# BER – Is it Bit Error Rate or Bit Error Ratio?

One of the most important ways to determine the quality of a digital transmission system is to measure its Bit Error Ratio (BER). The BER is calculated by comparing the transmitted sequence of bits to the received bits and counting the number of errors. The ratio of how many bits received in error over the number of total bits received is the BER. This measured ratio is affected by many factors including signal to noise, distortion, and jitter.

And there we have our answer – the correct term is Bit Error Ratio. However, BER is commonly referred to as Bit Error Rate, referring to the number of bit errors per unit time, which you can see is not the same as the equation above.

The most obvious method of measuring BER is to send bits through the system and calculate the BER at the receiving end. Since this is a statistical process, the measured BER only approaches the actual BER as the number of bits tested approaches infinity. Fortunately, for most cases, we just need to test if the BER is less than a predefined threshold. The number of bits required to accomplish this will only depend on the level of confidence needed and BER threshold. The confidence level is the percentage of tests that the system’s true BER is less than the specified BER. Since we cannot measure an infinite number of bits, and it is impossible to predict with certainty when errors will occur, the confidence level will never reach 100%.

To calculate the confidence level (CL), we use the equation:

For our purposes, we will only be concerned with the case where there are zero errors detected. This equation can be rearranged to calculate the number of bits required for a given BER and confidence level (CL).

As an example, if our specified BER is 10-12, and we require a typical confidence level of .95 (95%), the required number of bits to test without any errors is 3x1012. Once we have tested Nbits = 3x1012 without error, we can be sure that our actual BER is less than 10-12. Whether the actual BER is 10-12, 10-15, or 3.1x10-14 is unimportant. Again, note that this value is independent of the data rate that the bits are being tested at. To determine the test time required, the number of bits to be tested is simply divided by the bit rate (bits/second). Resulting in the equation:

At a standard 95% confidence level, we can substitute 0.95 in for CL and obtain a very useful function:

A bit error ratio tester (BERT) is used to measure BER to a given confidence level. BERT solutions allow the most accurate and efficient design verification, characterization, compliance, and manufacturing test of high-speed communication ports.

Learn more by checking out Keysight’s M8000 Series Bit Error Ratio Testers.