Field Trial of Optical Fibre Cable-TV System Optical Fibre System for ...

Terminology and Definitions
The terminology and definitions used in the
article are explained here. Some ot the terms
are in general use, others are specific to the
article. The definitions of reliability concepts
are based on the terminology in the report of the
Nordic Working Group 3 with certain
simplifications made for the relevant
application.
Availability
Instantaneous availability is the probability that
a unit is functioning at a certain moment of
time.
Complete failure
A failure that means total loss of the required
function.
Constant failure rate period
A period of time, between the early failure
period and the wear-out failure period, when
failures occur at an approximately uniform rate.
Degradation fa/lure
A characteristic exceeds the given tolerance
limits without constituting complete failure, but
the process is so slow that the failure could
have been anticipated by prior examination.
Down time
The time during which a unit should be working
but is faulty or being repaired. The down time
comprises fault detection time, administrative
time, waiting time and repair time. Net down
time includes only fault detection time and
repair time.
Early failure period
Early period with a noticeable decrease in the
failure rate.
Failure
Undesirable deviation of a certain characteristic.
In practical applications failures should be
defined in detail with respect to cause, consequences
and extent.
Failure rate
Instantaneous failure rate, z(t), is the limit value
of the ratio of the probability of failure during an
interval of time to the length of the interval when
the latter tends to zero.
The mean failure rate, z, is the average value of
the failure rate during an interval of time, i.e. the
ratio of the integrated instantaneous failure rate
during the interval to the length of the interval
and is given in 1/h.
In the US the unit FIT = 10~ 9 failures/hour is
used.
Failure that prevents operation
A failure which means that the unit does not
function. However, the failure does not
necessarily prevent transmission.
Function block
A part of the equipment with a specific function,
often in the form of a magazine or a shelf (e.g.
60-channel multiplexer, 30-channel PCM, 565
Mbit/s line terminal).
Intermittent failure
The function returns without any repair having
been made. The failure can be recurrent.
MTBF
Mean time between failures, which is the mean
value of the time intervals between failures.
During the constant failure rate period MTBF =
1/z.
MTTR
Mean repair time. In practical applications it
should be specified whether the Mean Time To
Repair is meant, i.e. net down time (fault
location, net repair and function test time), or
Mean Time To Restore, i.e. total down time
(fault location, administrative, waiting and
repair time). The mean time observed by
different administrations is usually given as a
ratio of the total duration of the failures to the
number of times failures occur during a given
period. In these cases the MTTR is the mean
time to restoration of function after a failure and
can be used direct in the calculation of the
mean availability.
Observed failure rate
A quantitative observation result, which during
the constant failure rate period (with an
approximately constant failure rate) is obtained
as the ratio of the number of occurring relevant
failures to the observation time.
Observed MTBF
A quantitative observation result, which during
the constant failure rate period is obtained as
the ratio of the observation time to the number
of relevant failures occurred.
Operating time
The time during which a unit is in operation.
Permanent failure
A failure that remains until it has been repaired.
Predicted failure rate or MTBF
Calculated failure rate or MTBF based on
Component failures can affect the function
of a unit in different ways. Conestimated
values from experiments or
observation of other or similar units.
Primary failure
Failure that is not caused by the failure of
another item.
Probability of failure
The probability that a failure will occur during a
given interval of time undergiven operating and
environmental conditions.
Redundancy
Redundancy means that a given function mode
is maintained by more than one means (e.g.
several parallel routes).
Reliability
The probability that a unit will work properly
during a certain interval of time under given
operating and environmental conditions.
Repair time
Time for fault location, fault correction and
functional test.
Shortage risk
The probability that a spare unit is not available
when needed.
Sudden failure
A failure that occurs so suddenly that it could
not have been anticipated by prior examination.
Telecommunication network
All lines and equipment used to set up
communication between a number of different
places. A network contains nodes (exchanges)
and links (transmission systems) between the
nodes.
Transmission equipment
Equipment (hardware) in the form of different
function blocks that form part of a system for
the transmission of information.
Transmission network
A network of links between nodes, i.e.
transmission systems between exchanges.
Transmission system
Various equipments or transmission media
connected together in a configuration that
makes it possible to transmit information (e.g.
multiplexer, lineequipment, cable, line system).
Wear-out failure period
Late period with a noticeable increase in the
failure rate.
rates for all components in the unit are
added. This means that from the point of
view of reliability all components in the
unit in question are considered as being
series connected without any internal
redundancy.
Practical calculations are usually made
for an operating temperature of +40° C
(104° F) and standardized stress models.
The operating temperature is considered
to be the result of a room temperature
of +25° C (77° F) with an addition of
15° C (27° F) caused by dissipated heat.
However, both temperature and stress
models can be varied within the limits of
the operating range if necessary.
sequently, different failure modes with
different failure rates arise, which can
be related to different functions in a
complex unit. Furthermore, each unit
can affect the function block in different
ways depending on its own failure
modes, the structure of the block, the
position of the unit in the magazine and
other units in the block.
For practical reasons certain assumptions,
conditions and limitations are introduced
when making predictions, resulting
in the following model: A failure
is considered to be primary (not caused
by another failure), total, sudden and
preventing function but not necessarily
preventing transmission. It is permanent,
and manual action is required to
restore function, it has occurred under