Asymmetric compression

Consider a wireless client similar to the Skiff exchanging English text with a server. All requests by the client should be made with its minimal-energy compressor, and all responses by the server should be compressed in such a way that they require minimal decompression energy at the client. Recalling Figures 4 and 5, and recognizing that the Skiff has no low-power sleep mode, we choose ``compress-12'' (the twelve-bit codeword LZW compressor) for our text compressor as it provides the lowest total compression energy over all communication speeds.

To reduce decompression energy, the client can request data from the server in a format which facilitates low-energy decompression. If latency is not critical and the client has a low-power sleep mode, it can even wait while the server converts data from one compressed format to another. On the Skiff, zlib is the lowest energy decompressor for both text and web data. It exhibits the property that regardless of the effort and memory parameters used to compress data, the resulting file is quite easy to decompress. The decompression energy difference between compress, LZO, and zlib is minor at 5.70Mb/sec, but more noticeable at slower speeds.

Figure 9 shows several other combinations of compressor and decompressor at 5.70Mb/sec. ``zlib-9 + zlib-9'' represents the symmetric pair with the least decompression energy, but its high compression energy makes it unlikely to be used as a compressor for devices which must limit energy usage. ``compress-12 + compress-12'' represents the symmetric pair with the least compression energy. If symmetric compression and decompression is desired, then this ``old-fashioned'' Unix compress program can be quite valuable. Choosing ``zlib-1'' at both ends makes sense as well - especially for programs linked with the zlib library. Compared with the minimum symmetric compressor-decompressor, asymmetric compression on the Skiff saves only 11% of energy. However, modern applications such as ssh and mod_gzip use ``zlib-6'' at both ends of the connection. Compared to this common scheme, the optimal asymmetric pair yields a 57% energy savings - mostly while performing compression.

It is more difficult to realize a savings over symmetric zlib-6 for web data as all compressors do a good job compressing it and ``zlib-6'' is already quite fast. Nevertheless, by pairing ``lzo'' and ``zlib-9,'' we save 12% of energy over symmetric ``lzo'' and 31% over symmetric ``zlib-6.''

Figure 9: Choosing an optimal compressor-decompressor pair