ZyXEL Elite 2864 User Guide - Page 124

3) Automatic Re-transmission Request (ARQ) Error Correction Once a data block is received error-free, the receiving modem will acknowledge this block immediately. The sending modem receives the acknowledgement and will check if any block(s) is(are) not acknowledged. Let us assume 18 blocks were transferred and the first 14 blocks were acknowledged. If no acknowledgement for block 15 is received in a given time, this block must have been damaged. Acknowledgements of blocks 16 through 18 are ignored. Blocks starting from 15 are resent. 4) ARQ Error Correction with Selective Reject ZyXEL modems support V.42 error control with selective reject. Using this feature, the modem only resends the erroneous data block(s), not all the data blocks starting from the one in error (just block 15 in the above example). This saves retransmission time and greatly improves efficiency in high error rate situations. Error control protocols can detect and recover almost 100% of all errors to make an error-free transmission possible. Data Compression Data compression works by representing the original data information in less bits and transmitting the reduced data bits through the data link. The receiver recovers the original information by reversing the representing process. The process of representing original data in less bits is called redundancy removing. Its effectiveness is both algorithm and data dependent. A random data file is not compressible. A data file with a high degree of predictability, like an ASCII English text file, graphic file, or database file, is suitable for data compression. In the modem, the data compression is activated between asynchronous to synchronous conversion in an attempt to reduce the number of bits actually sent. The receiving modem applies these techniques in reverse to recover the actual data from the compressed data stream. 2864 series modems support both V.42bis and MNP5 data compression protocols. Data compression needs an error-free data link to work correctly, otherwise the corrupted compressed data stream will ruin the decompression process. MNP5 is used with MNP4 error control and V.42bis is used with V.42 error control. MNP5 data compression utilizes the run-length encoding and adaptive frequency encoding techniques. V.42bis uses a string coding algorithm. The compression efficiency of V.42bis is generally higher than that of MNP5. In some cases it can be 50% to 100% higher and in other cases it is just slightly higher. In general, it is about 50% more efficient. 10-2