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SLM-3650 Satellite Modem
Maintenance
5.2.3.4
Receive Interface Faults
Fault/Alarm
BUFFER UNDERFLOW
BUFFER OVERFLOW
RX DATA/AIS
FRAME BER
BACKWARD ALARM
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Possible Problem and Action
Buffer underflow alarm.
Indicates the plesiochronous buffer has underflowed. Buffer underflow is
normally a momentary fault (there are clock problems if this alarm is
continuously present). This alarm is included in this section to be consistent
with the fault reporting system and to be correctly registered in the stored
fault memory. The time and date of the first 10 receive buffer underflow
faults are stored in battery-backed memory as an aid to troubleshooting. The
interval between stored overflow/underflow events can be used to determine
relative clock accuracies.
Buffer overflow alarm.
Indicates the plesiochronous buffer has overflowed.
The problems and actions in the buffer underflow section apply to this alarm.
Data or incoming AIS. The data monitored for RX data is coming from the
satellite.
When the AIS is selected for RX data fault in the Interface Utility menu, the
RX data/AIS is monitoring an alarm condition of all 1s from the satellite.
When data is selected for RX data fault in the Interface Utility menu, the RX
data/AIS is monitoring a fault condition of all 1s or 0s. This is referred to as a
data-stable condition (data is not transitioning). The fault indicates trouble in
receiving data from the satellite. The modem passes this signal
transparently, and can close a FORM C contact. The indication is a monitor
function only to help isolate the source of trouble in a system.
The receive decoded error rate has exceeded 10 -3 over a 60-second period
measured on the framing bits.
This is defined as a major (prompt) receive alarm by INTELSAT
specifications IESS-308. In a redundant system, a switch-over will be
attempted. Since some data must be correctly received to indicate this fault,
receive AIS will not be substituted. This fault is to be sent as a backward
alarm to the distant end. This must be wired externally, as faults other than
from the modem may need to enter the fault tree.
Backward alarm. Modem is receiving a backward alarm, indicating trouble at
the distant end. The alarm may be a result of improper transmission at the
near end of the link.
This particular alarm is reported and recorded, but the modem takes no
other action. In most cases, the fault is due to some receive problem with the
modem, so a real fault will probably be occurring if backward alarm faults are
being recorded.
5–16
Revision 3
MN/SLM3650.IOM
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