By definition Bit Error Ratio (BER) is
number of wrong elementary signals BER = --------------------------------------------- total number of received elementary signalsWhen measuring bit error ratio transmitter of the measuring equipment sends a known measuring signal to the link. In the receiver the received signal will be compared to the measuring signal. We count the differing elementary signals and the received elementary signals.
Similar to the bit error ratio one can define error ratio for composite of the elementary signals (character, block, message, etc.), too.
Bit error measurers use Pseudo Random Binary Seguence (PRBS) as a measuring signal sequence. The transmitter and receiver of the bit error measurers use PRBS generators of the same construction. At the beginnig of the measurement a synchonizer circuit drives the receiver generator to the same position as the transmitter, then the comparision can be started. Synchonizer circuit interrupts the loopback of the PRBS generator of the receiver and refill its shift register with the bits of the received PRBS signal, which was sent by the transmitter. Then closes the loopback again and checks the existence of the synchronized state.
In addition to the displaying of the BER bit error measuring equipments visualize the number of errored bites received during the measurement in tabular form numerically and in histogram form graphically.
Beyond the above mentioned visualization measuring equipments can interpret the measuring results even more form. E.g.: