The Uncompress Application on Distributed Communications Systems

The most popular compressors are based on Lempel-Ziv coding methods. Zip compressors and Unzip decompressors apply the sliding window method, while other applications as Compress and Uncompress under Unix and Linux platforms use the so-called LZW compressor and decompressor. LZW compression is less effective but faster than the zipping applications. We face the problem of how to implement Lempel-Ziv data compression on today's large scale distributed communications systems. Zipping and unzipping files is parallelizable in theory. However, the number of global computation steps is not bounded by a constant and a local computation approach is more advantageous on a distributed system. Such approach might cause a lack of robustness when scalability properties are required. Differently from the Zip compressors, the LZW encoder/decoder presents an asymmetry with respect to global parallel computation since the encoder is not parallelizable while the decoder has a very efficient parallelization. We show that, in practice, the number of iterations of the LZW parallel decoder (Uncompress) is much less than ten units. Since scalability and robustness are generally guaranteed if bounding the number of global computation steps is possible, LZW is more attractive than Zip for distributed communications systems in those cases (which are the most common in practice) where compression is performed only once or very rarely, while the frequent reading of raw data needs fast decompression.