3Com Courier, Справочник

Страница: 101 / 204

Error Control 13-5
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.32
terbo
:
16.8 and 19.2 Kbps, with an additional US
device-to-USR device speed of 21.6 Kbps.
■
V.32 bis : 14.4 Kbps.
■
V.32, V.22 bis , etc.: 9600 bps and lower.
ASL (used in V.32
terbo
and V.32
bis
modes) is a technique that allows the
Courier V.Everything 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
Courier V.Everything modems can employ error-control techniques during
data connections.
High-speed data calls are vulnerable to errors unless the data is protecte
by error control. If your Courier V.Everything 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) or
enable synchronous protocols. The Courier V.Everything modem and the
remote device
must use the same protocol.