Patent GB-1257623-A (GB)
IPC: H04Q11/04
Filed: 1967-12-11
Abstract: 1,257,623. Automatic exchange systems. POST OFFICE. 11 Dec., 1968 [11 Dec., 1967], No. 56149/67. Heading H4K. An exchange system switches PCM signals over send and receive time division multiplex highways between incoming and outgoing PCM time division multiplex inter-exchange trunks, the connection between a calling channel and an outgoing channel in the wanted direction being set up over an intermediate highway pair introducing no delay into the connection across the exchange as a first choice if an appropriate channel is free and accessible in the outgoing direction, or being set up over an intermediate highway pair introducing fixed delays in the send and receive directions of transit as a second choice if an appropriate channel is found free and accessible in the outgoing direction, or as a third choice using an intermediate highway pair into which it is possible to introduce any delays that are appropriate to a channel found free and accessible in the wanted direction. As shown schematically in Fig. 1, incoming 24 channel 8-bit PCM highways are served by system units 3 which synchronize incoming codes with the exchange cycle and, for reasons of security, routes odd channels to one supervisory trunk unit and even channels to another such unit. Each trunk unit 4 receives odd channels from one system unit and even channels from another such unit, and supervises its channels to detect calling conditions and connect a register 6 working in time division multiplex by way of highway switching matrix 5. The register, in co-operation with a translator 7, sends a mark to that system unit or those units 16 in the required outgoing direction. The even and odd channels of the outgoing 24 channel highways are slipped relative to their outgoing trunk units 17 in the same fashion as on the incoming side of the exchange. The mark from the register to the outgoing system units 16 results in all those channels free in the wanted outgoing inter-exchange highways being marked to accessible highways in highway switching matrix C, and those of the free channels accessible over the C switch being marked available to the B-C switch links. At the originating side of the exchange the register effects a forward signal to indicate the calling channel from the incoming trunk unit to the highway switching matrix A and from thence to mark all such channels available in the A-B links. The highway switching matrix BZ provides a connection in the calling channel if a suitable outgoing channel is found to match the calling channel. The highway switching matrices BI and BO provide connections using fixed delays if the forward indicate signals, on channels in the A-B links giving access to the calling channel, are fanned out over all such idle channels free in the fixed delay cords 14 and can be matched in the BO switching matrix with an accessible channel in the wanted outgoing direction. Connections over matrices BI and BO using variable delay cords involve selecting an outgoing channel accessible over B-C links and adjusting the delay cord to suit the channels. The delay cords are 8 Î 24 capacitative storage arrays each of the 24 rows of 8 serving a channel. Fixed delay cords read in and read out at set times whereas the times of read in and read out for a variable delay are established by storage of each channel code in the other channel time. Where a connection can be set up over any one of a number of available routes the route and B-switch selectors 19 and 20 select one for use. Synchronization of the incoming PCM codes is effected by distributing the code bits, by a three-stage counter running with the incoming pattern, on to a store from which the bits are read by exchange cycle pulses. The send channels are put in range of synchronization circuits by a 7-stage shift register by means of which the codes may be delayed any number of bits. A call trace facility is provided by stretching the first bit present on busy channels in both directions so that the condition is available to a scanner inspecting the system units. Connection of register.-Calling condition is detected in the incoming trunk unit from the presence of the first bit in a channel code in odd frames. The condition is scanned for at 3 ms. intervals and, if present for two consecutive scans, the trunk unit calls for connection of a register with a free time slot to match the calling channel. The register connector finds such a match and stores the incoming trunk identity in a register to operate the appropriate cross-point between trunk and register in the calling channel slot. The register connector reverts a signal to the trunk circuit where a sequence state store is updated to indicate a register in service and to cause connection of the incoming channel to the register. Signals present in the channel activate a hold-persistence store in the register to provide a hold signal for the connection until such time as an exchange originated control bit has been found absent for two consecutive 3 ms. periods. The register reverts a signal back to the originating exchange to indicate its presence. Digit reception.-The presence or absence of the first code bit is monitored by the register to detect valid makes and breaks to discriminate between Strowger impulses and inter-digital pauses. If the hold condition is present for more than 3 sees. with no valid break detected, NU tone is reverted. A valid make is signalled if two out of three consecutive 6 ms. scans indicate a make. A valid break is signalled if two consecutive 6 ms. scans indicate a break. Counters measure off intervals to discriminate pulses and inter-digital pauses. When one or more digits are recorded a "ready to set up connection" store is made active. A connection must be set up during an inter-digital pause and the attempt will be discarded and the register released if signals are received from the originating exchange before connection is complete. The connecting process is timed out and busy tone is reverted if no connection appears likely to mature due to register congestion. All channels served by the register and for which one or more digits have been received are identified in the "ready to set up" store and one of those channels is nominated for a connection attempt.  Register attempts connection.-Each register is given access to the switching network for setting up a connection during the period of an associated scanning pulse RS from a series of such pulses RS1 to RS28. These scanning pulses also provide periods during which the switching network is subjected to unspecified checking functions and routining. As described, only three registers are in service in the periods RS1, RS2 and RS3, respectively. Each RS scan pulse embraces 28 multiplex frames defining a pulse series E1 to E28 employed to sequence the connecting process. The forward channel indicating signal for the channel nominated for connection during the scan period, is repeated in the channel slot of every frame until continuity of the connection is proved by the receipt of a "circuit free" code present on the receive part of the seized outgoing channel. Tests for the presence of the 'circuit free" code are made in frames E19 and E21. The register responds by sending a seize signal forward to the distant wanted terminal from which the "circuit free code" is removed. As it does not involve the processes of selection this response takes place beyond the RS scan pulse as does a test for removal of the "circuit free code" after which the register releases by inhibiting the exchange hold signal applied from the register connector. If, by the third following register scan pulse RS, tests to confirm connection are negative a "second attempt" is ordered by preserving the nominated channel in the register until the next scan pulse RS for the register. If a second attempt fails, NU tone is connected. Register faults.-If register function is aborted by fault during a connecting process and such faults occur at random amongst the registers, it is presumed that the faults are external to the registers. A running count of faults in each register is restarted if a fault occurs in any other register. If a count of four faults is recorded in a register a warning lamp is lighted. If the count reaches seven without being erased by a fault occurring in any other register, the register is put out of service and can only be restored by a manual reset. A record is made of the number of calls lost. Route selection.-The forward channel indicate signal is passed over the register connector to the incoming trunk unit where it is passed on to the A-switch. At the called side the register marks the system units serving highways in the wanted direction and the units are strapped to distribute the mark to the outgoing trunk units from which they are accessible. In the trunk units the marks are received over separate channels, depending on whether they are in odd or even time slots, and establish which channels are available for connection in the wanted outgoing direction over the C switch. If an outgoing channel is free the first bit of its code is present in odd frames from the receive highway. A busy condition is deduced from the holding code which is the first bit present in even frames on the send highway. Busy channels on the highway serving each outgoing trunk unit into the C-switch are indicated by trunk unit codes circulating in the channel times in a C-switch channel store. Those channels of the store that are free indicate channels free to the switch and if they coincide with a channel on the outgoing highway in the wanted direction marked free over the trunk unit, the channel is marked forward to the B-C links as an available component for a complete route. The indicate signal at the calling side of the network causes the A-switch to be offered the incoming trunk unit identity for recording and the indicate signal is passed forward to s