8 Further constrains: Packet independence and packet length

As an example of global constrains to the on-board compression we discuss the problems related to Packets Independence and Packets Length.

Data from the LFI must be put into equal size packets before being sent to Earth. Packets independence is considered to be a requirement, then each packet must be self-consistent, its loss or its erroneous transmission must not interfere with the data retrieval from subsequent packets. More over each packet must carry in "clear'' format (i.e. uncompressed) all the information needed to decode its content. That is: each packet must contain its own decoding table or decoding information. A typical packet length is about some hundred of bytes, but a smaller length may be planned if required; at the same time a typical decoding table holds something less than a hundred bytes leaving limited room for data.

In addition, for a fixed length $\mbox{$L_{{\rm u}}$ }$ of a random input stream (expressed in bits) the output $\mbox{$L_{{\rm c}}$ }$ will not be a constant but will change in time with respect to the averaged length $\mbox{$L_{{\rm u}}$ }/\mbox{$C_{{\rm r}}$ }$. Of course, it is not possible to predict in advance what will be the final length of a given bit stream. So either $\mbox{$L_{{\rm u}}$ }$ is held fixed, loosing in compression efficiency, or $\mbox{$L_{{\rm u}}$ }$ is adapted with some interactive method, maximizing the compression efficiency but at the cost of a significant slowing of the compression process.

In conclusion, the packets independence plus limited packet length prevents from sending the decoding table, leaving only two possibilities open: i) send the relevant bytes only (Maris 1999a), ii) to use a predefined coding table (Maris 1999b), both methods are described in the next section.