Abstract: Digital data, in the form of blocks delimited by end-of-block words, that is to be coded into variable length codewords, is protected against errors occurring within a block, as well as errors crossing block boundaries, by modulating the respective end-of-block words with a bit, or bits, of a larger reference pattern which bit, or bits, have been conditioned to carry information on the content of VLC codewords coding a corresponding (eg. preceding) data block. The "modulation" may be by means of selecting between different possible end-of-block words dependent upon the value of the conditioned reference pattern bit, or bits, or by means of appending the conditioned bit(s) to the end-of-block word. The reference pattern may be a repeated pseudo-random sequence.

Abstract: An optical time-domain reflectometry method of sensing respective values of a physical parameter, such as temperature, at different locations along an optical fibre, in which back-scattered optical radiation is used to produce output signals indicative of the values being sensed, is improved by employing optical filtering means to ensure that the back-scattered radiation used is restricted to a preselected single spectral line resulting from inelastic scattering in the fibre, for example one of the Anti-Stokes Raman lines, or to two mutually adjacent such lines.