12-6 CHAPTER 12: HANDSHAKING, ERROR CONTROL, DATA COMPRESSION, AND THROUGHPUT
.
Example: Sending AT&M4 allows your modem to operate in Normal Mode
is an ARQ connection can’t be made.
Error control is available for calls at 1200 bps and above. It can be
disabled, although high-speed calls (9600 bps or higher) should always
be under error control. The operations defined in an error-control
protocol include:
■Establishing compatibility.
■Formatting data frames.
■Detecting errors using Cyclic Redundancy Checking (CRC).
■Retransmitting corrupt data frames.
The Business Modem is set at the factory to try for an error-control
connection and, if that isn't possible, to proceed with the call in Normal
mode. This means that &M4 is set.
For your modem to Command
Have no error control (Normal Mode)
Due to the nature of phone line channels, this is never
recommended for analog calls above 2400 bps
AT&M0
Operate in Normal mode if an ARQ connection can’t be
made (Normal/ARQ mode) AT&M4
Hangs up if it can’t make an ARQ connection (ARQ
Asynchronous Mode) AT&M5