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DWDM: Optical Networks in the Enterprise
Executive Summary
The purpose of this report is to inform management about the company’s difficulty
involving the network capacity being reached. The company must decide whether to
deploy new fiber, upgrade the Synchronous Optical Network (SONET) or use the Dense
Wavelength Division Multiplexing (DWDM) transmission protocol. Solutions that are
currently available in the telecommunications market to help ease congested networks will
be discussed as well as the associated costs involved such as training, performance,
deployment and management of the network.
The report also gives a brief overview of the DWDM technology, which includes
the basics of multiplexing and de-multiplexing wavelengths into one signal, the
components needed and the different types of DWDM networks currently in use. The end
discussion will cover how the technology addresses the company’s issues with limited
bandwidth by giving an outlook of and the markets in which DWDM is being used. The
initial costs and incentives such as return on investments, convergence of the network, and
increasing capacity will be discussed.
Issues and Background
The main issue is with the network reaching capacity as the demand for data,
video, and voice increases, which is referred to as the ‘Triple Play’ {“The Promise of the
Triple-Play” 1}. Currently management is looking into deploying more fiber to offset
this but laying more fiber may not be the answer. As concerns for additional resources to
keep up with the current demands are raised as well as for the future growth of the
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network, further fiber deployment will become more of a financial pressure and less of a
practical alternative. With the company’s interests, being greater than ever before, in the
process of increasing revenue and lowering costs, fiber deployment will not provide
immediate relief because of the added costs of extra equipment needed for the new fiber.
This will also require extra man-hours for the planning, optical budgeting (process of
calculating loss in the network due to splices, connectors, patch panels, etc), installing
and configuring the equipment into the network. In addition, testing and/or repairs of the
fiber will be needed during the process.
A viable solution to help alleviate the amount of congested traffic that is
present on the network must be pursued. One option is to determine whether to upgrade
the existing SONET network or to convert to a DWDM optical interface transmission
system. If the preferred solution is to upgrade the SONET network, the next Signal
Digital Hierarchy (SDH) level available will be OC-192. The associated start-up costs in
changing out the OC-48 equipment i.e., modules, plus the labor involved would be
substantial. A point of interest is that because SONET has high levels of overhead, this
will decrease the total data payload offered (Newton 685). Therefore, in order to meet
demands for future growth and current bandwidth requirements, SONET will not be able
to provide the relief that is needed for the network. Also, SONET works for voice
communications well but was not meant to handle the bursty nature of Internet Protocol
(IP) traffic.
One other viable solution to consider is using DWDM. This gives the ability to
use the existing network and transmit/receive over one fiber. The technology is also
protocol transparent meaning that it does not matter what is sent across the optical layer
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when using DWDM (Zeisler 1). According to Jerry Zeisler’s article, Optical Networks
For The Enterprise: The Broader View, some of the features that DWDM offers are,

Protocol Transparency/Low Latency--Most DWDM systems support Fibre
Channel, ESCON, ATM, FICON, GbE, SONET/SDH, Sysplex Coupling
Facility timers, and video. Little latency through the device is noticed
because only modest processing is required to direct the optical signal
from the input to the appropriate laser or vice versa (2).

Capacity, Bandwidth, and Speed, and lots of it--up to about 640Gbps
(Gigabits per second) per system today. Protocols and applications can all
run at their native speeds, i.e., 17MB (MB/sec) for ESCON, 1.062Gbps
for Fibre Channel (FC)--2.12Gbps soon--and 1.25Gbps for Gigabit
Ethernet (GbE)…(1)
With these features, the ability to grow the network over time is feasible as more data and
bandwidth demanding applications are used (Zeisler 3).
Brief Overview of the DWDM Technology
The basics of the DWDM network consist of multiplexing and demultiplexing
wavelengths of different frequencies and the specific components needed for this process.
Since DWDM involves dealing with various wavelengths, this requires the generating
and combining of wavelengths into one signal (“Fundamentals of DWDM Technology”
2-4). To do this, the signals must be transmitted and received by specific components.
The end result is the breaking down or demultiplexing of the signals back into their
respective wavelengths. See figure 1 below for reference.
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Figure 1 Wavelength
The components consist of optical multiplexers, demultiplexers, filters, gratings,
amplifiers and regenerators. A well-known example of an optical demultiplexer is the
prism. When white light hits the prism, it separates the light into its spectral components.
The demultiplexer works in the same way by using filters to break out the light into its
respective wavelength. Another component of interest are the Optical Add/Drop
Multiplexers (OADM). These types of multiplexers will allow certain wavelengths to be
added or dropped at certain points in the network while allowing the other wavelengths to
pass through. The advantage of this is the ability to add/drop data, voice, etc. from
certain nodes in the network thereby reducing signal loss in the network as opposed to
separating the entire signal, taking what is needed and then returning it to the network.
