility between every B switch The allotted register applies to the call control for the selection of inlet and every C switch outlet. The D switch is provided to ensure a free supervisory which, when found, causes the register to be a good grade of service on terminating calls without upsetting the connected via the register access switch, the chosen supervisory and modular construction of the A, B and C switches. All crosspoints the C, B and A switches to the calling line. have four reed inserts. Initially, the allotted supervisory may be either an O/G junction supervisory or an O/E supervisory, depending on the facilities 32.3.2 The supervisory and register area required and the distribution of traffic. For example, if the majority of the traffic is to the local tandem exchange, an O/G supervisory on this route will be the first choice. If there are no free supervisories Only three basic types of supervisory are needed although variations on this route, a second O/G group or O/E supervisory is allotted may be introduced to meet particular requirements. Each type instead. contains a transmission and signalling bridge and appropriate logic When the register sets up the initial connection, it determines from circuits. the class of service signal received if any special facilities are The outgoing supervisories provide facilities for routing dial pulse required, for example if the call is from a public call box. When the signals to a register, for onward transmission to other exchanges, for facilities have been provided, dial tone is returned and dialling can the receipt of called subscriber answer signals and service tones. proceed. They also generate metering signals in response to answering or, From the digits received, the register determines whether the call where appropriate, repeat metering from the called exchange. The incoming and local super