visories are also arranged to receive dial pulse signals and to route these to a register. Provision is made 1. An own exchange call. for ringing the called subscriber's bell, the transmission of ringing A local call to or through the tandem exchange. tone to the caller and detecting the answer signal. 3. A long distance (trunk) call. The registers associated with the supervisories are general pur- pose registers. Access is obtained via a register access switch (reed Assuming the tandem route is the first choice, in case 1, the relay). The registers may be arranged to receive and store either dial register proceeds after the first digit or, in the case of a linked pulse signals or multi-frequency coded push button signals. numbering scheme, a later digit to release the initial set up, to call in a free O/E supervisory and to reset the call, all during the 32.3.3 The control area inter-digital pause. The calling customer is not affected. In case 2, as soon as sufficient digits have been received to indicate that the call is to the tandem exchange, the register is The control apparatus is common to the exchange and is shown in block schematic form in Figure 32.1. It is duplicated to ensure released and dialling continues over the already established path. continuous service in the event of faults. The two controls are In case 3, the call will be released and reset to a supervisory in the trunk route. independent and are used alternately with a periodic changeover to ensure that both controls are exercised and that faults will not go Because all calls are connected from a supervisory to a customer's undetected for long periods. line, terminating calls are set up in a manner broadly similar to originating calls. Typically when a call arrives on an I/C supervi- When a customer or incoming junction originates a demand for sory, its terminal identifying number is generated by the calling access to a register, this demand is first routed to a calling number number generator and placed in a free register which then procee