Page 65 - Optical Switching And Networking Handbook

03_200023_CH02/Batesx 1/17/01 8:17 AM Page 50

50 Chapter 2

Backhoe fade This risk comes from heavy earth-moving
equipment such as backhoes and plows. A major hazard for
outdoor cables is cable-laying machines. In most countries,
the cables are laid along rights-of-way. The contractors used
by all the major telecommunications providers cut the
cables they are hired to install. Many of the cable cuts are
from the same contractors that initially laid them in the
first place.
Damage during installation Cable also must withstand
the stresses of being installed. The installation crews have a
job to do, and they do it. They have little regard for the
stresses, tugs, and snags they put in the cable. Their job is
to install x amount of cable per day, and they do just that.
Consequently, they bend, stretch, and snag/cut the cabling
in the process of installing it.
Water Water is the worst enemy of an optical fiber
system. Waterproofing the cable is often more important
than worrying about some of the other risks. Over time, the
fibers begin to degrade because of a chemical reaction
between the glass and water. The glass can change its
absorption rate, and this can cause significant loss of signal
strength. The change in the composition of the glass causes
it to cloud and changes the refractive and reflective
characteristics. Basically, this means that water is a big
problem. As access holes flood, the water can permeate
through the outer jacket of fiber cables and cause these
problems. Water also causes microcracking in the glass
fibers, producing light scatter.

Getting Fiber to Carry the Signal

We always try to define the bandwidth of a cable or a radio trans-
mission system. This is so because we relate everything to our
telecommunications discussions.We try to determine the bandwidth
of the optical signal.