Comtech CRS 1:1 Series Redundancy Switches for Legacy Modems

NTRODUCTION

Their purpose is to continuously monitor a pair of
modems in a redundant configuration, so that the unit
automatically switches data and IF signals from the
failed unit to the standby unit if an equipment failure or
undesired traffic condition occurs.
This fully protects traffic paths, and the operator can
have increased confidence that equipment failures will
not adversely affect system availability.
There are two types of switches. The first type routes
data and IF signals through the switch. The CRS-100
and are representative of this type of switching. This
switch has individual connectors mirroring the data and
IF interfaces available on the modems they support.
Operationally, a copy of the Tx and Rx traffic is
delivered to the offline modem so that both units see
identical traffic signals.
The second type of switch is a passive switch that
performs IF switching and allows the data to be
passively switched within the pair of modems
interconnected by a Y-cable. The net result is the same:
both modems see the identical Tx and Rx traffic signals
permitting the continual comparison of fault status. The
CRS-170A is this type of switch.
A significant feature of the switches is the Auxiliary
Serial connections between the two modems in the pair.
The online unit interrogates the standby unit at reg ular
intervals to determine its configuration. If a difference in
configuration is detected, the online unit automatically
reconfigures the standby unit, so that the configurations
are always synchronized. The advantage of this feature
is clear: If the standby unit is replaced, it does not have
to be reprogrammed to match the online unit — the
process is entirely automatic.

MANUAL AND AUTOMATIC SWITCHOVER
Manual switchover is enabled from the front panel or
remote control of the online modem.
Automatic switchover conditions are user-defined by
setting two switches at the front of the unit. The user
can select Unit Faults only, Unit Faults or Receive
Traffic Faults, Unit Faults or Transmit Traffic Faults, or
all three. This user-configured feature provides a great
deal of flexibility in the operation of the switches.

OPERATION
Only one modem in the pair (the online unit) is
permitted to transmit its IF carrier signal at any one
instant. For total security, the offline modem mutes its
TX carrier, and the switch provides further isolation by
using an RF relay within the unit. Unlike some other 1:1
redundancy systems, which use a passive power
combiner for this function (losing approximately 3.5 dB
in output power level), the switch does not introduce
any attenuation of output signal level.
The supplied G.703 interfaces support the T1, E1, T2,
and E2 standards, in both balanced and unbalanced
configurations. Support is also provided for ‘G.703 -like’
signals at 512 and 1024 kbps.
Operators do not have to configure the interface type –
control signals from the modems perform the selection
automatically.