Experimental considerations on adaptive clock method for transmitting microprocessor-based current differential teleprotection signals in ATM networks

This paper describes experimental transmissions of severely time-critical teleprotection signals in ATM networks using the adaptive clock method developed by the authors, in order to assess its potential effectiveness. They examined the transmission delay variations of a commercially available terminal adapter with the method, which not only converts a constant-bit-rate teleprotection signal into ATM cells, and vice versa, but also transfers clock information from one device to another in an asynchronous manner. The experiments showed that one-way delay ranged from 1.7 to 1.94 ms, resulting in a maximum two-way differential delay of 240 μs that leads to synchronization errors for the simultaneous sampling of currents at remote stations. The variations were mainly due to the irregularity of read timing of the receive cell buffer in transient states such as just after turning on the power. A method to reduce delay variations by correcting the initial read timing using cell arrival timings is proposed