User`s guide

AT Commands
Modem 3600 5-35
Dial Backup
Dial backup allows the modem to switch to a dial backup mode if the
data connection on the leased line is unacceptable for communications.
This can be accomplished in two ways:
• Automatic - backup due to extended loss of carrier or 4
unsuccessful retrains in 3 minutes
• Manual - user determined using front panel controls or AT
commands
In leased line operation, if both units have autodial backup enabled, one
must be configured for forced answer. This prevents both units from
dialing if the leased line fails.
IMPORTANT: Both methods will cause the modem to dial the
prestored autodial number. The originate modem will wait for five
seconds and then initiate the call. The answer modem will wait for a
ring. The modems then train and begin communicating over the dial-up
line. If the dial connection is unsuccessful after three attempts, a retrain
on the leased line will be initiated.
In manual mode, the return to leased line is only done when
commanded. In automatic mode the return to leased line is initiated after
the lookback time in register S28 has elapsed. To prevent unnecessary
termination of the dial line connection, a leased line lookback test is
performed. If the leased line is not acceptable, the dial connection is
resumed with a retrain. If the leased line is acceptable, the dial
connection is dropped and normal leased line mode is resumed.
When the unit attempts to return to leased line, the LCD displays
LEASE LOOKBACK. If the leased line has been restored to service,
data can be passed approximately 10 seconds after LEASE
LOOKBACK was initiated. The LCD will continue to display LEASE
LOOKBACK for slightly more than a minute. During this time the dial
line connection is maintained if a return to dial line operation is
required. When the LCD displays ON LINE again the dial line is
disconnected.
Note
A diagnostic test initiated during dial backup mode
terminates when the modem performs a leased line lookback.