Patent GB-1268959-A (GB)
Title: A network for the transmission of pcm signals
Inventor: LYGHOUNIS E
IPC: H04J3/18
Filed: 1969-06-23
Abstract: 1,268,959. Switching systems employing pulse code modulated channels. SOC. ITALIANA TELECOMMUNICAZIONI SIEMENS S.p.A. 23 June, 1969 [3 July, 1968], No. 31584/69. Headings H4K and H4L. A telecommunications switching network, in which nodal switching circuits are interconnected by time division multiplex highways employing pulse code modulation, provides for each highway incoming at a node a first buffer store on which incoming PCM codes are registered in respective storage units, each PCM code having a control bit which indicates if the code is the first or a subsequent repetitive appearance of the code, such repetitive appearances arising due to the multiplex rate of internodal highways being greater than the multiplex rate of modems serving lines local to a nodal switching circuit, a second buffer store being provided to which incoming PCM codes are transferred, to corresponding storage units, from the first store provided that the control bit indicates that the code is a first appearance, the second buffer store being read out channel-bychannel to an outgoing highway the control bits of transmitted codes indicating whether the code transmitted is a first or a subsequent repetition. As shown in Fig. 1 signals on local lines served by a nodal switching circuit are shaped at 2 with a multiplexing rate V 0 , a timer 3 set by synchronizing codes distributes the PCM codes of the channels of the multiplex over storage units of a buffer register 5, the storage units having 8 bits for each PCM code and a ninth bit set to "0" when a code is registered and set to "1" when non-destructively read-out by a scanner 12 for transmission to an outgoing inter-nodal highway over circuit 13. As the multiplex rate of the outgoing highway and scanner 12 is higher than the rate V 0  of the local multiplex terminal the scanner 12 may read out a channel code more than once before it is replaced in the buffer store 5 by the next code. The transmissions of a channel code other than the first are identified by the ninth control bit of each code. Each multiplex highway incoming to a nodal switching circuit, and having a multiplexing rate V<SP>1</SP>, is received in a shaping circuit 7 and by means of a timing circuit 8 set by synchronizing codes the received channel codes are distributed over corresponding storage units of a first buffer store 9. By inspection of the ninth control bit of each channel the timer 8 eliminates PCM codes and transfers to a second buffer store 10 only those PCM channel codes with control bits "0". When the channel storage units of store 10 are read out to the appropriate outgoing highway the control bit is again employed to indicate the first and subsequent transmissions of a code. Those channels for which a local termination is desired are switched, not to an outgoing highway as shown in Fig. 1, but to a termination unit shown in Fig. 2. The unit of Fig. 2 shapes the incoming channel multiplex at 22 and a timing circuit 24 distributes the PCM codes to corresponding channel units of a buffer store 23. By inspection of the control bits repetitive codes are suppressed and the rate at which the codes were multiplexed at their terminal point of entry to the network of nodal switching circuits is established as a channel signal V 01 ,...V 0K , by detecting the rate of occurrence of repetitive codes which equals the difference between the multiplex rate at the terminal point of entry and the multiplex rate determined by the scanner 12 reading out from the buffer store 10 of the terminal point of exit. The buffer store 23 acts as a decoder and converts the received PCM codes to amplitude modulation stored on channel capacitors. A channel distributer 28 gates the amplitude modulations to their respective local lines 1 to K at a rate corresponding to the rate V 01  to V 0K  indicated from the timer 24. If the multiplex highways employ deltamodulation, rather than provide a separate control bit for each channel, it is suggested that the control bit may be transmitted as a phase modulation.