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CCITT V.42 BIS ERROR CONTROL PROTOCOL
V.42 bis is an industry standard for error control adopted by the Consul-
tative 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 communi-
cations applications. MNP is Microcom Networking Protocol that has
become an industry standard by the 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 by AT commands.
Note
Error control protocols are only valid when using asynchro-
nous DTE options.
RELIABLE
When a 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.
AUTO-RELIABLE
In auto-reliable mode the modem negotiates to the highest protocol
(LAPM or MNP) common to both modems. However, if a reliable
connection cannot be established, auto-reliable allows the protocol to
fallback to normal mode.
V.3400
Chapter 6
Protocols
6-1
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