threshold is no doubt a general concept.
But for the case of BER in communication theory, I beleive it is taken to be a mid-point , because the bits are taken to be equi-probable. i.e, bits 0 and 1 have equal probability of being transmitted. And at the Rx, if the amplitude is above mid-point, it is considered as 1 and if it is less than the mid-point value, it is considered as 0.
And For Ranjeet, I would suggest you to post a screenshot of the BER Analyzer results where in there is some value. In the image you have attached, no data has been received or atleast detected. So not much can be inferred about how Optisystem has programmed the BER Analysis.