1. Design and Demonstration of a Reconfigurable Metro-Regional WDM 32 x 10 Gb/s System
Scaling beyond 500 km of G.652 fiber.
Cinzia Ferrari*, Rosanna Pastorelli*, Stefano Piciaccia*, Mauro Macchi*, Loukas Paraschis^, Ori Gerstel^
Optical Networking, Cisco Systems,
* Via Philips 12, 20052 Monza, Italy, mmacchi@cisco.com
^ 275 E. Tasman, San Jose, California 95134 USA, {loukas, ogerstel}@cisco.com
Abstract: We demonstrate an industrial-strength 32 x 10 Gb/s WDM system with multiple
reconfigurable optical-add/drop nodes that enables metro regional networks to cost-effectively
scale to > 500 km with span and installation/maintenance flexibility.
©2004 Optical Society of America
OCIS codes: (060.4250) Networks; (060.2330) Fiber optics communications
1. Introduction
The significant growth of enterprise and broadband data applications with extensive metropolitan
networking needs has placed increased emphasis on the scalability of the Metro Networks. Metro architectures have
thus evolved to multiple access fiber rings interconnected through a larger regional network (MRAN) [1]. An
MRAN “physical” fiber ring often extends to hundreds of km, to interconnect in “logical” star or mesh architectures
the multiple (typically 5-10) access-ring “hub” nodes [2]. A successful generation of systems addressed the initial
need for fiber relief in such mutli-service architectures, leveraging the advancements in electronics and 2.5 Gb/s and
10 Gb/s optical SONET/SDH transport [3]. More recently, WDM has been additionally utilized in MRANs to
enable optical add-drop multiplexing (OADM) architectures that transparently scale to hundreds of Gb/s. Unlike
Long Haul WDM, where the main design objective has been to maximize the capacity and reach of transport
networks with typically well-defined (often simple point-to-point) topologies, MRANs require “open” WDM
solutions that allow for both service, and span flexibility. An equally important requirement, the lack of which has
delayed large-scale deployments, is for installation, maintenance, and upgrade simplicity. In this paper, we present
an innovative and practical WDM system that leverages an automatic optical power control layer, and a
reconfigurable OADM (ROADM) architecture to cost-effectively meet the metro regional needs. We analyze the
performance of this 32 x 10Gb/s system in an actual 400km ring (no recirculation loops) with 8 optical nodes, and
demonstrate “stressed” system performance for a 10 Gb/s 480 km link in another network with 16 OADMs and
highly non-uniform spans. We also demonstrate that similar system performance is achievable with ROADMs that
offer the required service flexibility. All these results use commercially available optical components and standard
(G.652) fiber, and well exceed all previously published related demonstrations [4]. This WDM system offers
optical add/drop flexibility, span non-uniformity, and installation/maintenance simplicity, with 32 x 10 Gb/s
transport performance that scales to 15 x 20dB equal spans, for a total reach exceeding 1000 km.
2. WDM Transport Optimized for Metro-Regional Networks
The key to a robust and cost-effective implementation of WDM MRANs carrying hundreds of Gb/s over
hundreds of km, is to leverage the advancements in erbium-doped-fiber amplifiers, and chromatic dispersion
compensation technologies, initially used in LH systems. In this sense, optical amplification noise (ASE) and, to a
certain extent, chromatic dispersion at 10 Gb/s transport, are the primary physical impairments that limit the system
performance. At the same time, more complex MRAN OADM designs are also limited by OSNR penalty due to
amplifier gain ripple or transients, filter concatenation and loss variation, span loss variation due to ageing and
repairing, and in the case of mesh networks ASE noise lasing [2]. The evolving nature of metro networks, from
simple access to regional reach, places an additional significant challenge on the design of a MRAN WDM system
that offers span flexibility, operational simplicity, and low cost even at large number of nodes. Therefore, MRAN-
optimized solutions need to address the trade-off between service flexibility and OSNR, guaranteeing the BER
performance requirements, while minimizing inflexible channel banding and cost.
Most notably, the LH practice of having tight control of the channel power along the link using per channel
gain equalizer would lead to significant cost penalty which is unacceptable for MRANs. Building on the fact that
most physical impairments of interest impact the total optical power, we have developed a cost-effective WDM
design system that meets the metro regional performance requirements by monitoring and regulating the multiplex

2. power, unless the power of individual channels is already available, at selected critical point along the optical paths.
