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

211
Transmission system Dow n time
(100 miles = 160 km) l System
i ;min/year)
Multiplexer
(% of total)
140 Mbit/s coaxial
system (incl.
mechanical damage to
the cable)
140 Mbit/s coaxial
system (only
equipment failures)
12 MHz coaxial system
(only equipment
failures)
140 Mbit/s optical fibre
system (excl.
mechanical cable
damage)
140 Mbit/s optical fibre
system with
redundancy (1 standby
for every ten systems
in operation)
2170
251
49
696
19
0.4
2,5
47
1.3
1.3
Table 2
Down time for different transmission systems
over a line of 100 miles (160 km)
ability of the system will not be improved
significantly by redundancy in the multiplexing
system if the availability of the
line system is low.
Instead of introducing redundancy in individual
function blocks the availability
requirements for the network can be
met by means of distributed traffic, rerouting
or standby equipment at the link
or system level. Such measures are considered
advantageous since they also
increase the probability of the system
surviving sabotage, catastrophes and
war.
One exception that should be mentioned
is line systems for submarine cables.
Long repair times and high costs
justify redundancy for function blocks in
the form of standby equipment in repeater
stations at the bottom of the sea.
Some manufacturers apply the redundancy
principle for equipment at the
function block level as a general measure
to improve availability. It results in
increased volume and higher power
consumption (higher temperature) and
a larger number of failures overall.
However, failures in the duplicated
equipment do not prevent transmission,
and as a consequence longer repair
times can be accepted, which can have a
positive effect on the maintenance routines.
Operation and maintenance
of transmission equipment
The availability of the equipment and the
system is affected by the maintenance.
Modern transmission equipment requires
only corrective maintenance.
However, a certain amount of preventive
maintenance is required for the laser diodes
in optical fibre systems, in which
failures can start as degradation of certain
parameters. Continuous automatic
monitoring with alarm means that a unit
that has started to degrade can be exchanged
at a suitable time, before a
complete failure occurs.
The telecommunication networks of today
contain electronic equipment with
similar components for transmission
and switching. Technically (and financially)
it is thus possible to integrate the
operation and maintenance of these two
types of equipment. Most administrations
still have separate organizations
for the maintenance of transmission and
switching equipment, however.
Supervisory equipment in computercontrolled
exchanges makes it possible
to indicate and locate faults also in the
transmission system. The mean time to
repair, MTTR, is dependent on the policy,
organization and technical facilities
of the administration, and also the geography
and communication facilities of
the country. Adistinction should also be
made between MTTR for failures in
manned and unmanned exchanges and
in equipment and cables.
Thanks to a modular structure with
plug-in units (printed board assemblies)
the net repair time is very short and in
practice is reduced to just replacing the
faulty board. The down time is considerably
longer, however, because of the
various actions that are necessary during
the time from when the fault occurs
to when the equipment is back in operation,
see "Terminology and Definitions".
As a guide value for practical calculations
it may be assumed that the mean
times for restoration of function after a
failure has occurred is two hours for a
manned exchange, six hours for an unmanned
exchange and repeaters in
housings and twenty hours for cable
damage. For the sake of simplicity an
MTTR of four hours is often used in calculations
for all electronic equipment.
Repair in the form of a replacement of a
faulty printed board assembly requires
good administration of spare parts, with
a sufficient number of spares held at
strategic points. Tenders therefore include
a recommended list of spare
parts.
The calculation of the number of spares
is based on the failure rate for the unit
under the given environmental and operating
conditions, the number of units
in operation and their importance to the
transmission, and also the time it takes
to obtain replacements.
The number of spare units is set so that a
given shortage risk is not exceeded during
the period until the next replenishing
of the stock of spares 7 .