C H A P T E R
3
FIBER OPTIC
NETWORKS
JI M HAYES AND PHI L SHECKLER
One often sees articles written about fiber optic communications networks that
imply that fiber optics is new. That is hardly the case. The first fiber optic tele-
phone network was installed in Chicago in 1976, and by 1979, commercial fiber
optic computer datalinks were available. Since then, fiber has become common-
place in the communications infrastructure.
If you make a long-distance call today, your voice is undoubtedly being
transmitted on fiber optic cable, since it has replaced over 90 percent of all voice
circuits for long-distance communications. Transoceanic links are being con-
verted to fiber optics at a very high rate, since all new undersea cables are fiber
optics. Phone company offices are being interconnected with fiber, and most
large office buildings have fiber optic telephone connections into the buildings
themselves. Only the last links to the home, office, and phone are not fiber.
CATV also uses fiber optics via a unique analog transmission scheme, but
they are already planning on fiber moving to compressed digital video. Most large
city CATV systems are being converted to fiber optics for reliability and in order
to offer new services such as Internet connections and phone service. Only fiber
offers the bandwidth necessary for carrying voice, data, and video simultaneously.
The LAN backbone also has become predominately fiber-based. The back-
end of mainframe computers is also primarily fiber. The desktop is the only hold-
out, currently a battlefield between the copper and fiber contingents.
31
32 CHAPTER 3 FIBER OPTIC NETWORKS
Security, building management, audio, process control, and almost any other
system that requires communications cabling have become available on fiber
optics. Fiber optics really is the medium of choice for all high bandwidth and/or
long-distance communications. Let us look at why it is, how to evaluate the eco-
nomics of copper versus fiber, and how to design fiber networks with the best
availability of options for upgradeability in the future.
IT IS REALLY ALL A MATTER OF ECONOMICS
The use of fiber optics is entirely an issue of economics. Widespread use occurred
when the cost declined to a point that fiber optics became less expensive than
transmission over copper wires, radio, or satellite links. However, for each appli-
cation, the turnover point has been reached for somewhat different reasons.
Telephony
Fiber optics has become widely used in telephone systems because of its enor-
mous bandwidth and distance advantages over copper wires. The application for
fiber in telephony is simply connecting switches over fiber optic links (Figure 3-
1). Commercial systems today carry more phone conversations over a single pair
of fibers than could be carried over thousands of copper pairs. Material costs,
Long Distance
Local Loop (City)
Subscriber Loop
(Fiber to the Curb FTTC)
Fiber to the Home FTTH
Figure 3-1 Telephone fiber optic architecture.
CHAPTER 3 FIBER OPTIC NETWORKS 33
installation, and splicing labor and reliability are all in fiber s favor, not to men-
tion space considerations. In major cities today, insufficient space exists in cur-
rent conduit to provide communications needs over copper wire.
While fiber carries over 90 percent of all long-distance communications and
50 percent of local communications, the penetration of fiber to the curb (FTTC)
and fiber to the home (FTTH) has been hindered by a lack of cost-effectiveness.
These two final frontiers for fiber in the phone systems hinge on fiber becoming
less expensive and customer demand for high bandwidth services that would be
impossible over current copper telephone wires. Digital subscriber loop (DSL)
technology has enhanced the capacity of the current copper wire home connec-
tions so as to postpone implementation of FTTH for perhaps another decade.
Telecommunications led the change to fiber optic technology. The initial use
of fiber optics was simply to build adapters that took input from traditional tele-
phone equipment s electrical signals on copper cables, multiplexed many signals
to take advantage of the higher bit-rate capability of fiber, and used high-power
laser sources to allow maximum transmission distances.
After many years of all these adapters using transmission protocols propri-
etary to each vendor, Bellcore (now Telcordia) began working on a standard net-
work called SONET, for Synchronous Optical NETwork. SONET would allow
interoperability between various manufacturers transmission equipment.
However, the telephone companies (telco s) transition to SONET was slow,
a result of reluctance to make obsolete recently installed fiber optic transmission
equipment and the slow development of the details of the standards. Progress has
been somewhat faster overseas, where the equivalent network standard Synchro-
nous Digital Hierarchy (SDH) is being used for first-generation fiber optic sys-
tems. SONET is now threatened by Internet protocol (IP) networks, since data
traffic has surpassed voice traffic in volume and is growing many times faster,
mostly due to the popularity of the Internet and World Wide Web.
