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 By
Majid L. Riaziat Ph.D
One
bit of information is a yes or no answer to any question. Telecommunication
networks transmit bits of information by sending electronic 0 and
1 signals. These signals are used to reconstruct voice, images,
etc. at the destination point. A typical conversation is transmitted
through the telephone line by 64,000 bits of information per second
(64 kbps). In contrast, a TV channel uses approximately 6,000,000
bits of information per second or 6 Mbps (A picture is worth a thousand
words!).
Telecommunication networks transmit telephone conversations, FAX,
cable TV, Internet data traffic, etc. They use copper wires, radio
waves, microwaves, or more recently, laser light traveling in an
optical fiber. Each one is known as a transmission medium. Every
telecommunication network has a network capacity that is the maximum
bit rate it can transmit. Network capacity depends on the medium
and the technology used. Table 1 compares the capacities of a number
of communication technologies.
Telecommunication
networks consist of multiple communication links. Each link has
a transmitter, a receiver, and a transmission medium. Signals traveling
between the transmitter and the receiver are attenuated by the medium
and may need to be amplified or even fully regenerated along the
way. Amplification and regeneration add cost to the installation
and operation of the link. Optical fiber is the lowest-loss guiding
medium available today. Infrared light at the wavelength of 1550
nm experiences the minimum attenuation of approximately 0.25 dB
per kilometer of propagation. Figure 1 shows the attenuation of
infrared light through silica fiber as a function of its wavelength.
Telecommunication companies that historically provided telephone
service, and even those that provided cable TV are increasingly
relying on fiber links for information transmission
Table 1. Some telecommunication technologies and teh approximate
capacity associated with each.
Depending
on transmission distance and the clients served, fiberoptic networks
are referred to as submarine, long-haul, metro, access, or enterprise.
Long-haul networks typically transmit information through a few
hundred to a few thousand kilometers. Metro networks serve metropolitan
areas and employ links of a few tens to a few hundreds of kilometers.
Access networks span a few kilometers between a central office and
the consumer or the commercial customer. Finally, enterprise networks
provide connectivity within a building or multiple sites of a company.
Development Since the 1990’s
The dramatic increase in data traffic that started in early 1990’s
led to a revolutionary growth in fiber optic telecommunications.
Prior to 1990’s, network capacity was driven by voice traffic
that grew at the rate of 4 to 7 percent per year. In contrast, since
the late 90’s, data traffic has been growing at the estimated
annual rate of 70% to 200%. In year 2000 for the first time data
traffic volume in the US overtook voice traffic. Even in the current
economic downturn, data traffic continues to grow at the approximate
annual rate of 70%. It should be pointed out, however, that as of
this date, voice traffic is still where most telecommunication companies
make their profits. Residential data traffic in particular is not
a moneymaking venture.
The bulk of the growth in long-haul telecommunications has been
made possible by advances in fiber optic technology. Specifically,
the adoption of dense wavelength division multiplexing or DWDM (discussed
below) has played a crucial role. Currently, with DWDM technology,
a single strand of optical fiber is capable of carrying a data rate
equivalent to approximately five communication satellites (Table
1). The capacity of the optical fiber can easily increase by another
factor of ten within the next few years. Optical fiber provides
the largest capacity and, in many cases, the most economical medium
for long-haul telecommunications.
In order to appreciate the global pervasiveness of optical fiber,
consider the fact that by the end of year 2000 the total length
of installed fiber in the world grew to 180 million kilometers.
This is enough to circle the earth 4,500 times.
Since early 1990’s the information carrying capacity of a single
optical fiber has increased by a factor of more than 100 (Figure
2). The explosive growth of Internet data traffic created the demand
for this increased capacity. In 1992 the code for the World Wide
Web was first made available, and by 1996, the number of Web sites
had grown to over 230,000. It was apparent that the Internet data
traffic would grow much faster than the speed of the electronic
circuitry that had previously supported capacity growth (Moore's
Law). Data traffic growth required major innovations rather than
incremental electronic improvements. These major innovations came
in the form of DWDM and EDFA.
DWDM and EDFA
Wavelength Division Multiplexing, WDM, or its dense version, DWDM
places a number of discrete wavelengths or optical channels from
multiple lasers on a single optical fiber. Typically, the laser
light sources operate in the low-loss 1550 nm infrared window of
the optical fiber, and have narrow enough spectral widths to occupy
the same fiber without interfering with adjacent wavelengths. Adjacent
wavelength spacings have been shrinking over the years. Currently,
100 GHz spacing is typical. In 1989, a WDM system was deployed with
two channels carrying 3.4 Gbps per fiber. Today’s standard
DWDM employs over 128 wavelengths at 10 GB/s for 1.28 Terabit transmission
over a single fiber. This equals 165 million equivalent voice circuits.
The key enabler of DWDM was the EDFA or the Erbium-Doped Fiber Amplifier
that first appeared on the market in 1993. EDFA amplifies the optical
signal in multiple DWDM channels simultaneously without having to
convert them into electrical signals first. EDFA eliminates the
need to have a large number of expensive regeneration centers. The
cost reduction achieved this way makes it practical to combine or
multiplex tens to hundreds of channels or wavelength onto a single
optical fiber. The typical wavelength range of the EDFA defines
the “C” band in fiber optic communications shown in Figure
1.
Figure 1. Optical communication bands currently in use. the solid
curve is the typical attenuation of infrared light per kilometer
of propagation in silica optical fiber as a function of wavelength.
Technology
Trends
In order to meet the growing demand for transmission capacity, three
major areas of development are being actively pursued concurrently:
(1) Increasing spectral efficiency or information transmitted per
bandwidth used; (2) expanding utilized optical bandwidth; and (3)
extending transmission distance between signal regenerations. These
areas are addressed individually in the following discussion. The
question of transparent vs. opaque networks is also addressed.
