Switching systems

1,084,556. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. Oct. 29, 1964 [Nov. 5, 1963], No. 44106/64. Heading H4K. In a system comprising first and second offices interconnectible by tandem exchanges located in primary and secondary routes, in the event of congestion at the primary tandem a signal is reverted to the offices which causes the route selectors thereat to choose a secondary route. Thus, in a system employing local, toll, sectional and regional offices, a regional office may easily become congested due to a large number of calls awaiting processing by its common circuits although many incoming and outgoing trunks may still be available to its subsidiary offices. Consequently an office seizing such a free trunk may be subjected to considerable delay before the call on the trunk can be routed through the regional office, with the further possibility that the call may be timedout at the subsidiary office before it is dealt with. If, however, the tandem exchange informs the sub-office that it is overloaded by artificially busying the outgoing trunks at these offices, the latter can choose an alternative route until the regional office overload subsides. General description (Fig. 1).-A call from subscriber 110 in the Denver Toll centre area to subscriber 112 in New York is normally set-up via local and toll offices 106, 102 in the Denver area, trunks 114 to the St. Louis regional centre, trunks 124 to the New York toll area and then toll and local offices 104, 108. In the event that St. Louis becomes congested an overload detector circuit 132 operates so as to revert " centre busy " signals (frequency-shift telegraphy) over the signalling junction 133 to the various toll offices which use St. Louis as a primary tandem. The outgoing trunks, e.g. 114, are marked busy by these signals and the route selectors (register-translators) are thus caused to set-up an alternative route, e.g. via trunks 115 to the Chicago centre. The digits of the wanted number are transferred in full from exchange to exchange during setting-up of a call. Particular description. Normal call over first choice route (Figs. 2 to 6 arranged as in Fig. 7, not shown).-A calling subscriber 110 is connected via his local office and trunk 134 to the (crossbar) Denver toll centre when his local office determines from the ten dialled digits that a toll-call is involved. A sender 221 is connected to the trunk and receives the digits. Thereafter the sender is connected to the common de