ZyXEL Elite 2864 User Guide - Page 125

Run-length Encoding Run-length encoding is applied in an attempt to avoid sending long sequences of repeated characters (data). When three or more repeated characters appear in succession, only the first three tokens (representing the compressed format of that character) and a repetition count will be sent. Adaptive Frequency Encoding Adaptive frequency encoding is applied after removing repeated characters (data). In adaptive frequency encoding, a token is substituted in the data stream for the actually occurring character in an attempt to send fewer than 8 bits for each character. The token is generated from a dynamic tabulation of character appearing frequency. The total number of available tokens is 256, of which only the first 32 tokens are smaller than 8 bits, so random data will gain no advantage from this technique. String Coding Instead of sending each data character individually, a token for a character string is sent. The modem adaptively builds a dictionary of string tokens according to data that appears. 2864 series modems support a dictionary size up to 2K string tokens. The input data characters are combined and checked for a matching string in the dictionary. The token is sent for the longest matched string. Compressibility is high if there are some regularities of character pattern in the data. For the 2864 series of modems, the error control and data compression option can be enabled either from the front panel or the terminal. Control Level NONE MNP4 (MNP3) MNP4 + MNP5 V.42 (MNP4) V.42(b) + MNP4 (5)* AT Command AT&K0 AT&K1 AT&K2 AT&K3 AT&K4 Error Control none MNP4 MNP4 V.42 / MNP4 V.42 / MNP4 Data Compression none none MNP5 none V.42bis / MNP5 *. default &K4 is the default. For some applications, like BBSs, where transmission of already compressed files is usual, the MNP5 data compression process actually produces more redundant data and slows down transmission. Setting S38b5 will disable MNP5 protocol negotiation regardless of the error control setting. 10-3