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

Table 1
Comparison of predicted and observed MTBF for
some function blocks
Equipment
Channel/SG
SG/MG
MG/SMG
SMG/LG (2700)
30-channel PCM
2 Mbit/s line system
(2 terminals + 2 rep.)
8 Mbit/s line system
(2 terminals + 4 rep.)
Older
Predicted
5.2
27
30
36
10
-
-
Mean time between f; tilures, IV TBI- (years
des gn (M4)
New desig
Observed Predicted
11
7.1
27
33
31
31
27
36
11
18
-
-
26
19
i (MS)
Observed
10
71
28
31
18
39
29
Fig. 4
The correlation between predicted and observed
failure rate for function blocks (magazine = shelf)
x
Different function blocks
Observed value = Predicted value
prediction and the use of the data in
future calculations. Telecommunications
administrations and manufacturers
represent different viewpoints in
practical applications.
Telecommunications administrations
and users of telecommunication services
are mainly interested in the availability
of the service and equipment.
However, the availability is dependent
on external application and maintenance
factors as well as the built-in reliability
of the equipment. The calculation
model becomes complex because
of factors that vary with time and space
and also from one case to another.
The manufacturer of transmission
equipment, on the other hand, is only
responsible for the built-in reliability.
Application factors are beyond his control.
Under given operating conditions
the built-in reliability is only dependent
on components and design parameters,
and the calculation model is therefore
relatively simple. It is also easy to compare
equipment from different manufacturers.
The reliability parameters of the
equipment, i.e. the failure rate or mean
time between failures, constitute the
basic data for calculations concerning
systems, links and networks.
The following parameters have proved
suitable for use in most practical cases:
the failure rate z, the mean time between
failures MTBF, reliability R, availability
A, and down time DT. See the panel
"Terminology and Definitions".
Of the many possible ways of presenting
reliability data two extreme cases may
be mentioned. In the first case the whole
equipment is regarded as a single item
and all failures are counted without their
effects being considered. In the other
case a failure effect analysis is made, i.e.
the effect the failure has on the transmission
capacity of the equipment is
considered and the failures are structured.
The presentation method using structured
failures is considered most suitable
for transmission equipment 4 . A brief
description is given below.
Observed
failure rate
(failures/10 9 h)
10 5
Structured presentation
Different types of failures in equipment
can affect the transmission capacity in
different ways. With respect to its transmission
capacity the equipment can be
in different (but well defined) states depending
on which and how many functions
are being affected by the failure.
10 4
10 3 -
102
102
Worse than
predicted: 38%
x
X
X * /
/
X X X
X/
/
/
TT1—
103
w /
*
/
/
X
X
X