electronic telephone sets coaxial cable systems - The history of ...

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utilizing the cable for even larger systems.
Nowadays {he normal coaxial
cable (2.6/9.5 mm) is justified only for
60 MHz systems.
Small-core coaxial cable (1.2/4.4 mm)
is very suitable for 12 MHz systems and
even for 4 MHz systems when the
traffic requirement is limited. On the
other hand it is very doubtful whether
it will be possible to justify 1.3 MHz
systems in the future.
Fig. 6 shows a comparison of the route
construction costs for different coaxial
cable systems. The calculations have
been carried out for a line section of
100 km with normal and small core
coaxial cable respectively. The cost of
the cable and the laying costs have
been included and also the cost of the
repeater equipment and its installation.
The distribution of the route construction
costs per circuit kilometre is
shown in fig. 7.
The change to transistorized coaxial
systems also resulted in a substantial
reduction of the operational and maintenance
costs. Power consumption fell
by more than a power of ten, preventive
maintenance was eliminated and repair
costs were reduced. All in all the direct
operational and maintenance costs for
transistorized systems constitute only
about 5 % of the corresponding costs
for valve-type systems.
The future of coaxial
cable systems
Coaxial systems and radio-relay links
have been the dominating transmission
media in the long-distance network for
telephony for very many years. This
development can be expected to continue
for another ten years or so, before
digital systems with high capacity have
left the experimental stage and become
commercially competitive. Naturally
for certain applications satellites and
sea cables are attractive alternatives,
but quantitatively these are not so important.
The choice between coaxial systems
and radio-relay links is influenced by
a number of factors. As a general rule
radio-relay links have the advantages
that the route construction costs are
lower and the time needed to install
them is less. On the other hand the
coaxial cable alternative provides the
possibility of successively increasing
the number of circuits by converting to
larger systems. The cables that were
originally intended for 4 MHz systems
(960 circuits) have proved to be satisfactory
for 60 MHz systems (10,800 circuits).
The long life of coaxial cables,
the conversion possibilities, their reliability
and the low maintenance costs
makes for a very competitive annual
cost per circuit.
Even the traditional "radio-relay link
countries" have begun to evince an
increasing interest in coaxial systems
during recent years. Contributory reasons
for this are that
— the traffic requirement on certain
routes is so great that the capacity
of radio-relay links is insufficient
— the frequency bands that are allocated
for radio-relay links are already
exploited to such a high degree
that the introduction of further
radio-relay links is becoming increasingly
difficult. This applies
particularly in the vicinity of most of
the large cities and towns in the
world.
The coaxial cables that are installed
represent a very large capital investment,
and there are very strong reasons
for increasing their capacities
even further. Investigations that have
been undertaken recently indicate that
the existing tube construction can be
used for frequencies well above 60
MHz. Preliminary investigations of systems
with bandwidths in the region of
150 to 200 MHz are going on in several
countries. Another alternative, which
appears to be promising, is to introduce
digital coaxial systems with high
capacity. This development line is
favoured by a general tendency towards
the digitalisation of telephony.
A digital system with a capacity in the
order of 1 Gbit/s (equivalent to roughly
15,000 telephone channels) can become
a reality within 10 years. Compared
with waveguides and optical
fibres such a coaxial system should
prove to be extremely competitive.