Comtech EF Data SMS-758 - Manual (Page 54)

Operation SMS-758 Modem Protection Switch
3–16 MN/SMS758 Rev. 3
In the AUTO mode, the switch monitors the prime and backup modulator and
demodulator fault inputs, awaiting a failure. When no faults are present, the switch will
place the backup modulator(s) and demodulator(s) in the “Hot Standby” mode for the
highest priority prime(s), and a dash (-) will be displayed in the status display. In this
mode, the switch programs the backup modulator(s) and demodulator(s) for the highest
priority, lowest numbered prime modulator(s) and demodulator(s), and connects the
downlink IF to the backup demodulator(s). This action prepares the backup modulator(s)
and demodulator(s) for a minimal delay during the backing up of high priority channels.
When a fault does occur, the switch takes action according to the backup algorithm. The
backup algorithm selects the highest priority, lowest numbered faulted channel for
backup, and will take a lower priority channel backup offline, even if that channel is still
faulted, to back up a higher priority channel.
When a demodulator backup is required, the switch performs the following steps:
1. It verifies the configuration of the backup demodulator, and re-programs it, if
necessary.
2. If more than one demodulator is faulted, the switch selects the demodulator with
the highest priority. If more than one faulted demodulator is set to the highest
priority, the switch selects the demodulator with the lowest channel number.
3. The switch implements the sweep delay (it is not programmable by the user).
This delay is two times the sweep period of the demodulator. At the end of the
sweep delay or when the backup demodulator locks, the switch performs one of
the following operations:
a. If the backup demodulator fails to lock, the switch makes the back-up
demodulator available for other faulted channels.
b. If the backup demodulator locks, the switch commits the backup
demodulator to the faulted prime and starts the demodulator delay timer. An
asterisk (*) will be displayed in the status display if modem switch firmware
is installed. (Continue to Step 4.)
4. At the end of the demodulator delay, the switch again checks the fault status of
the prime demodulator.
a. If the prime demodulator still shows a fault, the switch places the backup
demodulator online for the faulted prime, and an arrow will be displayed in
the status display. The backup will then be available only to failed primes
having higher priority.
b. If the prime demodulator locks during the demodulator delay, the backup
demodulator is made available for other faulted primes.