CATV
In CATV, fiber initially paid for itself in enhanced reliability. The enormous
bandwidth requirements of broadcast TV require frequent repeaters. The large
number of repeaters used in a broadcast cable network are a big source of failure.
And CATV systems tree-and-branch architecture means upstream failure causes
failure for all downstream users. Reliability is a big issue since viewers are a vocal
lot if programming is interrupted!
CATV experimented with fiber optics for years, but it was too expensive
until the development of the AM analog systems. By simply converting the signal
from electrical to optical, the advantages of fiber optics became cost-effective.
Now CATV has adopted a network architecture (Figure 3-2) that overbuilds the
normal coax network with fiber optic links.
34 CHAPTER 3 FIBER OPTIC NETWORKS
Coax Network
Headend
Fiber Overbuild
Headend
Figure 3-2 CATV architectures before and after fiber overbuild.
Fiber is easy to install in an overbuild, either by lashing lightweight fiber
optic cable to the installed aerial coax or by pulling in underground ducts. The
technology, all singlemode with laser sources, is easily updated to future digital
systems when compressed digital video becomes available. The connection to the
user remains coaxial cable, which has as much as 1 GHz bandwidth.
The installed cable plant also offers the opportunity to install data and voice
services in areas where it is legal and economically feasible. Extra fibers can be
easily configured for a return path. The breakthrough came with the develop-
ment of the cable modem, which multiplexes Ethernet onto the frequency spec-
trum of a CATV system. CATV systems can literally put the subscriber on a
Ethernet LAN and connect them to the Internet at much higher speeds than a
dial-up phone connection. Adding voice service is relatively easy for the CATV
operator as well.
Local Area Networks
For LANs and other datacom applications, the economics of fiber optics are less
clear today. For low bit-rate applications over short distances, copper wire is
undoubtedly more economical, but as distances go over the 100 meters called for
CHAPTER 3 FIBER OPTIC NETWORKS 35
in industry standards and speeds get above 100 Mb/s, fiber begins to look more
attractive since copper requires more local network electronics and there are
many problems installing and testing copper wire to high speed standards. Abil-
ity to upgrade usually tilts the decision to fiber since copper must be handled very
carefully to operate at speeds where fiber is just cruising along.
Fiber penetration in LANs is very high in long-distance or high bit-rate back-
bones in large LANs, connecting local hubs or routers, but still very low in con-
nections to the desktop. The rapidly declining costs of the installed fiber optic
cable plant and adapter electronics combined with needs for higher bandwidth at
the desktop are making fiber to the desk more viable, especially using centralized
fiber architectures.
There are a large number of LAN standards today. The most widely used,
called Ethernet or IEEE802.3 after its standards committee, is a 10, 100 MB/s or
1 GB/s LAN that operates with a protocol that lets any station broadcast if the
network if free. Token ring (most often referred to as IBM Token Ring after its
developer) is a 4 or 16 MB/s LAN that has a ring architecture, where each station
has a chance to transmit in turn, when a digital token passes to that station.
These two networks were developed originally based on copper wire standards.
Fiber optic adapters or repeaters have been developed for these networks to allow
using fiber optic cable for transmission where distance or electrical interference
justifies the extra cost of the fiber optic interfaces for the equipment.
Most LANs have been designed from the beginning to offer the option of
both copper wiring and fiber optics. Several of these networks were optimized for
fiber. All share the common specification of speed: they are high-speed networks
designed to move massive quantities of data rapidly between workstations or
mainframe computers.
Fiber Distributed Data Interface (FDDI) is a high-speed LAN standard that
was developed specifically for fiber optics by the ANSI X3T9.5 committee, and
products are readily available. FDDI has a dual counter-rotating ring topology (
Figure 3-3) with dual-attached stations on the backbone that are attached to both
rings, and single-attached stations that are attached to only one of the rings
through a concentrator. It has a token passing media access protocol and a 100-
Mbit/s data rate. FDDIs dual ring architecture makes it very fault tolerant, as the
loss of a cable or station will not prevent the rest of the network from operating
properly.
ESCON (Figure 3-4) is an IBM-developed network that connects peripherals
to the mainframe, replacing bus and tag systems. ESCON stands for Enterprise
System Connection architecture. The network is a switched star architecture,
using ESCON directors to switch various equipment to the mainframe comput-
ers. Data transfer rate started at 4.5 megabytes/second but was increased to 10
Mbytes/second. With an 8B/10B conding scheme, ESCON runs at about 200
Mbits/sec.
