Black Box 3600 MODEM Chapter Protocols, Ccitt V.42 bis Error Control Protocol, Reliable Mode

Chapter 6

Protocols

CCITT V.42 bis Error Control Protocol

V.42 bis is an industry standard for error control adopted by the Consultative Committee for International Telephone and Telegraph (CCITT). The CCITT V.42 bis protocol incorporates two error control algorithms, LAPM and MNP. LAPM is a CCITT Link Access Protocol family member related to LAPB and LAPD, currently in use in other communications applications. MNP is Microcom Networking Protocol, which has become an industry standard by the large number of its users.

The use of V.42 bis requires both local and remote modems to be V.42 bis compatible. Error control protocol is transparent to the user and requires no special hardware or software. Data to be transmitted is put in a buffer so the modem can retransmit it if an error occurs. The modem also buffers data received from the remote modem in case an error occurs and the data is retransmitted. To avoid overfilling the buffer, flow control is used to control data between the modem and the terminal. V.42 bis protocol options can be set byAT commands.

Note

Error control protocols are only valid when using asynchronous DTE options.

Reliable Mode

When an LAPM or MNP link is established, the modem is in reliable mode. V.42 bis allows negotiation with a remote modem to the highest level of protocol common to both units. Both LAPM and MNP control data errors by retransmitting any block of data that was corrupted in transit. LAPM is assigned highest priority, and, if not supported, then an MNP connection is attempted.

Modem 3600 6-1