Accelerated measurement of bit error ratio

#This invention relates to measurement of bit error ratio (BER) in data transmission components and systems, and in particular to methods and apparatus for accelerating measurement of BER for components and systems which have a relatively low value of this parameter.

#In designing, manufacturing, deploying and maintaining data transmission systems, it is important to be able to test and measure various operational parameters. The accuracy of the measurements must be sufficient for the purpose for which they are made, but it is also desirable for the time taken for each measurement to be as short as possible, both for the sake of speed of the process involving the measurement and to ensure the measurement is available when it is required.

#One fundamental measurement in digital data transmission systems is bit error rate or ratio (BER), which is the number of erroneous bits in a digital data stream divided by the total number of bits transmitted, received, or processed over some stipulated period. As the data rate of transmission systems increases, it is typically necessary to reduce the BER of the system, so that the absolute number of errors does not also increase to an unacceptable level.

#A method and apparatus in accordance with this invention, for measuring BER at very low error ratios, will now be described, by way of example, with reference to the accompanying drawings, in which:

#FIG. 1 is a block schematic diagram of a known arrangement for measuring BER at very low error ratios; and

#FIG. 2 is a block schematic diagram of an arrangement for making such measurements in accordance with this invention.

#FIG. 1 shows an arrangement for accelerated measurement of BER in digital optical transmission systems, using a sinusoidal interference method. This method is described in more detail by P. Palacharla, J. Chrostowski, R. Neumann, and R. J. Gallenberger in a paper entitled “Techniques for accelerated measurement of low bit error ratios in computer data links”, Proceedings of the IEEE Fourteenth Annual International Phoenix Conference on Computers and Communications, Scottsdale, Ariz., Mar. 28-31, 1995, at pages 184-190, and in Standards Proposal No. 3696 dated May 26, 2000 of the Telecommunications Industry Association, “Accelerated Measurement of BER and Q-Factor in Digital Optical Transmission Systems Using the Sinusoidal Interference Method”.

#Referring to FIG. 1, a BER transmission pattern generator 10 supplies a test data signal to a laser transmitter 12. The optical output of the laser 12 is passed through an optical attenuator 14 and an optical combiner 16 to a photodiode optical receiver 18. The electrical output signal from the photodiode 18 is supplied to a BER receiver and error detector 20, which operates in known manner to derive a desired measurement of the BER of the data transmission link between the laser 12 and photodiode 18. In practice the BER generator 10 and the BER receiver 20 may be combined in a single piece of equipment.

#A sinewave generator 22 supplies a sinusoidal modulation signal to a second laser 24 which generates an optical interference signal that is added to the optical output from the laser 12 by the optical combiner 16. The addition of the interference signal increases the BER of the data transmission link in a controlled manner, enabling a measurement of the increased BER to be accomplished in a time which is sufficiently short for the measurement to be useful. The actual, very low BER of the link (absent the interference signal) can then be estimated by extrapolation.