V.42bis

V.42bis is an adaptive data compression standard in common use nowadays. It is implemented by the MODEM to reduce the amount of sent data modulated (modulation and bandwidth being the bottleneck in today's dial-up connections). It uses an adaptive Huffman tree and a dictionary buffer (if I'm not mistaken) and may go to "transparent mode" in which data is transmitted uncompressed. This protocol also conveys MODEM commands.