This document discusses the evolution from 10G to 100G networks for metro applications. It describes how 100G technology can complement existing 10G infrastructure by utilizing unused wavelengths on fibers. A key consideration is maintaining economic viability while providing greater bandwidth efficiency. Direct detection is seen as more suitable than coherent detection for metro due to lower cost and complexity. The document outlines various technological requirements for a 100G network such as 100G transceivers, DWDM multiplexers, optical amplifiers, dispersion compensation, and FEC. It provides an example architecture for upgrading an existing 10G system to incorporate 100G services over longer distances.

1. Evolution from 10G to 100G
for a Metro Network
White paper

2. Introduction
With data traffic volume increasing by around 40% each year, the prevailing 10 Gb/s
optical networks are quickly becoming saturated. As the long-haul technology research
tries to avert the ‘capacity crunch’ with novel fibers, advanced optical components and
sophisticated digital signal processing, the change in technology for metro networks,
enterprise and datacenters has to be smooth yet swift. For the roadmap to 100G metro
networks, economic viability is of paramount importance together with greater space,
power and bandwidth efficiency. It is essential that the upgrade takes advantage of the
current infrastructure with minimal disruption to existing services and is inherently flexible to
further accommodate newer equipment as per demand.
Overlaying flourishing 10 Gb/s services with additional co-propagating 10 Gb/s
channels in different colors, or wavelengths, is already common practice. Network
operators eager to increase capacity have begun overlaying 40 Gb/s channels onto
available fiber without leasing more dark fibers as the best pragmatic approach. With the
advent of 100G technology, the interest is now shifting from 40G to 100G installations.
This paper provides a primer in 100G technology developments and examines important
parameters and prerequisites supporting practicable 100G metro network solutions. An
example of complementing the existing 10 Gb/s services with new 100 Gb/s WDM
channels is also discussed.
Direct or Coherent Detection: Which one is for Metro?
The transmit signal strength is limited by laser heat dissipation and power consumption.
Consequently, significant research and development efforts have been undertaken to
improve the sensitivity of the receiver. Two technology alternatives exist: direct detection
or coherent detection.
Predominantly suited to trans-oceanic submarine or terrestrial long haul applications, the
performance of the coherent detection is undeniably superior to that of direct detection. In
the case of coherent detection, the opto-electric conversion process is linear. Thus the
phase information embedded in the optical signal is preserved permitting the straight
forward electrical compensation of fiber linear effects including chromatic dispersion (CD)
and polarization mode dispersion (PMD). However, the hardware required to perform
coherent detection is somewhat more elaborate and comprises a local oscillator, a 90°
Hybrid module necessary to discriminate the phase quadrature’s of the received optical
signal, and four balanced photodiodes to detect the signal from a single polarization as
seen in Fig. 2.
Figure 1
Preconditions and requirements
for 100G metro evolution

3. D-5116 Rev.A.1 3
Evolution from 10G to 100G
for a Metro Network
A direct detection technique, on the other hand, requires only a delay interferometer and
two single photodiode translating into a device of much lower cost and complexity. The
advantage of longer transmission distances (in the range of several 1000 km) enabled
using a coherent approach is overkill for deployments of much shorter reaches - ranges of
several 100 km. The direct detection technique is thus seen as an attractive alternative for
metro networks, enterprise and datacenters where economic viability plays a vital role.
Taking the minimalist approach, direct detection works without additional equipment in
most cases. When the application range approaches the limit of direct detection
technology, the simple addition of dispersion compensation may be used to augment
signal detection and extend reach.
Technological Requirements
Similar to 10G multiplexed networks, 100G networks begin with the use of 100G
transceivers and dense wavelength division multiplexing (DWDM) multiplexers or de-
multiplexers. Depending on the application scenario, 100G networks may require optical
amplifier, dispersion compensation modules and FEC enabled transponder cards. In this
section, we delve deeper into the details of these components.
The 100G DWDM-CFP transceiver is currently employed for applications associated
with distances of up to 20 km. For transmission reaches of up to 20 km, the optical
amplifier and dispersion compensation module are not required. The 100G CFP uses four
lasers, each tunable over the DWDM grid, such that the resulting link realizes 4 x 25
Gb/s with each 25 Gb/s travelling in a separate single mode duplex fiber.
Figure 2
Coherent detection using optical
90° hybrid and balanced
photodiodes
Figure 3
100G DWDM CFP
• Transports 4 x 25 Gb/s
• Four single mode duplex fibers
• Employs tunable lasers in the 50 GHz or 100 GHz ITU-T
channel grid (DWDM) with four receivers

