Patent GB-1128582-A (GB)
Title: Improvements in or relating to telecommunication exchanges
Inventor: DINWIDDY SIMON EDWARD
IPC: H04Q11/04
Filed: 1967-04-28
Abstract: 1,128,582. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. 28 April, 1967, No. 19771/67. Heading H4K. Junctions between a satellite and a main exchange are provided by PCM channels on a multiplex highway, each channel being associated with a storage section at the satellite whereby program words from the main exchange indicate the control operations required in respect of each channel during the processing of calls. As illustrated, the program word store PS serves 23 channels with a section of 4 bits for each. A store ENS provides 8 bit stores for each channel and is used to store the equipment number of the subscriber's line connected to a channel. The channel PCM from an incoming register IR terminating the receive highway RH, passes to a central register CR from which it is sent to a coderdecoder CODEC which supplies the appropriate subscriber with an amplitude modulation corresponding to the received pulse code. The appropriate subscriber is connected during the channel time slot by the cyclic appearance of the equipment number stored in ENS in the output store OPE, a decoder END converting the code to operate the individual subscriber gate. In the same time slot the subscriber's line is sampled and a code formed in CODEC is passed to the central register CR for transmission over outgoing register OR and highway GH. Newly received equipment numbers are stored in ENN from which they may be employed by a comparator COMP to see if such numbers are already set up in ENS. The 24th highway channel is used to synchronize the exchangers and the store ENN and COMP are employed in this process. For a free channel the stores PS and ENS store all O's; for a busy channel the store PS stores all 1's. The satellite is controlled by four program words from the exchange. Originating calls. Both for the purpose of obtaining wanted subscribers on the satellite and of searching for calling satellite subscribers, the exchange seizes a free multiplex channel (junction) and sends successive search code words. The code is recognized at the satellite by CR which instructs program control PRC and returns the code to the exchange to check transmission. The store PS is now given the code 0001 so that the next code received over the channel is treated as an equipment number of a line wanted for scanning and stored in ENN. The code in PS changed to 1001 to cause the comparator COMP to look for the wanted line in ENS. If found in ENS the line is busy and PRC sends a busy code over GH to the main exchange, the channel stores and ENN being cleared. If free, however, a free code is reverted, the equipment number is put in ENS, and code 1101 is written in PS. At the next occurrence of the time slot the subscriber's line is gated to CODEC to revert a single on-hook, off-hook reading to the main exchange; the channel being freed once this is done. If offhook condition is found the subscriber is deemed waiting for dial tone and a call sequence is started. Call sequence. A call sequence code from the main exchange is interpreted in PRC which writes 0011 in the store PS of a seize channel. In the next multiplex cycle the received code is consequently accepted as a wanted line number and is stored in ENN with 1011 in PS. As the call sequence is used to terminate incoming calls as well as connecting calling satellite subscribers, the comparator COMP searches ENS for the wanted number and reverts busy code if the number is found. If the line is free the code 1111 is written in PS, the equipment number is written in ENS and the wanted subscriber is thereafter gated to the highways in the channel time slot. Call trace. For the purpose of determining that the correct line has been connected at the satellite and for use as a means of tracing malicious calls, the main exchange transmits successive call trace codes. The satellite only responds if the store PS for the channel concerned holds 1111. If this is so the PS record is changed to 0101 and this is reverted to the main exchange. In the next cycle the satellite sends the equipment number in store ENS and writes 1100 in PS. This is repeated in the next cycle but PS now has 1111 restored to it so that normal speech connections are re-established. Interrupt. To release a connection the main exchange sends an interrupt code twice, the satellite responding to the first code by changing 1111 in PS to 0111 and by changing 0111 to 0000 in response to the second code. If the second code is missing the store PS is automatically restored to 1111. Of the sixteen codes possible in each channel store in PS five are not used and should one of the unused codes occur in a storage section of PS the control circuit PRC re-writes them automatically as 0000. Connections between subscribers at the satellite are made by connecting each over a junctor to the main exchange where connection between the junctors is effected.