Telecommunications began more than 160 years ago, with
telegraphs and telephones working through wires. We still use wires –
known as landlines, or the fixed network – but now a web of OPTICAL FIBRES, radio, and satellite links connects every place in the
world. You can control this machine yourself, simply by picking up a
telephone.
Pressing the keys on a telephone sends signals through wires to a
local telephone exchange. A numbering plan stored in a computer at the exchange
tells the exchange when a complete number has been dialled. If the phone you
are calling belongs to a different exchange, your exchange sends signals to
other exchanges to set up a route for your call.
Most calls from fixed phones travel to the local exchange through
copper wires. Each phone has its own line card – a circuit that is
permanently connected to the phone. This responds with a dialling tone when you
pick up the phone. It also converts your call into electrical pulses, so that
it can be handled by computers that route the call.
Nearly all calls between big cities now travel as laser light
through thin glass fibres, called optical fibres. The laser switches rapidly on
and off to send out high-speed digital codes. Clever coding squeezes as many
different calls as possible into each optical fibre, but allows them to be
sorted out again when they arrive at the next telephone exchange.
Some calls, particularly those to isolated areas, make part of
their journey by riding on a beam of microwaves. These very short waves are
focused by a dish-shaped reflector on a tower and sent from point to point in a
straight line. Microwave links are quick and cheap to set up, as there is no
need to dig tunnels or erect poles to carry fibres or wires.
Eventually the call reaches the local exchange that handles the
telephone you have dialled. There, it is directed to that phone’s line
card and the signal is changed back to analogue form. A pulsing current sent
down the line rings the phone. When the phone is picked up, a switch in the
receiver completes a circuit that cuts off the ringing current and connects the
call.
Light can be used to send signals – for example,
with a torch. However, light sent through air is stopped by objects in its
path. An optical fibre traps light inside a thin strand of glass. The light is
reflected back from the surface of the glass and cannot escape. An optical
fibre can direct pulses of laser light for many miles. Some fibres amplify the
light to send signals around the world.
Table 5. SOME MAJOR OPTICAL FIBRE LINKS
| NAME | DISTANCE IN KM | CAPACITY* |
|---|
| FLAG FEA (Japan–UK) | 14,000 km (8,700 miles) | 163,840 |
| Japan–US Cable Network | 10,500 km (6,500 miles) | 655,360 |
| FLAG FA (UK–USA) | 7,000 km (4,350 miles) | 1,310,720 |
| Atlantic Crossing 2 (UK–USA) | 7,000 km (4,350 miles) | 10,737,418 |
| *equivalent simultaneous phone calls | | |
Optical fibre glass is so pure than you could see through a mile
of it. It is even more transparent to the invisible laser light that it
carries. The inner core is covered with a layer of less heavy glass, and the
light is reflected (and so trapped) where the two kinds of glass meet. A
plastic coating on the outside makes the fibre tougher and easier to
handle.
Delicate optical fibres are heavily protected when laid on the sea
bed. Each cable contains several fibres. Some may not be needed at first but
these “dark fibres” will be brought into use when calls on the
cable route increase.