4. The DWDM multiplexer or de-multiplexer is the core technology providing a platform for
adding essentially unlimited co-propagating channels to existing 10G and 40G channels
with the 100G upgrades and eventually enabling the ability to upgrade all the channels
to 100G. In DWDM, multiple wavelengths are used to carry the optical signals through
the same fiber leading to higher spectral efficiency per fiber. Pass band bandwidth, pass
band flatness, stop band rejection ratio, transition slope from pass band to stop band and
insertion loss are the key specifications determining selection of the multiplexers and de-
multiplexers.
Traditional thin film optical filters based DWDM multiplexers provide excellent
wavelength selectivity. However, since these are free-space optical devices, much care
must be taken to precisely align individual optical sub-components to achieve perfect
collimation of the optical beams. Additionally, the complexity increases with the increase
in the channel count.
Thus, arrayed waveguide gratings (AWG) based on planar light wave circuit (PLC)
technology, are widely used for multiplexing large numbers of channels. The AWGs are
composed of integrated multiple waveguide delay lines to render multipath optical
interference. As depicted in Fig. 4, an AWG consists of input and output waveguides, two
star couplers and an array of waveguides inter-connecting the two star couplers. Each
waveguide within the array manifests a slightly different optical length, leading to different
interference at the output when the differing paths recombine. The AWG can be built into
a 19” rack type installation as shown in Fig. 5.
Depending on the transmission power budget available from the transceiver (e.g., current
DWDM 100G CFP are limited to a power budget equivalent to about 20km), an optical
amplifier can be introduced to overcome the additional loss encountered from the fiber
attenuation, optical power splitting, and other factors. For a DWDM application, an
erbium-doped fiber amplifier (EDFA) is the best choice as it provides high gain, high
optical signal power, low crosstalk between wavelength channels and convenient optical
coupling from and into optical fibers.
Figure 4
Composition of an AWG
Figure 5
DWDM unit for
19” rack type installation
….
….Input Outputs
1
2N
1
2
N
1st star
coupler
2nd star
coupler
Waveguide
array

5. D-5116 Rev.A.1 5
Evolution from 10G to 100G
for a Metro Network
The EDFA is composed of an erbium-doped fiber, a pump laser, an optical isolator, and a
wavelength-division multiplexer. The amplifier needs to have a wide optical bandwidth
and flat optical gain within the WDM bandwidth. Depending on the application and
length, the gain of the amplifier has to be adjusted so that the power at the receiver does
not exceed the receiver sensitivity limits and destroy the transceiver.
The phenomenon of temporal broadening of the signal while the signal travels through
the optical fiber is termed dispersion. A DWDM 100 Gb/s CFP typically offers a
dispersion tolerance of 400ps/nm which typically permits a reach of 20 km without
compensation. This optical phenomena causes overlapping of pulses called inter-symbol
interference (ISI) as shown in Fig. 6. Due to ISI, the detector cannot detect the data
correctly, increasing the number of error bits and consequently limiting the transmission
distance feasible.
The dispersion compensating fiber (DCF) module is widely used to overcome the
obstacle of ISI. A DCF module consists of a special fiber inducing a large value of
negative (normal) dispersion at the 1550 nm wavelength window. The advantage of
DCF for DWDM applications is that the DCF may be applied over a wide wavelength
window and allows tailoring the slope of chromatic dispersion to match with SMF-28 for
C-band channels.
Forward error correction (FEC) is a means of mitigating errors in data transmission to
effectively provide OSNR enhancements and thereby extending the transmission reach of
high data rate optical systems. Redundancy is added into the signal in terms of additional
control bits which are used at the receiver to identify and then correct errors in the
received signal. FEC coding can be implemented in the transceiver itself or in the
transponder line card hosting the transceivers.
Complementing the existing
10 Gb/s system with 100 Gb/s upgrades
The phenomenal growth in new applications such as cloud services, telemedicine, video
on demand etc., has ushered in the unprecedented outburst in data rate migration
encompassing the entire optical network. Core networks have deployed DWDM to
quench this bandwidth explosion. The application of DWDM in metro is not deployed in
exactly the same way as in core networks since with metro distances impairments tend to
be much less severe. DWDM-based architecture cost-effectively fulfills the bandwidth
boost requirements with the flexibility to continuously scale according to the evolving
market needs. Thus the question for service providers is not if they employ DWDM metro
networks but how and when.
1 1 1 10 0 0 0 00
1
1Distance
(acceptable)
L 2Distance
(unacceptable)
L
Threshold
1 1 1 10 0 0 0 0
t
Power
ISI
Figure 6:
Conceptual diagram for dispersion
showing pulse broadening along
the fiber [1]

