1515294 TDM switching systems POST OFFICE 30 May 1975 [10 June 1974 (3)] 25566/74 25567/74 and 25568/74 Headings H4K and H4M A digital telephone system comprises a concentrator which includes a switching network having a number of switching elements and a switching centre connected to the concentrator, data being transmitted as TDM PCM signals on a PCM highway interconnecting the concentrator and switching centre. The signals are formed from a succession of superframes, each superframe consisting of a number of multiframes, e.g. 4, each multiframe consisting of a number of frames, e.g. 16, each frame consisting of a number of time slots, e.g. 32 and each time slot consisting of a number of digit positions, e.g. 8. Switching network control signals are sent between the concentrator and switching centre using signalling digits occupying some of the digit positions of each of a multiplicity of time slots and using the particular frame and particular multiframe in which a time slot occurs to identify the switching element to which the signalling digits of that time slot relate. The multiplicity of time slots used for the signalling digits is in fact constituted by the corresponding predetermined one or more time slot in each of a multiplicity of frames. In the telephone switching system described, 1500 data terminals are connected to each concentrator 1 and each concentrator is linked by four PCM TDM highways 4 to respective group controllers 28 arranged in pairs in switching centres 5. The group controllers work independently in setting up connections which can be between customers on different concentrators. Each concentrator comprises a two stage switching network A and B (Fig. 6) with customers lines connected to the A switches and the outputs of the B switches connected to signalling units 13 in turn connected to PCM multiplexers 14. Each concentrator is arranged in sections; each section incorporates 3 A reed relay crosspoint switches and 5 B crosspoint switches interconnected by AB links. Each A switch consists of 25 5Î5 crosspoint arrays commoned at their inlets. Each 5Î5 switch array has access to 5 different B switches. The inlets (as viewed from the data highway side) of the B crosspoints are connected to 3 channels on each of 4 PCM systems. The outlets of each B switch are connected into the 25 inlets of an A array with 125 outlets. Thus each of 125 customers connected to an A array can be connected to 15 channels in each of four PCM systems, and 4 sections each containing 3 A arrays can handle 1500 customers. Signalling between concentrator and switching centre is of two types (i) customer and concentrator signalling in which customer's lines are scanned and marked at a slow rate and (ii) channel signalling in which, once a connection has been established between a customer and switching centre, the signalling unit in the concentrator signals data concerning the customer at a faster rate. The multiplex frames on each highway contain 32 channels (0-31), channels 0 and 16 being signalling channels whose significance is in accordance with the frame and multiframe number. Channel 0 in odd frames carries the frame sync pattern (a 7 bit code) and in even frames carries customer and concentrator control signalling, and superframe sync. For example, designating the multiframes within a superframe as MF0-MF3 and the frames within a multiframe as F0-F15, then channel 0 in MF0/F0 carries superframe sync, in MF0/F2 scan or route instruction, MF0/F4 carries section address, by flagging a particular bit position to indicate the required section address. Even frames in MF 1 and MF2 are used to signal block address, each column of a 5Î5 crosspoint array in an A switch being a block, bits 5 and 7 being flagged, e.g. MFI/F2 bit 5 signals block zero. Customer address (each row of a 5Î5 array in an A switch) is signalled using the first four bits of even frames in all multiframes, the first three representing Busy, Free and Mark condition and the fourth a Mark in the AB link. When signalling from concentrator to main switching centre, section address is given in the last four bits, 4 to 7 of MF0/F4. Channel 16 is used for multiframe sync in frame F0 of both odd and even multiframes. In frames F 1 to F15 in odd multiframes it is used for signalling between a main switching centre and a concentrator signalling unit and in even multiframes between a main switching centre and concentrator auxiliary unit. The control over the signalling unit (13, Fig. 6) which incorporates the A/D interface between the telephone data and the multiplexer 23 for the PCM highway, is such that the data will be inserted into the channel required by the switching centre. Auxiliary unit 12 provides an interface for special facilities, e.g. malicious call alarm, private metering, line testing &c. Four PCM highways are available for selection at each concentrator. On an individual highway there is provided after every four superframes a "hiccup" superframe in which no data is transmitted. This enables the superframes on the four highways to be staggered relatively in time such that there is no possibility of the same frame being simultaneously available on more than one highway, and thus no confusion arises in carrying out switching centre instructions. In the four multiframes of a superframe, the first carries superframe sync signals and the remaining three carry information indicating the superframe timing on the other three t.d.m. highways serving the concentrator. This enables the group control to successively signal to the concentrator via the different highways without confusion.