3Com U.S. Robotics 56K Voice, Command Reference Manual

Page: 101 / 210

Error Control 12-5
Lower-speed V.
Protocols
Older, lower-speed "V." protocols do not employ line probing. Instead
they use predefined answer tones to specify or identify speed capabilities.
These protocols define the following maximum speeds:
■
V.32terbo: 19.2 Kbps, with an additional USR device-to-USR device
speed of 21.6 Kbps.
■
V.32bis: 14.4 Kbps.
■
V.32, V.22bis, etc.: 9600 bps and lower.
ASL (used in V.32terbo and V.32bis modes) is a technique that allows the
Business Modem’s receivers and transmitters to act independently of each
other. One transceiver may slow down and then speed up without
affecting the data flow on the other. The result is more efficient line
operation.
While most modems on the market now use higher speeds, there may be
a problem in answering older, V.32 modems at 9600 bps. Use Register
S28 to modify the duration of the extra tones used in V.32 negotiations,
in the rare instance that this may be necessary. See Appendix A,
S-Registers.
Error Control
Business Modems can employ error-control techniques during data
connections.
High-speed data calls are vulnerable to errors unless the data is protected
by error control. If your Business Modem connects with a remote device
at a high speed, but without error control, and if you are not using an
error control protocol for your call, you may lose data.
Automatic Repeat Request (ARQ) is a method used in many error-control
protocols to ensure that any data that has been corrupted in transit is
retransmitted. We use the term to designate a connection under error
control.
Error-Control
Commands
You can use Error-Control commands to enable ARQ (error control). The
Business Modem and the remote device
must use the same protocol.