Comtech EF Data CRS-100 - Manual (Page 28)

Functional Description CRS-100 1:1 Redundancy Switch
4–2 Rev. 1.2
hence lose approximately 3.5 dB in output power level) the CRS-100 does not introduce
any attenuation of output signal level.
The Rx IF signal is fed to both units simultaneously, using an internal power divider.
This does introduce a loss of approximately 3.5 dB, but given the wide dynamic range of
the demodulator in both the CDM-500 and the CDM-550, this is not considered to be a
problem. The advantage of this scheme is that the demodulators in both Online and
Standby units are locked, and therefore, if a switchover does occur, there will be no delay
while waiting for the demodulator to acquire lock, which greatly speeds the time for the
switchover to occur.
Fault status information is fed from each of the two modems, via the Data Interface
connector. The Controller State Machine decides, based on the fault status, which of the
two units, ‘A’ or ‘B’ is to be the Online unit. It will assert a control signal to the Standby
unit, which mutes its Tx IF carrier, and simultaneously indicates to the microcontroller
within the Standby unit that the unit is no longer ‘Online’. This results in the ‘Online’
LED on the front panel of the unit being extinguished. This status is also reported over
the remote control bus, so an external M&C system can determine the state of the
redundancy system. At the same time, a GREEN LED will illuminate on the front of the
CRS-100 to indicate whether the ‘A’ or ‘B’ unit is Online.
The User determines the conditions which cause an automatic switchover. This is
controlled by two switches at the side of the unit, and the User can select between Unit
faults only, Unit faults or Receive Traffic faults, Unit faults or Transmit Traffic faults, or
all three. This is covered in detail in Section 7. With the ‘bridging’ architecture of the
CRS-100 (whereby identical traffic signals are routed to both Online and Standby units)
the Controller State Machine can avoid un-necessary switchovers. By examining the fault
status of both units, it can infer if the fault is external to the system.
For example, suppose that the CRS-100 has been configured to switch following Unit
faults or Transmit Traffic faults, and that the modems are configured for external clock
operation. Now suppose that the external equipment (network, mux, router, etc) fails.
The standby and the Online units will show a Transmit Traffic fault (No Clock Detected
from Terrestrial Port). The CRS-100 Controller State Machine will see that these faults
have occurred at the same time (in fact, within a 0.5 second window) and infers that the
fault is external. Therefore, no un-necessary switchover is initiated.