A method of encoding fixed length data bit strings includes receiving and sequentially encoding a sequence of data bit strings. For a data bit string immediately following a preceding data bit string, this includes obtaining a reference bit string, and a mask bit string and a tracking bit string as present at the end of a previous encoding of the preceding data bit string, identifying bits that differ form corresponding bits in the reference bit string, determining, as unpredictable bits, all those bits in the data bit string that are indicated as not predictable by the mask bit string and are not in bit positions indicated by the tracking bit string, generating a sequence of position indicators, generating an encoded data packet that includes representations of values of the unpredictable bits and the sequence of positon indicators, and periodically updating the mask bit string and the tracking bit string.