ZyXEL Elite 2864 User Guide - Page 32

V.25bis Command Set V.25bis is a set of auto-dialing commands defined by the ITU-T. V.25bis supports both synchronous and asynchronous data interfaces and is mainly used in Europe and in IBM mainframe or mini-environments which use synchronous communication. Error Correction In synchronous data communication, data is checked and corrected in the host by the so-called "link layer protocol" to ensure data integrity. Normally, no data checking is provided in the host for asynchronous data communication. However, file transfer protocols which include error checking are available with asynchronous hosts. Nowadays, intelligent modems incorporate error correction protocols inside the modem for asynchronous data communication and it is transparent to the host. The modem packs the asynchronous data characters into blocks. The data is transmitted as synchronous data between the modems. The data blocks are error checked at the remote end, and an erroneous block is retransmitted. The commonly used error correction protocols are MNP® and V.42 protocols. Data Compression Intelligent modems use redundancy removing methods to reduce the number of data bits actually sent for asynchronous character transmission. The full character data is recovered at the other end, thus the total throughput of the modem data transmission is increased. The data compression protocols commonly used by modems are MNP® class 5 and V.42bis. (See definitions below.) Data compression works on redundancy removing and its efficiency is dependent on both the compression algorithm and data statistics. MNP® Protocols MNP® (Microcom Network Protocol) is a set of protocols first introduced by Microcom, Inc., and later used by many modem manufacturers. It consists of many classes. Classes 1 to 4 are for error correction, and class 5 is for data compression. MNP class 1 and 2 are obsolete. Normally class 4 is used for error correction and it uses class 3 internally. MNP class 5 is a data compression protocol with a maximum efficiency of 2 to 1. MNP-5 is used with MNP-4 for error correction. 2-6