Patent GB-1393309-A (GB)
Title: Pcm time-division-multiplex telecommunications networks
IPC: H04Q11/04
Filed: 1972-11-10
Abstract: 1393309 Multiplex pulse code signalling SEIMENS-ALBIS AG 10 Nov 1972 51906/72 Heading H4L Means are described for calculating the capacity of the buffer store to prevent the storage time from falling below a predetermined limit and also to prevent overflow of the store in a system for interconnecting PCM t.d.m. exchanges each having their own independent clock, each exchange being provided with a buffer store and bit-frequency differences being compensated for by the use of blind bits, i.e. bits which carry no information. As shown in Fig. 1, exchanges Vst1 and Vst2 are connected by lines L and operate with bit frequencies f1, f2 respectively. Each line includes a delay V1, V2 and terminates in a buffer store P1, P2 respectively. If in a buffer store, the storage time falls below a lower limit (the blind-bit limit), the associated exchange generates a blind-bit by interrupting the read-out from the buffer store for the duration of one bit, whereby the storage time increases. A remote exchange on receiving a blind-bit interrupts the writing into its buffer store for the duration of said bit whilst continuing the read-out, thus reducing the storage time. The difference between the maximum and minimum bit frequencies in the system equals the maximum blind-bit frequency. A blind-bit is transmitted by inserting it between a signalling or synchronizing channel and a PCM information channel and is announced by a bit of the preceding signalling or synchronizing signal, Fig. 2 (not shown). Lines L have a transmission time #, delay elements V1, V2 each have a transmission time x and stores P1, P2 have storage times # 1 , # 2  respectively and the total (forward and return), propagation time is made equal to an even number of frames expressed in bits. The instantaneous storage time of both stores P1, P2 must not be allowed to fall below the blind-bit limit, thus producing simultaneously mutually compensating blindbits. This is prevented by arranging that the bit storage capacity of each store must not be less than twice the blind-bit limit plus one bit period multiplied by the maximum divided by the minimum bit frequency. In addition, an overflow of a store must be prevented by adding to the previous quantity the maximum number of blind-bits which can be generated on the associated line during a time interval corresponding to the total propagation period and further adding the maximum variation (expressed in bits), of the total transmission time which can occur in use. Further, the blind-bit limit may be arranged so that with the maximum permissible bit frequency of the associated exchange and the minimum permissible bit frequency of the remote exchange, when blindbits are transmitted the buffer storage time between two successive blind bit generated at a minimum interval remains greater than zero. This is achieved by making the blind-bit limit equal to the minimum interval between two blind-bits (expressed in bits), multiplied by the ratio of maximum and minimum bit frequency and reduced by one bit. The writing in to a buffer store is controlled by a counter operating at the external bit rate freed from the received blind-bits (external counter), and the read-out is controlled by an interval counter operated at the internal bit rate, the read-out for all buffer stores at an exchange being effected simultaneously. Means are described, Fig. 4 (not shown), for preserving the correct phase relationship between the external and internal counter. The setting of the internal counter is compared at the start of an internal frame with that of the external counter at the start of an external frame and if these settings do not correspond the external counter is delayed by one bit period, this process being repeated until the counter positions correspond.