FREE ESSAY ON DWDM

DWDM

Bandwidth is the capacity for carrying digital information. In the past few years,
bandwidth has been increasing at roughly twice the rate of computing power. Data traffic
has been doubling every 100 days (according to the U.S. Department of Commerce) and it is
predicted that that Internet traffic will reach 16 million terabytes by 2003. Fiber is
the transport medium of choice. It out performs copper, coaxial cable, and high frequency
radio in data-carrying ability. As service needs increase, fiber exhaust becomes a
problem. In some areas it is not economically feasible to lay new fiber. In these areas,
it is important to look at alternatives to provide economical solutions. One of the
biggest areas of concentration is how to increase the amount of bandwidth available. 
The bandwidth of an optical network is principally determined by how fast the light can
travel through the glass as well as how much light the fiber can have capacity for at any
given nanosecond. Although higher transmission speeds are trying to be achieved, the
speed of light is constant. Furthermore, the fiber increases in efficiency as the
diameter decreases. This makes efforts to increase bandwidth focus largely on how to push
more data through the same fiber cable at any given time. 
Multiplexing, by definition, is the process where multiple channels are combined for
transmission over a common transmission path. In the early 1990s, fiber could only carry
one wavelength, or color, of light at a time. Lasers were used by quickly turning them on
and off. By the mid 1990s wave division multiplexing could split the light into two
colors. The number of colors rapidly grew and today as many as 160 colors can be carved
out by using the most advanced systems, in what is now called dense wave division
multiplexing (DWDM). In other words, DWDM combines multiple optical signals so that they
can be amplified and transported over a single fiber. A DWDM network can merge signals
operating at different rates. An example would be a DWDM network with a mix of SONET
signals operating at OC-48 (2.5 Gbps) and OC-192 (10 Gbps) over a DWDM infrastructure can
achieve capabilities of over 40Gbps. The reliability of the system is maintained
throughout this process.
DWDM networks are self-regulated at the bit-rate and format level. They can also accept
any combination of interface rates on the same fiber at the same time. This greatly
increases the flexibility of the system. The communication industry can become fully
integrated, using multiple vendor interfaces with distinct technologies into one physical
infrastructure. The fiber itself would remain transparent to the protocol or type of
information. If a carrier operates both ATM and SONET networks, it is not required that
the ATM signal be multiplexed up to the SONET rate. 
A key feature of the DWDM network is that it exists at the Physical Layer. An all-optical
network implies that the service provider will have optical access to traffic at various
nodes in the network. Wavelengths can be added or dropped to or from a fiber without the
aid of a SONET terminal. Optical cross-connects offers service providers the ability to
have admittance to the network at these locations.
The fiber-optic amplifier section of the DWDM system rids you of the need to change the
optical signals into electrical when amplifying. An optical fiber is used that has been
treated with the element erbium. The pump laser is used to energize the erbium with light
at a specific wavelength. The erbium acts as a gain medium that amplifies the incoming
optical signal. If a connector were to be used rather than a splice, slight amounts of
dirt on the surface could cause the connector to become damaged. Fluoride or Silica based
fiber amplifiers could also be used. In the 1530 to 1565 nm range with filters, silica
based amplifiers and fluoride based amplifiers work equally well. However, they cost
more. Fewer amplifiers are needed due to the multiplexed system, using a broad range of
wavelengths in the 1.55 um region. A multiplexed system with 16 wavelengths on a single
fiber can decrease the number of amplifiers by that number at each regenerator site. 
It becomes clear that service providers can establish infrastructures were the capacity
could be increased as needed by combining multiple technologies. In optical networking,
DWDM could be compared to accessing the unused lanes of the highway. With each individual
color being a lane. The lanes of the highway are blind to the types of traffic that
travel on them. If you increase the capacity for data traffic within the lanes as well as
the number of lanes themselves, you have greatly enhanced the capacity of the fiber path.
