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degree of reliability. Particulars of existing installations using the type of
equipment offered are attached.
The most critical component in the system is the central computer. The
solution offered is based on a Hot Standby solution, which has been used in a
number of AWOS systems.
The failure rates of all components were considered. Note that neither
Compaq nor IBM would publish MTBF figures for their workstations or servers
(even though we are a Compaq distributor). Consequently we have used
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estimates based on our experience with this equipment. Assuming that the
MTBF of the PC is as low as 5,000 hours (about half a year) and assuming that
the MTTR for the PC is one hour (swapping with spares). In this case
duplicating the server will increase the MTBF beyond the lifetime (20 years) of
the system.
The sensor interface units offered are identical with the ones used in the
Almos Automatic Weather Stations. These stations are currently used by the
Australian Bureau of Meteorology. Almos has been the exclusive supplier of
Automatic Weather Stations to the Australian National Weather Network
since 1993. Based on official service records MTBF calculations are shown
below (Error! Reference source not found. MTBF Evaluation), which show
MTBF better than 30 years.
On the basis of above considerations the Mean Time Between Critical Failures
(MTBCF), which would stop operation of the Central Computer or one of the
Remote Stations is larger than 20 years. Mean Time Between Failures (MTBF),
which includes wind sensor errors and potential display errors is estimated to
be 10,000 hours.
3.1.2 Mean Time Between Corrective Maintenance Action (MTBCMA)
The only component that requires regular maintenance in the proposed
system is the wind sensor. MTBCMA = 1 year
3.1.3 Built in Test and Fault Isolation capability
Due to the requirements to keep repair to the absolute minimum, the impact
on operational readiness and the maintenance concept adopted, automatic
failure detection and isolation, referred to as built-in-test equipment (BITE) will
be implemented on all equipment and monitored. BITE functions are mostly
performed by software resources. The primary objective of BITE is to correctly
detect system malfunctions and accurately fault-isolate to a single replaceable
unit so that maintenance staff with a minimum of training is able to perform
this function. Failures undetected by BITE will be kept to a minimum. The
BITE will be capable of isolating faults down to LRU (SRU) level in at least 80%
of fault cases.
BITE function will be integrated into the different equipment in order to avoid
external test equipment for stimuli and measuring purposes. BIT functions will
not interfere with the operational of the system.
BITE functions include:
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• Sensor data is checked both for data format errors and for data quality
errors. Error checks include sampling the status information provided by
intelligent sensors.
• Communication ports are tested by heart beat packets sent between
computers
• Fatal system errors are tested by Watchdog cards in all PCs and Remote
Stations. Automatic restarts are generated after error detection.
• RAM memory is tested in PCs and the Automatic Weather Stations at
system reset.
• All memory components in the Remote Stations are checksum tested at
system reset.
3.1.4 Mean Time To Repair (MTTR)
The computer equipment will be repaired by swapping complete units with
spare, which will take average 15 minutes.
Remote Stations will be repaired by swapping cards. Using the advanced
Almos Diagnostic Tools faults can be diagnosed and cards swapped within 30
minutes plus traveling time.
Sensors will be repaired by swapping them with spares. Due to the fact that
access to some sensors is cumbersome the repair time would take one hour
average.
Unit Fault
detection
Isolation False Alarm
Sensor yes The system will detect most
sensor faults
no
Remote
Station
yes As above. Communication link
fault can be isolated by
communication LEDs
no
Server
computer
yes Automatic. Communication link
fault can be isolated by
communication LEDs on modems
no
Display
Workstation
yes As above no
Printer yes Automatic no
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