Technology is proposed for estimating one or more characteristics of a main signal transmitted via a communication link, wherein the main signal is composed from a number of component signals possibly comprising respective data signals and transmitted via a number of respective communication channels. The technology comprises providing an analyzing circuit independent from the communication link, transmitting a probe data signal in the analyzing circuit, monitoring a bit error rate (BER) parameter associated with the probe data signal and introducing into the analyzing circuit at least a portion of energy of the main signal as an interference signal. The technology allows judging about characteristics of the main signal based on changes in the monitored BER parameter of the probe data signal.