Detection of variable message lengths for NATO Improved Link Eleven using CRC codes

An unconventional approach to the problem of binary message boundary determination is investigated for possible use on the NATO Improved Link Eleven (NILE), and compared with more familiar approaches. NILE is a HF/UHF data link between tactical computers. Frame check sequences (FCS), which are the error-detecting parity bits generated by cyclic redundancy check (CRC) codes, are used to determine between a small number of allowable message lengths. This is done by checking the validity of the FCS at each potential message length. An invalid FCS could mean either an incorrect message length, or a correct length with at least one bit error. The need to determine between these two possibilities leads to the development of several candidate algorithms for choosing the correct message length based on this information. The feasibility of these algorithms is demonstrated, and they are compared with a variety of conventional methods