Patent GB-1285720-A (GB)
Title: Improvements in or relating to digital communication systems
Inventor: DUERDOTH WINSTON THEODORE
IPC: H04J3/06
Filed: 1968-12-18
Abstract: 1285720 Pulse-code modulation systems POST OFFICE 16 Dec 1969 [18 Dec 1968] 60201/68 Heading H4L Synchronization between switching centres with digital data links (using for example pulse code modulation techniques) is achieved by comparison of signals generated at two centres and representing the detected phase-difference of their clocks, and adjusting the frequency of the clock oscillator in one centre when the result of the comparison exceeds a fixed threshold. Fig. 1 shows exchanges A, B linked over lines 1, 2 and illustrates schematically the synchronizing arrangement for exchange A. Data received over 2 passes through 5 where its clocking pulses are derived to control through lines 11 and 13 the entry of the data into an eight-bit buffer store 7. Clock pulses on 11 advance 8 to enter bits into successive cells, and pulses on 13 act to reset 8. The local clock is derived from oscillator 3 which acts in similar manner to read out the data from 7, the time delay between reset pulses on lines 13 and 14 representing the number of bits held in the buffer. This is measured by using the "set" period of bi-stable 15 to gate pulses into counter 18, the most significant bits of which are compared with corresponding data from exchange B and held in stores 21 by a subtractor 23. If the difference from 23 exceeds a certain threshold a signal on line 27 opens gates 29, 30 to allow the sign of the difference to pass as an advance (A) or retard (R) signal to adjust the oscillator frequency. The A or R signals (if present) and the data from counter 18 are loaded into store 73 for transmission to exchange B, and any A or R signal from exchange B is also passed on to the R or A lines (the reversal is necessary since a relative advance of one oscillator is a relative retard of the other). If both A and R signals are present (one generated internally and the other received from B) a trigger 37 prevents any control signal passing gates 39, 40. The arrangement enclosed in broken lines D generates control-disabling signals- and "cut link" signals for the A-B link or other links of exchange A, in response to "out-of-limit" signals from threshold comparators for the links. These signals are generated when the output of subtractors such as 23 are greater than a second (and higher) threshold. In the arrangement illustrated the A-B link is of lower rank than the three other links to exchange A, and hence if any two links generate "out-of-limit" signals (on lines 68-70) and an A and R signal are also both present, gate 54 will pass a signal to inhibiting gates 43, 44 and to store 73. This prevents adjustment of oscillator frequency until higher-rank links have eliminated their out-of-limit signals. The Specification describes the assigning of appropriate ranks to a network of main, district and group switching centres, the use of different thresholds for different ranks, and special inhibiting methods of arranging for the inhibition of A and R signals so as to deal with lack of synchronization in particular nets of switching centres.