GB1255514A - Improvements in or relating to telecommunications exchange systems - Google Patents

Abstract

1,255,514. Automatic exchange systems. SIEMENS A.G. 29 Aug., 1969 [30 Aug., 1968], No. 43034/69. Heading H4K. In an exchange system in which a subscriber's number does not identify the exchange to which he is attached, calls being set up by passing a wanted number as hunting data from an originating exchange to all adjacent exchanges and from these onwards until the wanted number is recognized, the hunting data is sent over free inter-exchange channels nominated for such signalling, each exchange receiving hunting data keeping a record of the channel over which the data was received, and/or is to be sent, so that such channels may be seized for a connection when a free terminal identity has been ascertained. The connection may be established backwards from the exchange found to be serving the wanted subscriber, the channels identified as those nominated for sending the associated hunting data, should they still be free, being seized exchange-by-exchange until the originating exchange is reached. Since this process may result in parallel inter-exchange paths being set up over alternative routes, the originating exchange sends a message in all outgoing directions to cancel unwanted routes. Alternatively the wanted subscriber's exchange may revert an availability signal which causes the originating exchange to steer a connection over the catalogue of hunting data channels while discarding alternate routes. The reversion of an availability signal may effect seizure of hunting data channels in that it precipitates nomination of new hunting data channels before the connection is set up from the originating exchange. If the wanted subscriber is busy a message may be reverted to clear hunting data records at intermediate exchanges while at the same time carrying the busy signal to the originating exchange. Alternatively, if no connection matures within a fixed time the originating exchange sends an instruction to clear records pertinent to the call. In working between some exchanges of the system the identity of an incoming group of channels may be recorded in place of the channel specifically nominated for receipt of hunting data, a connection being set up in response to hunting data by seizure of one of the group of channels rather than the specifically nominated one. When a channel nominated for hunting data transmission is seized for a connection, a new channel is nominated for such data transmission and the identity of the new channel is substituted for that of the old channel in respect of connections for which hunting data has already been received over the old channel. The inter-exchange channels may be individual inter-exchange circuits, frequency division multiplex channels, or time division multiplex channels, and the data may be transmitted as multi-frequency signals or as pulse amplitude or pulse code modulations.