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Timing of GSM audio connections by generating UTC(LT) traceable digital time stamps

Technology for measurement of durations of GSM audio calls has been developed. Call duration was measured by using end-to-end method, making calls between two GSM terminals (Siemens MC35i). One GSM terminal was mobile and equipped with a test signal generator. Pseudo noise DTMF-like signals were used as test signals transmitted through the GSM channel. Receiving GSM terminal was connected to the PIC16F84A type microcontroller, which was used as a generator of digital time stamps at opening and closing of the GSM audio channel. Traceability of generated time stamps with UTC(LT) was assured by supplying the PIC microcontroller with pulse per second pulses (PPS) and 10 MHz clock signal from Cs clock HP5071A keeping UTC(LT) time scale. The uncertainty of generated digital time stamps was 103 mus. The obtained combined uncertainty of call duration measurement was about 100 ms. The technology developed was used for testing and verification of timing systems of GSM operators which are to meet the requirements of legal metrology.