Modern Fibre optic cables can carry
many millions of telephone calls, together with huge amounts of
video and internet data. Approximately 95% of the world's
telecommunication requirements are met by submarine cables. Cables
are used because of their high reliability, capacity and security.
Cables have an insignificant delay compared to satellite and are
most cost-effective on major routes, hence communications costs are
cheaper than satellites.
Since the inception of fibre optic
cables in the late 1980's, it is likely that in excess of 1 million
kilometers of cable has been laid. The longest cable system;
Sea-Me-We 4 (South East Asia to Western Europe) is over 40,000
kilometers end to end, with many landing points in the various
countries situated along the route.
A Fibre Optic Cable consists of an
inner optical core encased within a high tensile steel strength
member, clad within a copper power conductor. This package is then
insulated with polyethylene. This package is the basic deep
water cable (water depths greater than 2,000 metres) and is usually
17 - 21mm diameter.
|Cross Section of a Telcoms Cable
The optical (light) signal is
carried on pure glass fibre cased within the optical core. The
light is guided by internal reflection within the glass fibre. Each
fibre, of which there may be many, is much smaller in diameter than
a human hair.
Because the light signal loses
strength en route, repeaters are required at regular intervals to
restore it. Repeaters are now based on optical amplifying
technology, which requires short lengths of erbium-doped optical
fibre to be spliced into the cable system. These are then
energized by lasers, thus boosting the incoming light signal.
Repeaters need to be spaced every 50 to 80 kilometres apart and
since each repeater requires electrical power to operate it, this
is why long length submarine cables are powered and the voltages
carried may be very high. Damage to, or loss of a repeater can
result in a very expensive repair or replacement, as well as
breakdown in commercial business communications.
Shorter submarine cables, such as
those across the Irish Sea or English Channel, do not need
repeaters and have amplifiers at either end. It is possible
to have a single span of around 240km without the need for a
Cable systems may also have devices
called Branching Units inserted into them. The Branching Unit looks
like a 'Y' shaped cable connector and are very large. The branching
unit can be used to spilt the cable path to enable the cable to be
routed to two or more different locations thus allowing diversity
of connection for the cable system. More than one Branching Unit
can be used in a cable system and indeed, many cable systems use
multiple Branching Units to allow multiple connections.
In water less than 2,000 meters
deep, additional protection is added against environmental, fishing
and anchor damage in the form of external steel wire armour clad
within a polypropylene serving. There may be one or more layers of
armour applied to the cable.
The heaviest form of armoured cable
may have in excess of 70 tonnes breaking strength. While the cable
may only break or part at high tensions, damage to the optical path
or to the electrical insulation (the polyethylene) can occur at
much lower tensions such as when fishing gear may become engaged
with the cable. This is because the cable becomes bent to a radius
less than its minimum safe limits (usually 1.5 metres radius/3
metre diameter). Armoured cables vary in size depending on whether
one or more armour layers are used but may be up to 50mm diameter.
Breaking strains vary from 20 - 70 or more tonnes.