Patent GB-1344001-A (GB)
Title: Communication systems
IPC: H04Q3/00
Filed: 1971-04-19
Abstract: 1344001 Automatic exchange systems WESTERN ELECTRIC CO Inc 19 April 1971 [9 Feb 1970 (2)] 21121/71 Heading H4K In a communication system in which substations with both audio and wideband (video) equipment are connected to separate audio and wideband switching networks, the system responds to a calling condition by selecting audio and wideband trunks on the other side of the networks, independently selecting idle routes through the networks between the substation line and the trunks and after completion of both selections initiating establishment of paths through the networks over the selected routes. The specification states that ferreed switching networks may be used but the embodiment described uses cross-bar switches. The figure shows a system in which the audio network comprises four switching stages 126, 127, 128 and 131 which serve all subscribers of the exchange and can provide two wire audio paths between a calling and called subscriber via intra-office trunks such as 115A and 108A. The video (wideband) network comprises three main stages 132, 133 and 134 associated with all video subscribers and an auxiliary switch unit 125 used only for those video subscribers such as 101 and 102 which are grouped at a remote location. The video network provides a four wire path between calling and called video subscribers. The auxiliary switch co-operates with an intra remote switch trunk to interconnect the video equipments of calling and called subscribers if both are at the remote location and acts as a concentrator reducing the number of video links 119 which connect to the exchange for video calls involving only one subscriber at the remote location. For video calls in which both subscribers are adjacent the main exchange 100 the main stages 132-134 alone provide the video path. Call establishment.-When any subscriber initiates a call the audio subset is coupled in conventional manner via the audio switching network to an idle originating register 135 and on return of dial tone the subscriber transmits in multi-frequency form the directory number of the called subscriber preceded by a special prefix if a video connection is required. Each subset has twelve push-buttons, ten for multifrequency dialling and two special service buttons. The register seizes a marker which on detection of the video prefix verifies that the calling customer is equipped for video service and if not equipped connects the call to a tone trunk. The area code of the called subscriber as dialled by the caller and the equipment number of calling line are forwarded to the marker which seizes the audio trunk link frame 130 and for video calls the video trunk link frame 134 via connectors 136 and 137 and selects an appropriate idle audio trunk. Some trunks such as 113A deal only with outgoing calls and are associated with corresponding video outgoing trunks 113WB while other trunks such as 115A and 108A deal only with intra-office calls and are associated with corresponding video trunks 115WB and 108B, trunks 108 being used if both subscribers are at the remote location (i.e. terminated on the remote switch unit) and trunks 115 being used when the calling or called subscriber is connected to the main exchange. The marker selection circuit is such that given the same selection of idle trunks a different one would be selected in successive attempts to ensure even use of the trunks. Due to the one to one relationship separate selection of a video trunk is not necessary. The blocking factors of the audio and video networks are such that if an idle audio path exists then an idle video path also exists. When the called subscriber number has been dialled, the marker translates this in an appropriate number group translator into the audio equipment number of the called subscriber and transmits part of this number to the video line control which determines the video equipment number of the caller. Using the audio equipment number the marker tests the audio and video equipment of the called subscriber and if both are idle initiates selection of routes through the audio and video switching networks from the selected trunks to the called line. These routes are selected independently. Selections of an idle audio route (which is only idle if all three matching components comprising an audio line link 150, an audio junctor 141 and an audio trunk link 130 are idle) is effected by circuitry (Figs. 20 and 21, not shown) which can test ten routes at a time any busy component link of a route putting a marking on a conductor (such as 2000) common to the components links of that route via a diode resistor arrangement. The selection circuit selects the first idle route on a fixed priority basis. The selection of a route through the video network is effected by circuitry (Fig. 27, not shown) which includes a relay (such as 27 WCTO) for each route which is actuated over a resistive diode path if any component link of that route is busy. This selection takes place one at a time on a fixed priority basis. When both routes have been selected the marker initiates simultaneous establishment of these routes, a common timing device being employed to ensure adequate delay between operation of the cross-bar select and hold magnets. The marker then proceeds to establish a similar connection between the calling station and the intraoffice trunks. Once both channels (video and audio) are established a supervisory signal is transmitted over the video channel to turn on both video sets. Also ringing is transmitted from the audio intraoffice trunk to alert the called customer. Testing and checking.-Numerous check relays are provided to check such matters as that only one marker at a time seizes the trunk link frames and that stray potentials do not exist on particular wires. Certain overall check relays  check that a group of check relays are in their normal states. The paths are tested for continuity after selection. In particular the conductors of the video paths are tested in pairs for stray potential and a 12K.Z. signal is transmitted over one conductor pair and returned over the other to test continuity. A master test frame not detailed which permits maintenance personnel to simulate calls and perform tests is provided. The master test frame can remove the video facility from a subscriber. Circuitry for setting a magnetically latching relay associated with the line to indicate this fact is described.