Spectral Efficiency:
The one terabit per second capacity of optical fiber cited above
(100 channels at 10 Gbps data speed per channel) occupies 10 terahertz
of bandwidth. This corresponds to a spectral efficiency of 0.1 bps/Hz.
The practical limit of spectral efficiency using current protocols
is thought to be 3 bps/Hz. Therefore, capacity enhancement by a
factor of 30 can be achieved by improved spectral efficiency alone.
One way to improve spectral efficiency by a factor of 4 is to increase
the data speed per channel from 10 Gbps to 40 Gbps while maintaining
100 GHz channel spacing. Major experiments and field trials are
currently underway to accomplish this network upgrade. Figure 3
shows optoelectronic integrated circuits designed to work with 10
and 40 Gbps optical links.
Utilized Bandwidth:
Utilized communication bands of the optical fiber are the medium
and long wavelength bands shown in Figure 1. The most efficiently
used band is 40 nanometers around 1550 nm wavelength known as the
C band. Wavelengths close to 1400 nm are not used due to the absorption
peak caused by residual moisture in the fiber. Wavelengths around
1300 nm are sparsely used primarily for metro applications. L and
S bands in the long wavelength region will be fully utilized in
the near future as new fiber amplifiers become available. The water
absorption peak will be eliminated in future fibers. There is no
fundamental barrier against using the full bandwidth of 1000 to
1700 nm. This wavelength span is over 80 times what is utilized
today. Combined with improved spectral efficiency, fiber capacity
can grow by over 2000 times with incremental advances in technology.
If and when this potential is realized will depend on economic factors.
Figure 2. Laboratory demonstration of fiber capacity and the
product of fiber capacity times distance since 1993.
Transmission Distance:
The presence of dispersion and cross-talk place limits on transmission
distance between signal regenerations. Since higher data rates are
more severely affected, a tradeoff exists between system capacity
and transmission distance. In order to gauge the progress in optical
network technology, it is often the capacity times distance that
is monitored. Figure 2 shows the highest capacity times distance
products achieved in laboratory experiments since 1993. Note the
improvement by a factor of 100.
Figure 3. 10 and 40 Gbps optoelectronic integratea circuits fabricated
at OEpic, Inc.
Opaque vs Transparent Optical Networks:
There is a definite advantage in avoiding electronic regenerations
in the network. Electronic regeneration is expensive and the equipment
needs to be upgraded every time data transmission speeds increase.
For this reason some researchers would like to see networks that
are fully “transparent”. This means that optical information
is not converted to electronic information anywhere between initial
generation and final destination. All switching and routing is done
in the optical domain. As a consequence, the system is insensitive
to data rate. On the negative side, signal impairments accumulate
along a transparent network. This requires sophisticated centralized
planning of each link. Furthermore, a transparent network still
lacks system intelligence, i.e., no information is available about
data traffic. Performance monitoring and fault detection are quite
complicated.
At the other extreme is the “opaque” optical network where
electrical regenerations are abundant. All system functions including
traffic monitoring and fault detection are done electronically in
a straightforward manner. The system is expensive and resistant
to data rate upgrades. This was the only architecture available
in the early days of optical networks. Today’s optical networks
are a “hybrid”, meaning that there are “islands of
transparency” but still many system functions are performed
electronically. There is a steady trend toward generating more transparency.
However, practical communication systems are likely to remain of
the hybrid type for the foreseeable future.
Cost
Optical fiber is made of silica glass and costs much less than copper
wire or coaxial cable used in telephone and cable TV transmission.
Also, as data rates per fiber increase, the price per bit of transmitted
information drops. Nevertheless, the price of a broadband connection
has not dropped far enough yet for the average household to subscribe
to a fiber connection. It is envisioned that a 100 Mbps connection
to the home will satisfy the entire household’s multiple-service
information needs, including telephone, Internet, and video-on-demand
for the foreseeable future. The average household is willing to
pay approximately $100 per month for such a connection. Currently,
a provider’s cost to deliver this bandwidth is between 50 to
100 times higher. One can therefore conclude that once cost reduction
by a factor of fifty is achieved, a very large new market will open
for fiber-to-the-home services. Fiber-to-the-home will provide a
versatile and scalable conduit for bi-directional transmission of
all types of information.
Conclusion
We are observing the beginnings of a revolution in fiber optic communications.
The enormous information carrying capacity of the optical fiber
has been known for a long time but was not demonstrated in field
implementations until the mid 1990’s. It was during the last
decade that the telecommunication fiber surpassed the communication
satellite in information carrying capacity. We are still far from
reaching the capacity limits of the optical fiber. An additional
improvement by a factor of 2000 can be achieved without introducing
any radically new technology. Cost reduction goes hand in hand with
increased capacity, and in the near future, the average household
will be able to afford the advantages of a broadband fiber optic
connection.
Majid L. Riaziat is the CTO and a cofounder of OEpic, Inc., a manufacturer
of high-speed integrated optoelectronic devices for fiber-optic
communications. He was previously the director of Central Research
at Varian Medical Systems in Palo Alto, California. He has 19 years
of experience in technology development and acquisition. Dr. Riaziat
received his Ph.D. in Applied Physics from Stanford University in
1983.
Majid
L. Riaziat is the CTO and a cofounder of OEpic, Inc., a manufacturer
of high-speed integrated optoelectronic devices for fiber-optic
communications. He was previously the director of Central Research
at Varian Medical Systems in Palo Alto, California. He has 19 years
of experience in technology development and acquisition. Dr. Riaziat
received his Ph.D. in Applied Physics from Stanford University in
1983.
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