Ascom Tech Ag, Gesellschaft Fur Industrielle Forschung + Technologien

Thomas Martinson

Rainer Fehr

Fabrice Bonvin

In digital data packets comprising a first datum from which a second datum can be unequivocally computed, e.g. an error correction value, one or a plurality of additional data can be introduced by manipulating said second datum without altering the structure of the data packet. For this purpose, one or a plurality of bits of the second datum are modified in an unequivocally reversible manner, e.g. inverted, according to the additional datum which is also digital. The recuperation of the additional data is effected by comparing comparative values which are obtained by reversing all possible combinations of additional data in a received data packet to a value which is computed from the first datum of the data packet: The presence of the corresponding combination of additional data results from the consistency of the latter with one of said comparative values. Another possibility is to limit the inquiry to a fraction of the possible combination. A preferred application is the recognition of endlessly circulating ATM cells in a ring-shaped communication network. The one-bit additional datum is inserted in the HEC byte by a monitor node, and each ATM cell which reaches said monitor node and includes said additional datum is recognized as being undeliverable.

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Freiburg, Switzerland

Thomas Martinson