1.4.1
Performance Requirements
1.4.1.1
Reliability
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NOTE: Reliability and maintainability are relative
terms, and the attainable level will, therefore,
depend upon the type, configuration, and degree of
redundancy. System availability is a function of
reliability and maintainability and is defined as
the long-term average fraction of time that a system
is satisfactory in service. System availability
should be as high as economically feasible and may
be calculated as follows:
MTBF
A = -----------
MTBF + MTTR
Where:
A = Availability
MTBF = Mean-Time-Between-Failure
MTTR = Mean-Time-To-Repair
Designer should give serious thought and
consideration in specifying MTBF and MTTR.
Additional information are as follows:
Nonrelevant failures: a. Installation damage; b.
Accident or mishandling; c. Failures of external
test facility or tests; d. Equipment failures
caused by an externally applied overstress condition
in excess of approved operational test requirements;
e. Normal operating adjustments, nonfailures, as
prescribed in the approved equipment operating
instructions.
Relevant failures which requires maintenance:
a. Three accumulated failures in the common and
redundant equipment that do not inhibit full
operation; b. Accumulated failures in equipment
other than the common and redundant equipment, equal
to one per thousand of the equipped lines and one
per hundred of the equipped trunk and T1 span
equipment quantities that do not cause a complete
switch outage; c. Each software malfunction,
including inability to perform any one or more of
the system monitoring and maintenance diagnostics
functions or functional requirements; d. A complete
switch failure, no calls being processed; e.
Inability to maintain busy hour traffic; f.
Simultaneous loss of operations of all maintenance
consoles or all attendant consoles, or all printers
or all test/diagnostic equipment.
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Electronic Private Automatic Branch Exchange (EPABX) shall have a minimum
acceptable system Mean-Time-Between-Failure (MTBF) of [4000] [_____] hours.
SECTION 16720N
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