Fiber In Communications Fiber
has become the communications medium of choice for telephones, cell
phones, CATV, LAN backbones, security cameras, industrial networks,
just about everything. Why use fiber?
 The
biggest advantage of optical fiber is the fact it is the most cost
effective means of transporting information. FIber can transport more
information longer distances in less time than any other
communications medium, as the photo on the left from the late 1970s
illustrates so well. The bandwidth and distance capability of fiber
means that fewer cables are needed, fewer repeaters, less power and
less maintenance. In addition, fiber is unaffected by the interference
of electromagnetic radiation which makes it possible to transmit
information and data with less noise and less error. Fiber is lighter
than copper wires which makes it popular for aircraft and automotive
applications. These advantages open up the doors for many other
advantages that make the use of optical fiber the most logical choice
in data transmission.
These
advantages have led to fiber becoming the transport medium of choice
for practically all data, voice and video communications. Both
telcos and CATV operators use fiber for economic reasons, but their
cost justification requires adopting new network architectures to take
advantage of fiber's strengths. LAN and premises network designers and
installers now realize that they must also adopt new network
architectures too. A properly designed premises cabling network can
also be less expensive when done in fiber instead of copper. Conversion
from copper networks is easy with media converters, gadgets that
convert most types of systems to fiber optics. Even adding the cost of
the media converters, the fiber optic network will usually be less than
copper when the proper architecture is used.
Telephone Networks
Telephone
networks were the first major users of fiber optics. Fiber optic links
were used to replace copper or digital radio links between telephone
switches, beginning with long distance links, called long lines, where
fiber's distance and bandwidth capabilities made fiber significantly
more cost effective.
Telcos use fiber to connect all their central offices and long distance
switches because it has thousands of times the bandwidth of copper wire
and can carry signals hundreds of times further before needing a
repeater - making the cost of a phone connection over fiber only a few
percent of the cost of the same connection on copper. They even use
fiber to connect cell phone towers to save limited radio spectrum. 
After
long distance links were converted to fiber, telcos began replacing
shorter links between switches with fiber, for example between switches
in the same metropolitan area. Today, practically all
the telephone networks have been converted to fiber. Telcos and other groups are now running
fiber right to the home, (FTTH) using low cost passive optical network
(PON) systems that use splitters to share the cost of some fiber optic components among as many as 32 subscribers. More on FTTH, FTTH PON types and FTTH network architecture.
 Even
cell phone networks have fiber
backbones. It's more efficient and less expensive than using precious
wireless bandwidth for backbone connections. Cell phone towers with
many antennas will have large cable trays or pedestals where fiber
cables connect to the antenna electronics. The Internet
The
Internet has always been based on a fiber optic backbone. It started as
part of the telephone network which was then primarily voice but has
become the largest communications network as data traffic has outgrown
voice traffic. Now the telcos are moving their voice communications to
Internet protocol (IP) for lower costs. CATV
Most CATV systems are using fiber backbones too. CATV
companies use fiber because it give them greater reliability
and the opportunity to offer new services, like phone service and
Internet connections.
CATV
used to have a terrible reputation for reliability, not really a
problem with service but with network topology. CATV uses very high
frequency analog signals, up to 1 GHz, which has high attenuation over
coax cable. For a city-wide system, CATV needed many amplifiers
(repeaters) to reach the users at the end of the system; 15 or more we
common. Amplifiers failed often, meaning that subscriber downstream of
the failed amp lost signal. Finding and fixing failed amps was
difficult and time consuming, causing subscriber complaints.
The
development of highly linear distributed feedback (DFB) lasers allowed
CATV systems to be converted to analog optical systems.
CATV companies "overbuild" with fiber. They connect their headends with
fiber and then take fiber into the neighborhood. They lash the fiber
cable onto the aerial "hardline" coax used for the rest of the network
or pull it in the same conduit underground. The fiber allows them to
break their network into smaller service areas, typically fewer than 4 amplifiers deep, that prevent large
numbers of customers from being affected in an outage, making their network more
reliable and easier to troubleshoot, providing better service and
customer relations.
The fiber also gives CATV operators a return path which they
use for Internet and telephone connections, increasing their revenue
potential. Most current CATV systems still use AM (analog) systems which
simply convert the electrical TV signals into optical signals. Look for
them to convert to more digital transmission in the future.
Premises Networks
Premises
networks, mostly computer LANs (local area networks) use fiber optics
primarily in the backbone but increasingly to the desk and to connect
wireless access points. The LAN backbone often needs longer distances
than copper cable (Cat 5/5e/6/6A) can provide and of course, the fiber
offers higher bandwidth for future expansion. Fiber's ability to handle
network upgrades meant that one fiber type outlived nine generations of
copper cables in LANs. A new fiber type (OM3) offers future potential
for upgrades while copper continues to struggle with network speed
increases.
Until
recently large corporate LANs use fiber backbones with copper wire to
the desktop. LAN switches and hubs are usually available with fiber
optic ports but PCs have interfaces to Ethernet on copper. Inexpensive
media converters allow connecting PCs to fiber. Fiber to the desk can
be cost effective if properly
designed using centralized fiber architecture without local switching
in the telecom closet, but many users no longer want to be "tethered"
to a network cable. Desktop computer sales are declining and laptops
are the PC of choice for most users, with wireless connections to the
network. Generally only high data users like engineers and graphics
designers use desktop workstations; everybody else gets a
wireless-connected laptop.
