Patent GB-781020-A (GB)
Title: Alternative trunking in telephone systems controlled by overflow trunks and common directors
IPC: H04Q3/00
Filed: 1955-04-28
Abstract: 781,020. Automatic exchange systems. AUTOMATIC ELECTRIC LABORATORIES, Inc. April 28,1955 [May 3,1954], No. 12324/55. Class 40 (4). At an exchange provided with direct trunks to a number of distant exchanges, an outgoing selector on finding all the direct trunks to the wanted exchange busy seizes an overflow trunk leading back to a first selector and a common register-sender is taken into use to control the extension of the call over one of several alternative routes according to their availability. The invention is described with reference to routing between Vancouver and Winnipeg, the direct route being supplemented by three alternative routes via Calgary, Regina and Saskatoon respectively, as indicated in Fig. 1. The layout at the Vancouver exchange is shown in Fig. 2. The direct trunks to Winnipeg are available from the 6th level of the 3rd selector 3S1 and the later contacts of that level are connected directly or over secondary switches such as OS5 to overflow trunks. The overflow trunk OF4, for example, terminates in a 1st selector 1S5 and when seized the common registersender is associated therewith over a switch SSW. The busying arrangements are shown in Fig. 3. Relay 310 falls back when all the direct trunks to Winnipeg are busy and connects up HG in the overflow trunk circuit. Similar all-trunks-busy relays 340, 350, 360 are associated with the groups of trunks to Calgary, Regina and Saskatoon and, provided HG is up, control AR1, AR2, AR3. Thus one of relays H, R1, R2, R3 is operated according to the route available, or OF is operated if no route is available. Relay NB removes the busy marking from the overflow trunk when an alternative route is to be used and relay 330 prevents useless hunting of the secondary switch when no route is available. Assuming that the call is to be routed via Calgary, relay R1 marks the banks of a code switch in accordance with the routing digits to be sent, viz. 55572, and the code switch in turn controls a sending switch in the registersender. The incoming digits which comprise a 3- digit office code followed by numerical digits and are delayed pending the seizure of the register-sender, are received from the operator's toll board or incoming toll line and stored, the first on a switch in the overflow trunk circuit and the remainder on relay sets in the registersender. The routing digits are sent as soon as the first incoming digit is registered and the retransmission of the registered digits follows at once or as soon as the third code digit has been registered. The switch registering the first digit marks the code switch in the same way as relay R1, but the relay registers control the sending switch directly. The register-sender finally drops out and the connection is switched through from 3S1 to 1S5. Since the first incoming digit is registered in the overflow trunk circuit the register-sender may be re-seized before it is completely restored. Relay R1 locks up when the register-sender is seized, and if a direct trunk becomes idle before this occurs relay H operates in place of R and causes the transmission of the routing digits appropriate to the direct route. If the call is to be routed via Regina, which is a cross-bar exchange, relay R2 is operative and after the first two routing digits 57 have been sent by decimal impulsing to control the local selectors, subsequent digits are sent by code impulses of two out of 5 frequencies with a prefix and end-of-dialling signal. On account of the faster sending speed the retransmission of the incoming digits is delayed until the last or penultimate numerical digit is registered. But if the incoming digits represent an " operator code " comprising 3 or 4 digits with the first digit 1, retransmission starts after the third digit is registered. If all routes become busy after an overflow trunk is seized, the operation of relay OF returns a busy signal.