3Com REMOTE ACCESS SYSTEM 1500, Management Manual

Page: 138 / 300

136 C
HAPTER
10: A
DVANCED
M
ODEM
C
ONFIGURATION WITH
CLI/AT
COMMANDS
Configuring Error
Control Options
Error control can be accomplished in different ways. Error control is
available for calls at 1200 bps and above. It can be disabled, although
high speed calls (above 2400 bps) should always be under error control.
The operations defined in an error control protocol include the following:
■
Establishing compatibility
■
Formatting data frames
■
Detecting errors using Cyclic Redundancy Checking (CRC)
■
Retransmitting corrupt data frames
CAUTION: High speed calls are vulnerable to errors unless the data is
protected by error control. The operations described below take place
even if the RAS 1500 is not configured for error control. If the modems in
an RAS 1500 module connect with a remote device at high speed, but
without error control, and if you are not using an error control protocol
for your call, you may lose data.
Error Control
The RAS 1500 is set at the factory to &M4, causing it to try for an error
control connection and, if that is not possible, to proceed with the call in
Normal mode. The RAS 1500 first tries to make a V.42 connection, then
an MNP connection. The following information is based on the setting of
&M4.
Automatic Repeat Request
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.
V.42 Error Control
This international standard includes a two-stage handshaking process:
■
A Detection phase that is based on an exchange of predefined
characters.
■
A LAPM (Link Access Procedures for Modems) negotiation phase,
during which the devices identify their capabilities concerning
maximum data block size and the number of outstanding data blocks
allowed before an acknowledgment is required.