Patent GB-1356590-A (GB)
Title: Time division multiplex exchange
IPC: H04Q11/04
Filed: 1971-11-25
Abstract: 1356590 Automatic exchange systems INTERNATIONAL BUSINESS MACHINES CORP 25 Nov 1971 [24 Dec 1970] 54694/71 Heading H4K  A TDM system comprises a plurality of completely autonomous exchange modules which each incorporate an associative memory that is used in store and forward fashion for interconnecting them over a superhighway. The system serves telephone subscribers using (8 + 1) bit PCM of delta modulation and/or data subscribers using teleprinters, data collectors, input keyboards or displays. In the PBX shown in Fig. 1, the modules 11 are each associated with a group of subscribers 13 who may be arranged along one or more looped TDM buses 15, or in star formation i.e. with separate analogue lines which are time multiplexed in a concentrator 17. Certain functions such as public exchange line connections or conference calls are handled by separate modules PN, FM as are connections to a data processing system via modules DP. One module may be kept in reserve so as to take over the function of any other module under fault conditions. The superhighway 19 interconnecting the modules may be looped and parallel coding is used thereon. Timing.-Highways 15 utilize a 125 Ásec. frame containing 32, nine bit time slots of which the last 29 are allotted as required to subscribers while the first three are reserved for synch. and slow speed data or signalling purposes. The latter consists in the transmission of the address of the sender or the receiver during the second slot and then, a flag bit indicating a data word or a signalling word plus the word itself in the third slot (c.f. Fig. 2, not shown). The superhighway contains 32 major time slots equal in length to those on the highways 15. Each slot is divided into at least as many segments as there are exchange modules thereby ensuring a one-to-one relationship between channels on the highways and those on the superhighways. The first three major time slots are used for low speed data signalling wherein in the first slot, in the segment reserved to a sending module, the address of the required receiving module is written, in the corresponding segment of the second slot is written the address of the required receiving channel, and in the corresponding segment of the third slot is written the data word. All the major time slots preferably contain segments reserved for inter module signalling. Operation.-Voice or high speed data signalling. An indication that a subscriber is calling is transmitted during time slots 1 and 2 of his local highway to the module input buffer 75 (Fig. 7). The latter converts the serial data into parallel form and passes it via switches ES1, ES2 to the module controller 85 via opening register 89. Controller 85 reverts the callers address and a "request-to-dial" data word during highway time slots 1 and 2 via output signalling buffer 95 (of which more below), switch MS1 and parallel serial converter 77. The caller transmits the wanted party's address, again in the highway slots 1, 2. The controller having translated the address if necessary transmits it together with the address of the wanted module via switch MS3 on to the superhighway during any available inter module signalling time slot. These slots are monitored by all modules so that one of them, which may be the same as the calling module, recognizes the module address and via line 115 requests controller to determine the free/busy states of the wanted party and to allocate a free time slot on the local highway. This information is reverted over the superhighway to the calling module which then and only then allocates a free time slot to the caller and transmits this data to the called module. The transmitting controller writes into store 87a, the fact that the allocated time slot is now busy (the second column of storage locations), and the address of the called party in terms of the major time slot and the segment thereof which correspond to the allocated channel on the receiving local highway while the receiving controller performs a similar function except that the address is in terms of the calling party's allocated time slot. Thereafter a speech sample from the caller is inserted via switch ES1 into the store location corresponding to his time slot and is read out therefrom on to the superhighway when a match is found between the output of a superhighway time slot counter and the stored receiving address associated with the sample. At the receiving module, the sample is picked off the superhighway automatically since the receiver thereof is opened during a same specified segment of every major time slot (except the first three) and is passed via switch MS1 and the parallel series converter 77 on to the allocated receiving channel which is of course always related to a particular major time slot. Samples from the called party are transmitted in an analogous manner. It may be noted that the sample writein and the sample read-out circuits of the store 87a are controlled by a match circuit associated with but separate from the time slot counters and the time slot comparators (Fig. 5, not shown). The memory is of the content addressable i.e. associative, type.  Operation: low speed data.-As before, the controller is apprised of the caller's identity and the receiver's identity during the submultiplex i.e. second and third time slots, of the local highway. These identities are inserted into store 87b on a same row thereof and at the same time they are transmitted to the required receiving module during a superhighway signalling time slot so that they may be similarly inserted in its store 87b. Thereafter whenever the sender's address occurs in slot 2 of the local highway the corresponding receiver address is read from store 87b into register 93 while the data in slot 3 is written into an associated section of register 91. The address and the data pass through the registers and out on to the superhighway during the first three major time slots thereof. The relevant receive module picks off the data upon recognizing its address and passes it to register 95 where it is stored together with the address of the particular terminal for which it is intended. During a second and third highway time slot the address and the data are passed out of register on to the local highway for transmission to the called party. Synchronization is effected by superhighway time slot counter 97 and local highway slot counter 75 which cause the various switches ES, MS and WS to open only at their allotted times in a frame in dependence on clock pulses provided by a central source. Conference call can be set up by transmitting a general address code over the superhighway instead of a specific address code.