More on premises networks and fiber in premises networks. - Centralized Fiber LANs
When most contractors and end users look at fiber optics versus
Category-rated UTP cabling for a LAN, they compare the same old copper LAN
with fiber directly replacing the copper links. The installed cost of a fiber optic
cable plant comparable to the cost of Cat 5/6/6A, but fiber often requires medial conversion electronics
which add cost to the link for fiber.
-
However, the real difference comes if you use a centralized fiber
optic network - shown on the right of the diagram above. Since
fiber does not have the 90 meter distance limitation of UTP cable,
you can place all electronics in one location in or near the
computer room. The telecom room is only used for passive connection
of backbone fiber optic cables, so no power, UPS, ground or air
conditioning is needed. These auxiliary services, necessary with
Cat 5 hubs, cost a tremendous amount of money in each telecom room.
If designing a new building, you do not even need the cost of the telecom room itself.
-
In addition, having all the fiber optic hubs in one location
means better utilization of the hardware, with fewer unused ports.
Since ports in modular hubs must be added in modules of 8 or
16, it's not uncommon with a hub in a telecom closet to have
many of the ports in a module empty . With a centralized fiber
system, you can add modules more efficiently as you are supporting
many more desktop locations but need never have more than a one
module with open ports.
- More on fiber in premises networks and fiber versus copper, generally and in LANs
Other Applications For Fiber
Many
other networks use fiber. CCTV often uses fiber for it's distance
capability and security, especially in large buildings like airports
and metropolitan networks. Security systems are more secure on fiber.
Practically any network today has a fiber optic option.
Metropolitan Networks
Many
cities have incorporated fiber optics into their communications
networks. Metropolitan networks use fiber for many other applications
besides CCTV surveillance cameras, including connecting public service
agencies such as fire, police and other emergency services, hospitals,
schools and traffic management systems. Cities can install cables to
strategic locations so various services can share the fibers in the
cables, saving installation costs. Cities are also learning to bury
conduit every time a roadway is dug up so when cables need installing,
no further construction is needed.
Industrial Networks
Industrial plants use fiber
for it's ruggedness, distance and noise immunity.
In an industrial environment, electromagnetic interference (EMI) is
often a big problem. Motors, relays, welders and other industrial
equipment generate a tremendous amount of electrical noise that can
cause major problems with copper cabling, especially unshielded cable
like UTP. In order to run copper cable in an industrial environment, it
is often necessary to pull it through conduit to provide adequate
shielding. Fiber is also very flexible, so many industrial robots use
fiber for controls, often plastic fiber.
Fiber optics has complete immunity to EMI. You only
need to choose a cable type that is rugged enough for the installation,
with breakout cable being a good choice for it's heavy-duty construction.
The fiber optic cable can be installed easily from point to point,
passing right next to major sources of EMI with no effect. Conversion
from copper networks is easy with media converters, gadgets that
convert most types of systems to fiber optics. Even with the
cost of the media converters, the fiber optic network will be
less than copper run in conduit.
- Utility Networks
- Utilities
use fiber for communications, CCTV surveillance and network management.
Electrical utilities take advantage of fiber's immunity to noise
also, even running fiber inside high voltage power distribution cables.
Some utilities install fibers inside their high voltage distribution
networks and lease fibers to other telecommunications companies.
Utilities use fiber in one non-communications application; fiber optic
sensors allow monitoring high voltage and current in their distribution
systems. The interest in "smart grid" management of power distribution
to enhance efficiency is based on using fiber optics for network
management.
- Military and Platforms
- The
military uses fiber everywhere, on bases, platforms (ships and planes),
and on the battlefield because it's hard to damage, tap or jam.
Airplanes use fiber for its reliability and noise immunity, but also
like the lighter weight of fiber. Even millions of cars have fiber
networks connecting all the electronics because fiber is immune to
noise and saves weight.
Fiber Optic Links
Fiber
optic links work by sending optical signals over fiber. Fiber optic
transmission systems all use data links that work similar to the
diagram shown above. Each fiber link consists of a transmitter on one
end of a fiber and a receiver on the other end. Most systems operate by
transmitting in one direction on one fiber and in the reverse direction
on another fiber for full duplex operation. Transmitters are
semiconductor LEDs or lasers and receivers are semiconductor
photodetectors.
For more information on fiber optic links, read these pages on datalinks and transceivers. Wavelength Bands Used For Fiber Optic TransmissionDesigning Fiber Optic Networks This
is a big topic so we have a complete section
on the subject later in the book.
Fiber's extra bandwidth and distance capability makes it possible to do things
not possible with copper wire or wireless.
First and foremost, it's necessary to understand thoroughly what
signals are to
be transmitted over the fiber and the specifications of the
transmission equipment. Then map and visit the work site to understand
where the fiber optic cable plant needs to be installed. Know
the standards but use common sense in designing the
installation. Consider what are the possible problems and work around or
prevent them. Don't cut corners which may affect performance or
reliability. Document everything completely. Plan for future expansion
and restoration in case of problems. There is no substitute for
experience and common sense here!
More on Designing Fiber Optic Networks
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