Whenever there is an optical to electrical to optical conversion, it creates not only a point
of failure but also attenuation (decrease in signal strength).
Fiber Bragg Grating (FBG) is defined as “a narrowband reflection filter
permanently written into the core of single-mode optical fiber that enables the type of
wavelength precision that is necessary for DWDM systems to successfully combine,
control, and route multiple ‘colors’ of light within a single optical fiber” (Newton 294).
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In other words, according to Newton, the gratings are sections of optical fiber that have
changes in their refractive index.
Fiber Amplifiers and Regenerators are necessary for long haul circuits. The
difference between the two is that amplifiers only take the signal and boost its levels,
which often includes noise. This can have an effect with the signal to noise ratio that will
show up as attenuation on the signal levels, which are measured in decibels to one milliwatt (dBm). As for the regenerators, they will take the incoming signals, break them
down to their electrical properties, clean up the 1’s and 0’s, and convert them back to an
optical signal re-transmitting them back out again (Newton 619). In this case, the 3R
generators, which “reshape, retime, retransmit” (Islam et al. 283) are the preferred type of
components to have in the network.
How the Technology Addresses the Issue
In order to explain how the technology addresses the issue, an overall outlook
must be made in regards to where DWDM is found in the market as well as the current
costs to deploy it. At first, the Baby Bell companies used DWDM in long haul networks
to help alleviate fiber exhaustion and the costs involved in running new fiber from one
city to another. But over time, telecommunication companies and private enterprises
have begun to view DWDM as an option for the metropolitan network, which had
predominantly been SONET based. SONET technology is a proven transmission system
that has management, provisioning, error checking, etc embedded into it. However, the
main issue with SONET is the expense involved to upgrade to the next higher bit rate.
All systems within the network must be upgraded at the same time in order for SONET to
work correctly. In the example of going from OC-48 to OC-192, the initial costs would
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be great and the available bandwidth would be more than what is needed in the
beginning. In return, the amount to be covered for the costs will be out of pocket since
there will be bandwidth that is not in use, meaning not being charged. This is where
DWDM becomes cost affective. With the ability to add on to the existing SONET
network using DWDM in areas reaching capacity, this will give a company a scalable
network or ‘pay as you grow’ (“Fundamentals of DWDM technology”). It is much easier
to lease new wavelengths as opposed to laying new fiber.
Since DWDM is becoming ideal in the metropolitan market, carriers can use
reconfigurable optical add/drop multiplexers (ROADMs) to offer even more services to
the enterprise at lower costs to the carriers (Field 35). Using roadm’s will allow
telecommunication companies to add and/or drop wavelengths wherever they are needed.
No longer will the complete signal have to be demuxed just to add or drop 1 one
wavelength.
Finally, in regards to the long haul networks, companies can upgrade to OC-192
when the need so pays for itself. This will give the added bandwidth using DWDM
which future proof the network and allow years of available capacity. The need for
networks to re-compensate the company or have short turn-around times has become
increasingly important. With DWDM, the full potential of fiber can be met with modest
costs for start-up and overtime can be used along side the upgrades to higher bit rates.
Summary
In reading this report, using a DWDM solution shows added benefit by allowing
for more capacity on fiber-exhausted areas. Also of importance is the low initial costs
which can allow a “pay as the network grows” (Field 35) option. No other technology
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offers this ability and DWDM can be used in parts of the network rather than required in
the entire network before it will work. The technology of wavelengths will continue to
grow as more vendors develop additional channels that be used. Currently, there are 40
channels/wavelengths in OC-192 systems that allow an aggregate bandwidth in the
terabit range.
Works Cited
Field, Barry. "ROADM Spurs DWDM Scalability." Network World. 01 Jan. 2005: 35.
Proquest. 07 May 2005. Keyword: DWDM.
Fuller, Meghan. Timing right for ROADMs? 01 May 2005. Senior
Editor for Lightwave. 24 May 2005
<http://lw.pennnet.com/articles/article_display.cfm?Section=ARTCL&C=I
ndus&ARTICLE_ID=228501&KEYWORDS=ROADMs>.
Hicks, Brady. "Optical Switching and Routing Market Trends." Faulkner Information
Services. 07 Apr. 2004: 1-12. Faulkner. 05 May 2005. Keyword: DWDM.
Islam, Mohammed, Mohammad Masum, and Mizanur Rahman. "DWDM Technology:
Implemation on a Unidirectional System Through Algorithmic Apporach."
Fiber and Integrated Optics. 2003: 282-285. IEEE. IEEE via
DeVry. 12 Apr. 2004. Keyword: DWDM.
Newton, Harry. Newton's Telecom Dictionary. New York: CMP Books, 2002.
Zeisler, Jerry. "Optical Networks for The Enterprise: The Broader View." Computer
Technology Review. 2001: 35. Proquest. DeVry. 21 Apr. 2005. Keyword:
DWDM.