High dynamic range error rate monitor

There is provided an error rate monitor (10) incorporating a counter (30) which is incremented as errors are detected in a data stream passing through an interface unit (20) connected to the counter (30). The counter (30) is also simultaneously decremented at a rate proportional to an exponential of a value presently stored in the counter (30). The monitor (10) thereby provides from the value stored in the counter (30) a measure of error rate in the data stream, the value substantially proportional to a logarithm of error rate in the data stream, thereby providing the monitor (10) with a relatively large error rate monitoring dynamic range.