The following description is one possibility of how to increase capacity without
increasing the amount of fiber.
The various digital signals come from homes, businesses, phone companies, and many other
sources. The signals up until this point can be light or electrical pulses. They first
need to be brought into what is known as a time division multiplexing box. A time
division multiplexer (TDM) provides for the transmission of multiple signals over a
common path by using successive time intervals. A preset time interval is used and the
discrete data streams are sampled and interleaved by the TDM to provide a single data
stream at a much higher data rate than the input data stream.
This can then be multiplexed even further using the DWDM system. A classic DWDM system
would have a light source on the transmitter side that operates at a specific wavelength
from 1530nm to 1565nm at .8nm spacing. A DWDM multiplexer is then used to combine the
wavelengths onto the same transport fiber, where the information carried by each is
converted into a different color. The collection of colors is then shipped down different
lanes along a single piece of fiber. Fiber amplifiers are spaced through the system. They
operate at the same wavelength band as the source laser. The amplifier increases the gain
of the incoming signal for retransmission at equal power levels for each wavelength
channel. A demultiplexer is used at the receiving end to separate the wavelengths. It
would then be necessary in this scenario for the signals to be processed in a time
division demultiplexer. There are many different types of receivers that can then take
the optical signal and convert it into lower speed electrical signals for distribution.
This scheme is just one way utilized in creating more bandwidth. It could even be taken a
step further by adding frequency-division multiplexing (FDM). A lens sits in the nodes
between the strands of fiber, between the TDM and the DWDM multiplexers. The FDM box
converts light beams from the TDM boxes into radio frequencies which are packed closely
together by staggering them for about a billionth of a second. The frequencies are then
stacked creating multiple lanes of information within one composite radio wave. The FDM
box then converts the radio wave into a continuous analog wave of light that travels to
the DWDM. Of course, the signals would again have to be demultipluxed at the receiving
end.
DWDM systems in the past were predominately deployed in long haul and submarine cable
markets. Long haul optics are essential due to cost-cutting advantages and increased
efficiency. However, these systems have increased in use for the metropolitan markets. To
date North America has led in the deployment of WDM systems. The fastest growth portion
occurring in the metropolitan applications. On the other hand, in Europe, the span
lengths are greatly reduced which may cause systems to be extensively deployed in the
future. In 1998 the worldwide demand for DWDM systems was estimated be at $2.1 billion
and was expected to grow to as much as $12.1 billion by 2005 (as forecasted by
Electronicast).
The growth into the metropolitan areas is driven largely by the transparency of the
network and the opportunity for revenue generating services. Competitive Local Exchange
Carriers (CLECs) use lines that are leased from the regional Bell Operating Companies
(BOCs). The CLECs are driven to use the new technologies in order to maximize their
profits. In turn, the BOCs must utilize the new technologies in order to stay as
efficient and therefore competitive with the CLECs. 
In conclusion, when the different ways of using multiplexing are combined it improves the
amount of bandwidth dramatically. These technologies are extremely important in today's
society were we are seeing high demands for data traffic. By having an all-optical
network, we are provided with the ability to integrate the many different technologies
into one system. The DWDM network enables the service providers to go long distances as
well as provide for data lines and other growth in metropolitan area. There are more ways
to multiplex than are discussed in this paper and different combinations can be used. 
Bibliography
References Used:
All are Internet URL sites 
1) http://telecom.tbi.net/mux1.htm 
2) http://www.columbia.edu/cu/cie/techlists/patents/4926423.htm 
3) http://www.cis.ohio-state.edu/~jain/talks/h_5opt.htm 
4) http://www.techguide.com/comm/dwave.shtml 
5) ftp://ftp.netlab.ohio-state.edu/pub/jain/courses/cis788-99/h_aipwd6.pdf 
6) http://www.iec.org/tutorials/dwdm/topic08.htm 
7) http://www.ert.rwth-aachen.de/Projekte/Theo/OFDM/node6.html