6. A typical metro network scenario portrays an existing infrastructure often relying on
multiple 10 Gb/s or 40 Gb/s services multiplexed onto a single fiber pair.
Complementing these existing services with higher data rate services requires careful
network planning. The optimal network solution is constructed using the building blocks
presented in this paper and the architecture depends on the particular geographic
location, traffic and expected network growth.
For example consider a legacy system with fully functioning DWDM 10G services in a
100 GHz frequency grid. The 10G services from the line card are aggregated with the
aid of a 100GHz DWDM multiplexer as shown in Fig. 7 (upper part).
To maximize the bandwidth utilization and prevent higher cost, one may opt to add one
or more 100G services into the same fiber via the same DWDM multiplexer by
deploying the new service via the unused wavelengths. However, due to the distance
limitation of currently available 100G transceivers, the maximum reach of the aggregated
service is restricted by the 100G service, i.e., up to 20km. To enhance the transmission
reach to several 100km, the optimal network architecture would be as depicted in Fig. 7
(lower part).
Since the 100G services are more susceptible to dispersion, they would require extra
dispersion compensation and optical power boost. Thus an extra 100GHz DWDM
multiplexer is first used to combine all the 100G services together followed by a
combined dispersion compensation and amplification stage. The grouped 10G and
100G services can then be bundled together with the help of a 50/100 GHz
interleaver. Such an architecture makes bridging distances greater than 100 km possible
given that the existing 10G channels already supported that distance. Nonetheless, the
exact distance depends on the amplifier gain, amount of dispersion compensated and the
transceiver performance. To further guarantee a decent transmission distance for this
system, FEC can be enabled in the 100G switches (transponders) or transceivers.
Additionally, this architecture conveniently supports the ‘pay-as-you-grow’ model for
service providers. If and when the bandwidth is exhausted, the existing legacy 10G
channels may be seamlessly interchanged with 100G services. The same remaining
components can even be reused to extend the data rate up to 2.4 Tb/s.
Figure 7
Complementing the existing
10 Gb/s system with
100 Gb/s upgrades

7. D-5116 Rev.A.1 7
Evolution from 10G to 100G
for a Metro Network
This scenario would require 24 differently colored 100 Gb/s DWDM CFP transceivers
deployed together with the already existing 48 channel 100 GHz DWDM and
interleaver as shown Fig. 8. All the 100G services are first multiplexed together such that
only one dispersion compensation and amplification stage suffices. Clearly, such a
network architecture provides higher density with capability to salvage existing
infrastructure with flexibility while remaining cost friendly.
Conclusion
The rapidly increasing traffic demand will soon make the operators face the age old
problem: lease more dark fibers or find a way to increase capacity of the existing fibers.
With the growing developments in the 100G transceiver technology and the rise in
interest for installing 100G products, the metamorphosis of current network architecture to
100G systems is inevitable and only a matter of time. This paper, thus, delves into the
100G technologies and network solutions supporting seamless evolution to 100G while
easily coexisting with previous 10G services. However, as outlined, care must be taken
to limit disruption of the existing 10G services while maximizing reuse of existing
equipment.
About the Author
Susmita Adhikari has a M.Sc. in Digital Communication with over six years of research
and product management experience from long-haul communication to metro. At
HUBER+SUHNER Cube Optics, she works as a Product Placement Manager and
ensures a good visibility of newly introduced products via technical marketing and
customer interactions.
Figure 8
Complementing the existing
10 Gb/s system with
100 Gb/s upgrade, stage 2

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55129 Mainz
Germany
phone: +49-6131-69851-0
fax: +49-6131-69851-79
sales.cubo@hubersuhner.com
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It is exclusively in written agreements that we provide our customers with
warrants and representations as to the technical specifications and/or the fitness
for any particular purpose. The facts and figures contained herein are carefully
compiled to the best of our knowledge, but they are intended for general
informational purposes only.
D-5116 Rev.A.1