User`s manual

U.S. Robotics I-modem Command Reference
Handshaking, Error Control, Data Compression, and Throughput • Page 11-11
Error control is available for calls at 1200 bps and above. It can be disabled, although
high-speed analog calls (9600 bps or higher) should always be under error control. The
operations deÞned in an error-control protocol include:
¥ Establishing compatibility.
¥ Formatting data frames.
¥ Detecting errors using Cyclic Redundancy Checking (CRC).
¥ Retransmitting corrupt data frames.
The I-modem is set at the factory to &M4, causing it to try for an error-control connection
and, if that isn't possible, to proceed with the call in Normal mode.
The I-modem Þrst tries for a V.42 connection, then an MNP connection. The following
information is based on the I-modem's setting of &M4.
V.42 Error Control
This international standard includes a two-stage hand-shaking process:
¥A Detection phase that is based on an exchange of predeÞned characters.