09-06-2012, 11:17 AM
Understanding Optical Communications
Optical Communications.pdf (Size: 5.24 MB / Downloads: 1)
Introduction
The use of light to send messages is not new. Fires were used for signaling in
biblical times, smoke signals have been used for thousands of years and flashing
lights have been used to communicate between warships at sea since the days of
Lord Nelson.
The idea of using glass fibre to carry an optical communications signal originated
with Alexander Graham Bell. However this idea had to wait some 80 years for
better glasses and low-cost electronics for it to become useful in practical
situations.
Development of fibres and devices for optical communications began in the early
1960s and continues strongly today. But the real change came in the 1980s.
During this decade optical communication in public communication networks
developed from the status of a curiosity into being the dominant technology.
Weight and Size
Fibre cable is significantly smaller and lighter than electrical cables to do
the same job. In the wide area environment a large coaxial cable system
can easily involve a cable of several inches in diameter and weighing many
pounds per foot. A fibre cable to do the same job could be less than one
half an inch in diameter and weigh a few ounces per foot.
This means that the cost of laying the cable is dramatically reduced.
Material Cost
Fibre cable costs significantly less than copper cable for the same
transmission capacity.
Information Capacity
The data rate of systems in use in 1998 is generally 150 or 620 Mbps on a
single (unidirectional) fibre. This is because these systems were installed in
past years. The usual rate for new systems is 2.4 Gbps or even 10 Gbps.
This is very high in digital transmission terms.
In telephone transmission terms the very best coaxial cable systems give
about 2,000 analog voice circuits. A 150 Mbps fibre connection gives just
over 2,000 digital telephone (64 Kbps) connections. But the 150 Mbps
fibre is at a very early stage in the development of fibre optical systems.
The coaxial cable system with which it is being compared is much more
costly and has been developed to its fullest extent.