36 CHAPTER 3 FIBER OPTIC NETWORKS
Counter Rotating
Primary
Concentrator (DAC)
Node
(DAS)
Secondary Nodes
(SAS or SAC)
Figure 3-3 Fiber distributed data interface (FDDI).
Mainframe
Director
Peripheral
Peripheral
Director
Director
Director
Director Director
Figure 3-4 Enterprise system connection (ESCON) architecture.
CHAPTER 3 FIBER OPTIC NETWORKS 37
Optically, ESCON and FDDI are similar. They use 1300-nm transmission for
the higher bandwidth necessary with high-speed data transfer rates. Both single-
mode and multimode cable plants are supported and distances up to 20 kilome-
ters between directors.
Fibre Channel and High Performance Parallel Interface (HIPPI) are both
high-speed links, not networks, that are designed to be used to interconnect high-
speed data devices. The link protocol supports most fiber types and even copper
cables for some short runs.
FIBER OR COPPER? TECHNOLOGY SAYS GO FIBER, BUT . . .
Fiber s performance advantages over copper result from the physics of transmit-
ting with photons instead of electrons. Fiber optic transmission neither radiates
radio frequency interference (RFI) nor is susceptible to interference, unlike cop-
per wires that radiate signals capable of interfering with other electronic equip-
ment. Because it is unaffected by electrical fields, utility companies even run
power lines with fibers imbedded in the wires!
The bandwidth/distance issue is what usually convinces the user to switch to
fiber. For today s applications, fiber is used at 100 200 Mb/s for datacom appli-
cations on multimode fiber, and telcos and CATV use singlemode fiber in the
gigahertz range. Multimode fiber has a larger light-carrying core that is compati-
ble with less expensive LED sources, but the light travels in many rays, called
modes, that limit the bandwidth of the fiber. Singlemode fiber has a smaller core
that requires laser sources, but light travels in only one mode, offering almost
unlimited bandwidth.
In either fiber type, you can transmit at many different wavelengths of light
simultaneously without interference; this process is called wavelength division
multiplexing (WDM). WDM is much easier with singlemode fiber, since lasers
have much better defined spectral outputs. Telephone networks using dense wave-
length division multiplexing (DWDM) have systems now operating at greater
than 80 MB/s. IBM developed a prototype system that uses this technique to pro-
vide a potential of 300 Gb/s on a LAN!
Which LANs Support Fiber?
That s easy, all of them. Some, such as FDDI or ESCON, were designed around
fiber optics, whereas others, such as Ethernet or token ring, use fiber optic
adapters to change from copper cable to fiber optics. In the computer room, you
can get fiber optic channel extenders or ESCON equipment with fiber built in.
Where Is the Future of Fiber?
The future of fiber optics is the future of communications. What fiber optic offers
is bandwidth and the ability to upgrade. Applications such as multimedia and
38 CHAPTER 3 FIBER OPTIC NETWORKS
video conferencing are driving networks to higher bandwidth at a furious pace.
Over wide area networks, the installed fiber optic infrastructure can be expanded
to accommodate almost unlimited traffic. Only the electronic switches need to be
upgraded to provide orders of magnitude greater capacity. CATV operators are
installing fiber as fast as possible since advanced digital TV will thrive in a fiber-
based environment. Datacom applications can benefit from fiber optics also, as
graphics and multimedia require more LAN bandwidth. Even wireless communi-
cations need fiber, connecting local low-power cellular or personal communica-
tion systems (PCS) transceivers to the switching matrix.
The Copper Versus Fiber Debate
Over the past few years, the datacom arena has been the site of a fierce battle
between the fiber people and the copper people. First, almost 10 years ago, fiber
offered the only solution to high-speed or long-distance datacom backbones.
Although fiber was hard to install then and electrical/optical interfaces were
expensive, when available at all, fiber was really the only reliable solution. This
led to the development of the FDDI standard for a 100 Mb/s token ring LAN and
the IBM ESCON system to replace bus and tag cables.
By 1989, FDDI was a reality, with demonstration networks operating at con-
ferences to show that it really worked and that various vendors hardware was
interoperable. In 1990, IBM introduced ESCON as part of the System 390 intro-
duction and fiber had become an integral part of their mainframe hardware.
Everybody thought fiber had arrived.
