# The difference between BER and BER

-September 05, 2014

In 2013, I posted a poll on DesignCon Community—the predecessor to DesignCon Central—asking which definition for BER engineers used. The choices were bit-error rate or bit-error ratio. The poll results show that 84% of respondents say bit-error rate.

That confirmed what I suspected. In general, Keysight Technologies uses "bit-error ratio" when referring to their BERTS. Take, for example, the N4903 J-BERT. The data sheet uses "ratio." In contrast, Tektronix and Anritsu say "bit-error rate."

All this time, I assumed that bit-error rate and bit-error ratio were identical. Now, I see further.

The other day, I opened my copy of Digital Communications Test and Measurement by Derickson and Müller, looking for questions for my weekly quiz on EE Times. On Page 170, I learned the difference between BER and BER.

According to the authors, "The bit error ratio (BER) is a measure of the percentage of bits a system does not transmit or receive correctly." They go on to say "Instead of viewing BER as a percentage, we can also regard it as a probability for a single bit to be received in error. Either way, we can calculate the average number of errors in any number of transmitted bits as follows:
NErr = NBits × BER."

On page 171, the authors write "A second measure for the error performance of a digital transmission system is the bit error rate. It's different from the bit error ratio in that it relates the number of errors to the test time, rather than the number of bits compared during a test.
BERate = NErr / t."

Derickson and Müller conclude the section by saying "The terms bit error rate and bit error ratio are often mixed up in everyday useage, especially since the BER acronym is used for both. However, since the bit error rate is hardly used in practice these days, it is usually safe to assume that the bit error ratio was intended."

Bit-error rate is hardly used? The poll results seem to contradict the authors. But, this raises more questions. For example, if you measure BER on the same system with a Keysight BERT and with a Tektronix or Anritsu BERT, will your results differ? Even worse, what if you use a Keysight BERT and your customer use an Tektronix or Anritsu BERT and the results differ? Have you seen this happen? Who's right?

Now I pose this question to BERT manufacturers. Does Keysight use the ratio method while Tektronix and Anritsu use the rate method? I really want to know.

Martin Rowe, Senior Technical Editor