Patent GB-1317867-A (GB)
Title: Switching system
IPC: H04Q11/04
Filed: 1972-01-11
Abstract: 1317867 Automatic exchange systems INTERNATIONAL BUSINESS MACHINES CORP 11 Jan 1971 [25 Feb 1971] 1135/72 Heading H4K In a TDM exchange, time slot interchange is effected with the aid of a memory that is read/ written in a different manner during successive exchange time frames. The memory contains one section in respect of every communicating subscriber and each section is divided into two fields of which a first contains the identity of the other section involved in a two-way communication while the second contains speech information. During a first time frame the first fields are cyclically read out so as to assess the second fields associated with the identities in these first fields. The contents of the second fields are then read out to the subscribers identified by the sections which were originally interrogated and data from these subscribers is then written in to the vacated second fields. During an immediately succeeding time frame, the second fields are cyclically seamed and information therein is read-out to the subscribers corresponding to the identities stored in the first field followed by a writing-in operation from these same subscribers. When the subscribers are telephone subscribers their speech is sampled and converted into PCM format and then processed through the memory in parallel mode. In the private exchange illustrated, provision is made for n subscribers who are connected by respective incoming lines Li and outgoing lines L 0  to the system. Each line may be a TDM multiplex or may simply carry analogue or data signals. In any event, the incoming lines are monitored for calling condition by control circuit 44 so that in response to dialling signals the desired destination of a new connection may be entered in the first field A of that section of the time switching store allocated to a particular line. Thereafter during cyclic scanning of the lines by multiplexor 41, any signals thereon are coded in a serial PCM code in time slots particular to the lines and are submitted in turn to series/parallel converter 42. It is assumed that a mode I time frame is just commencing. Address generator 45 is thus set to time slot t 1  i.e. line L1, and gate 47 is enabled so that the A field of section 1 of the store is read-out into register 46. The contents of the latter viz. the identity of the other subscriber e.g. Lx to the connection with line L1 in terms of the store section e.g. x allocated to Lx, are used to access the B field of section x. The contents thereof are read-out during a last portion of the existing time slot t1 into parallel/series converter 50 and from their after demodulation in multiplexor 51 are passed to the outgoing wire Lol pertaining to subscriber L1. Meanwhile the speech sample from incoming wire Lo 1 has been processed through the multiplexor 41, and converter 42 so that during a first portion of time slot t2 it can be entered into the just vacated space in the B field of section x. During the remainder of t2 and first portion of t3 the above process repeats for line L2 and section 2 of the store and then similarly repeats for the remaining lines. At the end of the frame when all lines have been scanned, a new frame of node II type commences and gate 48 is enabled. During t1 of this frame the output of address generator 45 is entered into register 46 so that field B of section 1 is read out to the converter and thus to outgoing wire Lo1 while at the beginning of t2 the coded sample derived from Li1 during t1 is written into the first vacated B field of section 1. As before the process repeats so that at time slot x the B field of section x which contains the coded sample derived from line L1 during the previous frame is read-out to line Lx and is replaced by an incoming sample from this line. At the end of this frame a new mode I frame begins.