However, at the same time, the copper wire manufacturers had developed
new design cables that had much better attenuation characteristics at high fre-
quencies. Armed with data that their Category 5 unshielded twisted pair (UTP)
cables could transmit 100 150 Mb/s signals over 100 meters and surveys that
showed that most desktop connections are less than that distance, they made a
major frontal assault on the high-speed LAN marketplace. Simultaneously, other
high-speed LAN standards, high-speed Ethernet and asynchronous transfer mode
(ATM), which deliver FDDI speeds on copper wire, became popular. Now cop-
per manufacturers are offering proprietary designs for copper cables that promise
250 MHz bandwidth, although the designs are years away from standardization.
Many potential users continue to postpone making the decision to go to fiber.
So How Do You Decide Between Fiber and Copper?
Some applications are really black and white. Low bit-rate LAN connections at
the desktop with little expectation of ever upgrading to higher bit rates should
use copper. Long distances, heavy traffic loads, high bit rates, or high interference
environments demand fiber. So if you have a backbone and Ethernet or token
ring on the desktop, a fiber backbone and Category 5 UTP to the desktop makes
CHAPTER 3 FIBER OPTIC NETWORKS 39
good sense. If you already have a mainframe in the computer room and are using
channel connections, you probably will use bus and tag cables for connections.
But if you are extending those connections outside the computer room or buying
a new mainframe, you will be getting fiber optic channel extenders or ESCON.
If either media will work in your application, it really comes down to eco-
nomics which solution is more cost-effective. But cost is a combination of fac-
tors, including system architecture, material cost, installation, testing, and
opportunity cost.
More end users are realizing that in a proper comparison, fiber right to the
desktop can actually be significantly cheaper than a copper network. Look at the
networks (Figure 3-5), and you will see what we mean.
The Traditional UTP LAN
The UTP copper LAN has a maximum cable length of 90 meters (about 290 ft.),
so each desktop is connected by a unique UTP cable to a network hub located in
a nearby telecom closet. The backbone of the network can be UTP if the clos-
ets are close enough, or fiber optics if the distances are larger or the backbone
runs a higher bandwidth network than can be supported on copper. Every hub
connects to the main telecom closet with one cable per hub.
Cat 5 Copper Fiber Optics
Horizontal
Telecom Closets
(Hub, power, UPC,
interconnections)
Fiber Patch Panels
Backbone
Main
Cross-Connect
Figure 3-5 Fiber and copper use different network architectures.
40 CHAPTER 3 FIBER OPTIC NETWORKS
In the telecom closet, every hub requires conditioned, uninterruptable power,
since the network depends on every hub being able to survive a power outage. A
data quality ground should be installed to prevent ground loops and noise prob-
lems. It will probably also have a rack to mount everything in (and the rack must
be grounded properly.) Cables will be terminated in patch panels and patch cords
will be used to connect cables to hubs.
The Fiber to the Desk LAN
Fiber optics is not limited in distance as is UTP cable. It can go as far as 2 kilo-
meters (over 6,000 ft.), making it possible to bypass the local hubs and cable
straight to the main telecom closet. It is likely there will be a small patch panel or
wall box connecting desktop cables (probably zipcord) to a large fiber count
backbone cable. At least 72 desktops can be connected on one backbone cable,
which is hardly larger than one UTP cable.
So an all-fiber fiber network only has electronics in the main telecom
closet and at the desktop nothing in between. That means we do not need power
or a UPS in the telecom closet we do not even need a closet! Managing the net-
work becomes much easier since all the electronics are in one location. Trouble-
shooting is simpler as well.
The Myth That Fiber Is More Expensive
The myth that fiber is more expensive has been copper s best defense against fiber
optics. In a typical cost comparison, the architecture chosen is the typical copper
one, and the cost of a link from the telecom closet to the desk, including elec-
tronics, is always higher for fiber although by less and less each year.
But that is not a fair comparison! In a real comparison, we would price the
complete networks shown in Figure 3-5. It would look more like Table 3-1.
So what happens if we total up the costs with this comparison? One estimate
on a bank with no building construction costs had fiber costing only about $9
more per desktop. Another estimate had fiber costing only two-thirds as much as
UTP. Several new construction projects claimed saving millions of dollars by
eliminating all but one telecom closet in a large campus and thereby saving large
amounts in building construction costs.
Fiber also saves money on testing. For fiber, it is a simple matter of testing
the optical loss of the installed cable plant, including all interconnections to
worldwide standards. The test equipment costs less than $1,000 and testing takes
a few minutes per fiber.
