Error Control 12-7V.42 Error Control The Business Modem first tries for a V.42 connection, then an MNPconnection. The following information is based on the Business Modem'ssetting of &M4.This international standard includes a two-stage hand-shaking process:n A Detection phase that is based on an exchange of predefinedcharacters.n A Link Access Procedures for Modems (LAPM) Negotiationphase, during which the devices identify their capabilities concerningmaximum data block size and the number of outstanding data blocksallowed before an acknowledgment is required.MNP Error Control The Microcom Networking Protocol (MNP) is supported by the ITU-T V.42Recommendation. MNP is based on special protocol frames. If the remotedevice doesn't recognize an MNP Link Request, error control isn'tpossible. (In HST asymmetrical mode, 3Com devices use a proprietaryscheme similar to MNP.)Error Control andFlow ControlFlow control of data from the computer is required under error control fortwo reasons:n The transmitting device buffers a copy of each frame it transmits tothe remote end until it is acknowledged by the receiving device.n If errors are encountered, retransmission activity can cause a steadystream of data from the computer to overflow the buffer.
