Patent GB-1529594-A (GB)
Title: Communications systems
IPC: H04M7/06
Filed: 1976-06-08
Abstract: 1529594 Automatic exchange systems WESTERN ELECTRIC CO Inc 8 June 1976 [13 June 1975] 23737/76 Heading H4K A communication switching system comprises at least three switching offices linked together in sequence to form a chain by protection channels originating and terminating in adjacent offices in the chain, said offices also being linked by regular service channels to be protected and originated and terminated in offices not all of which are adjacent in the chain; receiver terminals terminating each protected regular channel and including means for initiating the transmission of a protection switch message comprising the identity of the service channel for which substitution is required and the busy/free status of the protection channel to the next upstream office in the chain; means at each intermediate office in the chain for evaluating a protection switch message to determine whether the service channel to be substituted for originates in that office and if not to forward the message to the next upstream office in the chain; and means at. the originating end of each protection channel for controllably effecting a transmit end switch from a regular channel to a protection channel. The figure shows four exchanges interconnected in a chain by protection channels P1, P2 and P3 and fully interconnected in pairs by regular channels 110, 111, 112, 120, 121 and 122. The upstream end of each channel is originated in a transmitter 101, 103 or 105 and the downstream end is terminated in a receiver 102, 104 or 106. If for example channel 120 becomes defective, defective signals are detected in receiver 106 which transmits the identity of that channel and the busy/free state of protection channel P3 to the transmit unit 105. This is effected by multiplexing a signal into a protection channel, not shown, forming part of a second protection chain similar to P1, P2, P3 but operative in the opposite direction. At transmit unit 105 the received identity is evaluated and when it has been determined that this identity relates to a channel not at that exchange a direct connection to its receiver 104 is effected and the identity forwarded to the transmit unit 103 of the next upstream exchange, the busy/free condition of the protection channel P2 is also determined by receiver 104 which modifies the busy/free indication received concerning the channel P3 and forwards a busy/ free indication for the combined series linked channel P2, P3 to the upstream exchange 102, 103 from where it is similarly forwarded to the exchange 101. As the link 120 to which the transmitted identity relates originates in this exchange the identity is recognized at the transmitter 101 and if the received busy/free indication indicates that protection channels P1, P2 and P3 are all available for serial use a switch is effected at the transmit end from the regular channel 120 to the protection channel P1, P2, P3. At the receive end of channel 120 the switch at the transmit end is detected and a receive end switch is effected. To maintain a switch to a protected channel the receive end must periodically transmit the protection switch signal. Signals on the protection channels are transmittted in DS4 format (see Specification 1,501,608) and circuitry for checking the parity bits and other characteristics of the format and for switching back to the regular channel and/or operating alarms is described. Also described is the manner in which selected bits in each format in a series of twenty four frames are used to provide said protection switch method and the manner in which transmitting terminals are synchronized with the subframe and superframes thereof.