Testing Category 5 or 6 UTP requires $3,000 to $50,000 in equipment and
very careful control of testing conditions. Standards for testing are still continu-
ously developed to keep up with new product development. If you consider the
cost of testing, copper will probably cost a lot more than fiber!
CHAPTER 3 FIBER-OPTIC NETWORKS 41
Table 3-1. Comparison of Fiber and Copper Networks
UTP Copper Fiber
Desktop Ethernet Network Interface Ethernet Network Interface
Card for Cat 5 Card for fiber
Horizontal Cabling Cat 5 cable, jacks, wall box, Fiber zipcord, connectors,
patch cord wall box, patch cord
Telecom Closet Patch panel, patch cord, rack, Wall mount patch panel
hub, power
connection, UPS,
data ground
Backbone Cabling One Cat 5 cable per One multifiber cable per
connection consolidation point
Main Telecom Closet Patch panels, patch cords, Patch panels, patch cords,
electronics, power, UPC electronics, power, UPC
Building Space for large bundles of Not needed
(relevant for new cable, large floor or wall
construction or penetrations, big telecom
major renovations) closets, separate grounding
for network equipment
FUTURE-PROOFING THE INSTALLATION
As fast as networks are changing, always to higher speeds, future-proofing is a dif-
ficult proposition. When the decision to install fiber is made, follow up is needed
in the planning phases to ensure that the best fiber optic network is installed.
Planning for the future is especially important. You can easily install a cable plant
for your LAN today that will fill your current needs and allow for network
expansion for a long time in the future.
Follow industry standards such as EIA/TIA 568 and install a standard star
architecture cable plant. Install lots of spare fibers since fiber optic cable is now
inexpensive, but installation labor is expensive. Those extra fibers are inexpen-
sive to add to a cable being installed today, but installing another cable in the
future could be much more expensive.
What fibers should be installed? For multimode fibers, the most popular
fiber today is 62.5/125 micron, since every manufacturer s products will operate
optimally on this fiber. However, most equipment is also compatible with
50/125 fiber, which has already been installed in some networks, especially mil-
itary and government installations in the United States and throughout Europe.
All singlemode fiber is basically the same, so the choice is easier, although for
42 CHAPTER 3 FIBER-OPTIC NETWORKS
most applications the specialty singlemode fibers (e.g., dispersion shifted or flat-
tened) should be avoided.
Paying a premium for higher bandwidth or lower attenuation specifications
in multimode fibers can allow more future flexibility. Very high-speed networks
have forced fiber manufacturers to develop better fibers for gigabit networks.
Installing that fiber today may make migrating to gigabit networks easier in the
future.
How many fibers should be installed? Lots! Installation costs generally will
be larger than cable costs. To prevent big costs installing additional cables in the
future, it makes good sense to install large fiber count cables the first time; how-
ever, terminate only the fibers needed immediately, since termination is still the
highest labor cost for fiber optics.
Backbone cables should include 48 or more fibers, half multimode and half
singlemode. If you are installing fiber to the desktop, 12 fibers, again half and
half, will provide for any network architecture now plus spares and singlemode
fiber for future upgrades.
The new generation of gigabit networks may even be too fast for multimode
fiber over longer distances and they will use lasers and singlemode fiber to
achieve >1 GB/s data rates. If you want to use fiber for video or telecom, you may
need the singlemode fiber now. But you may not want to terminate the single-
mode fiber until you need it, since singlemode terminations are still more expen-
sive than multimode; however, they are getting less expensive over time.
Fiber optics has grown so fast in popularity because of the unbelievably pos-
itive feedback from users. With proper planning and preparation, a fiber optic
network can be installed that will provide the user with communication capabil-
ity well into the next decade.
REVIEW QUESTIONS
1. Three areas in which fiber is used:
1. ________________
2. ________________
3. ________________
2. Match the application with the main reason fiber is the choice of transi-
tion medium.
______ LAN a. upgradeability
______ CATV b. reliability
______ Telecom c. high bandwidth and distance advantages
3. FTTC stands for ________________ .
4. FTTH stands for ________________ .
CHAPTER 3 FIBER-OPTIC NETWORKS 43
5. The development of ________________ made fiber cost-effective for
CATV applications.
a. repeaters
b. FM systems
c. AM analog systems
d. enormous bandwidth
6. Match the following LAN standards with their counterparts in the right
column.
______ Ethernet a. dual counter-rotating ring
______ ESCON b. most widely used LAN
______ FDDI c. connects peripherals to a mainframe
______ Token ring d. originally developed for copper networks
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