The Bit Error Rate tester generates a pseudo random bit stream. It also accepts such a pattern, synchronises to it, and checks the bit error rate in this stream.
The Bit Error Rate tester generates a bit stream, with a repeating 2047 bit pseudo random pattern, using an 11 stage polynomial generator. It also accepts such a pattern, synchronises to it, and checks the bit error rate in this stream. If the error rate is excessive the tester assumes synchronisation has been lost, and it attempts to resynchronise with the stream.
The bit error rate is continuously assessed against decadic ranges - > 1 in 10^2 > 1 in 10^3 > 1 in 10^4 > 1 in 10^5 > 1 in 10^6 > 1 in 10^7 < 1 in 10^7 To ensure fairly smooth results from this assessment, each decadic level is assessed over 10/error rate bits. That is, to assess if the signal's BER is above or below 1 in 10^5 the software looks over 10*10^5 => 10^6 bits.