15
V.42 is the ITU error correction protocol, also known as LAPM. V.42bis adds BTLZ data compression
to the V.42 protocol. It offers compression of up to 4:1. V.42bis connections require V.42 error
correction.
Both your modem and the remote modem must support the same data compression/error
correction standards if they are to make a data compressing/error correcting connection.
Using ZModem, XModem, or Pre-Compressed Files
Error Correction:
It is usually advantageous to use MNP 2-4 or V.42 error correction for ANY file transfer, regardless of
compression or software-based error correction standards such as Zmodem, Xmodem, etc. The error
correction standard strips start and stop bits from each character (to improve throughput) and
performs error correction on smaller chunks of data than software-based error correction standards.
Thus if an error does occur, less data has to be retransmitted, which takes less time.
V.42bis Data Compression (Mode Default)
V.42bis recognizes that there is no benefit to compressing software-compressed data further, so it
does not attempt to use V.42bis compression if no additional compression is possible. Use V.42bis
rather than MNP 5 whenever possible to receive these benefits.
For better performance, disable MNP 5 data compression (
%C
) when using an MNP Reliable
connection and transferring files that are already compressed by compression software such as ARC,
.ZIP, or StuffIt. When files compressed by these programs are transferred using MNP 5, the files may
take longer to transfer than files transferred using a Reliable connection and no data compression.
The default settings in your modem require the modem first attempt to establish a V.42/V.42bis
connection, and only attempt an MNP connection if the V.42/V.42bis connection is not possible.
C
ONNECTION
T
YPES
All Normal and Reliable connections use flow control, which allows your computer and modem
rates to be different. With Direct connections, the rates must be the same.
You can only use data compression if you have made an error-correcting connection (MNP or
LAPM) with another modem that supports data compression. MNP 5 is only used with MNP error
correction, and BTLZ is only used with LAPM error correction. If a data compression connection
cannot be made, or if data compression has been disabled, only error correction is attempted.
The commands for controlling connections (not to be used during an active connection) are listed
in AT Commands, under
AT\N
and
AT&Qn.
