Bit error ratio

Note 1: Examples of bit error ratio are (a) transmission BER, i.e., the number of erroneous bits received divided by the total number of bits transmitted; and (b) information BER, i.e., the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits.

Note 2: The BER is usually expressed as a coefficient and a power of 10; for example, 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 out of 10⁵ or 2.5 × 10^-5.

Note 3: On good connections you have an BER above 10E-9. The test time for a 95% confidence Level on a:
STM-256 / OC-768 = 1 sec
STM-64 / OC-192 = 3 sec
STM-16c / OC-48c = 12 sec
STM-4c / OC-12c = 48 sec
STM-1 / OC-3 = 3.2 minutes

Source: from Federal Standard 1037C and from MIL-STD-188