Patent GB-1467823-A (GB)
Title: Time division switching networks
IPC: H04Q11/04
Filed: 1974-03-07
Abstract: 1467823 Passing data envelopes through t.d.m. switching networks J-Y QUEFFEULOU 7 March 1974 [8 March 1973] 10330/74 Headings H4M and H4K In a time division multiplex switching network through which synchronous digital information is transmitted in the form of n-bit PCM words (e.g. of 8 bits) in allotted time slots of TDM frames, data in the form of envelopes having a greater number of bits m than the time slot capacity n, is also transmitted in the time slots. Each envelope includes an envelope framing bit, a status bit and m-2 data or signalling bits (depending on the value of the status bit). The envelopes are reconstituted after passing through the network from the sequence of n bit words from the allotted time slots, due account being taken of the frame shifts through the switching network (equal to an integral number n f  of TDM frames) experienced by the words in at least some time slots. The bits in the sequence are formed into m bit groups, each having an envelope framing bit, by a shift register, the groups being selectively delayed by 01 ... (m-1) bits. Local envelope framing bits are generated and compared with the envelope framing bits of the delayed groups, the result of this comparison controlling the delay. The integral number n f  is stored and an additional delay nf (m-n) bits is applied to the groups to allow for frame shifts through the network. Received allotted time slots are selected alternately by the units 301, 302, even slots going to the unit 302, under the control of time slot frequency signals from the time base 300. The words pass serially to a 10 position shift register 305 (assuming that the data envelopes originally held 10 bits) which provides a number of serial outputs, the words being delayed between 0 and 9 bits, under the control of bit frequency signals from time base 300. The register outputs pass in parallel fashion to three further registers 306-308, the connections to these being such that register 306 receives the various delayed outputs in the same order as that in which they leave register 305 but registers 307, 308 receive them with a two and four bit shift respectively. An output from register 306 is selected by an address register 324 and compared at 320 with local envelope framing bits supplied by the time base 300, these bits being provided at the frequency of a multiframe containing both a whole number of TDM frames and a whole number of data envelopes (only part of a data envelope will be transmitted in each TDM frame). The comparator output controls a bi-stable 323 and thus the address register 324 so that all 10 bits in the register can be successively compared with the envelope frame. The search goes on through successive even time slots until the envelope framing bit in the received data is located, when the system locks to this and the address register stops. To enable data to be distinguished from signalling information when the data envelopes may contain either, the particular word in the register 305 can be read-out over line 309 by gate 334. This is controlled from a logic matrix 332 and address responsive circuitry 333, in dependence on the local envelope and PCM time slot signals from the time base 300 to provide new time slot signals related to slots in the data envelope. The output from the gate 334 is applied to the control unit 41. In the control unit is stored a record of the delay experienced by TDM frames passing through the switching network. This is applied via register 329 to a decoder 330 which selects appropriately the output from one of registers 306-308; depending on whether the delay was 0, 1, or 2 frames. The selected output is applied at the unit 40 to the data switching network.