 The properties of metering signals differ between the telephone administrations in various countries.Some systems use alternating current pulses superimposed on the direct current (battery) levels of the local loop, while others may use a reversal of tip-ring polarity of the wire pair.Common frequencies of the A.C. pulses are 50 Hz, 12 kHz, and 16 kHz, with typical durations of the signal varying from several tens to hundreds of milliseconds.