Simple optical tap couplers and photodiodes are used to monitor the optical power, and an automatic control
software has been developed to regulate optical power within each node, by adjusting appropriately the settings of
in-line variable optical attenuators (VOA), and the optical amplifier gain. This metro-optimized WDM design and
control, along with currently available EDFA sub-systems with variable gain, and variable-loss mid-stage access,
have enabled 32x10Gb/s non-return-to-zero WDM transport to cost-effectively scale to > 500 km (with forward-
error-correction). Furthermore, the control layer is able to self adjust each node, enabling automatic “plug and play”
node setup during the installation phase. The same system design innovation enables automatic adjustment for
ageing effects, and/or traffic changes.
We build the WDM system using commercially available optical components, and sub-systems. We
employ 8 x 4-skip-1 channel bands in the standard (C-band) ITU 100 GHz grid to allow for the most efficient and
flexible 1/2/4 channel OADMs based on the established thin-film-filter technology. The cost-effectiveness of AWG
technology is used for the 32 channel (muxing/demuxing) at the hubs. In mesh WDM networks, ASE is suppressed
by the use of standard ASE-filtering, at least once (anti-lasing hub) per ring. We use cost-effective 2-pump variable
gain EDFA technology with 17dBm maximum total output power, and msec transient suppression (with constant
gain operation). When required, we introduce dispersion compensating fiber (DCF) with approximately 100 FOM,
cascaded in a mid-stage access of a 3-stage variable gain EDFA, with maximum gain of 28 dB, and typical noise
figure around 6dB at 20dB gain. We finally employ a 100 Mb/s optical-supervisory-channel (OSC) at 1510nm for
management and control of our WDM system intelligence.
3. Network Performance
Figure-1: 8 node, 409 km demo. (a) Spans (left), (b) BER performance (center), (c) Received spectrum at the hub.
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We have extensively analyzed the performance of our WDM system in an actual 409 km ring with one hub
(ful-demultiplexing), and 7 four-channel basic OADM nodes with the non-uniform spans as described in Figure 1a.
We used multiple configurations with 10 Gb/s channels employing standard (nominally zero-chirp) LiNbO3
transmitter with 2 dBm modulated output power, and APD receiver without FEC. Figure 1b shows the BER
performance (and the residual dispersion penalty compared to back-to-back) for our longest reach channel as a
function of OSNR (0.2 nm resolution bandwidth) for different received power levels. After 318 km of propagation,
from the hub to node 2 (spans 2-8), 10-12
BER is guaranteed with OSNR > 16 dB (at -18 dBm). We have been able
to maintain OSNR > 20 dB typically for all paths, and all configurations, by leveraging our intelligent optical power
control that corrects the accumulated ASE noise to correctly account for the true signal power, and successful post-
compensation of the EDFA gain tilt. Figure 1c shows one such experimental result of 21 different channels received
at the hub. The system performance further allows for a “healthy” optical path penalty margin, which would include
1 dB for residual dispersion and NLE, another 1 dB for channels crosstalk in a fully populated 32 channel system,
and filter concatenation penalty remained well below 1 dB in this 8 node ring. Our analysis, has further confirmed
sufficient OSNR margin under EDFA transient suppression.
To explore the limits of our WDM system, we have also demonstrated a 16 node, 3 hubs, multi-channel
OADM ring network, with more than 500 km of G.652 fiber (Figure 2). We show the spectra for two 10Gb/s
channels in the worse-case optical path with 120 dB loss. The initial (left) OSNR degrades by more that 12 dB (here
filter concatenation accounts for 1dB penalty), but it remains at the receiver (right) > 23 dB (0.1 nm bandwidth),
which is well above the required (non-FEC) levels. These results well exceed the previously published related
demonstrations [4]. They also establish a 32 x 10 Gb/s WDM system performance that –leveraging the > 6dB of
OSNR gains with FEC (even more with Enhanced FEC) – would achieve BER in networks scaling to 15x20dB

3. (>1000 km) when all nodes are fully de-multiplexed (hubs). Even more important than such long reach, the present
results establish the span (OADM placement), and operational flexibility required to enable “open” WDM metro
regional architectures.
Figure-2: Spectra for two 10G channels in the worse-case path (120 dB loss) in the 16 node MRAN (left).
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4. Flexibility Considerations
We believe that flexible, automated system design that further supports unforeseen traffic patterns is the most
critical aspect to the success of WDM MRANs. Most initial WDM systems were limited to optical pass-through at a
waveband level. While this reduces filter concatenation penalties, it severely restricts the flexibility of the solution,
especially if the traffic is distributed, since much of the bandwidth in a waveband may remain stranded if there is no
sufficient demand between the corresponding end-nodes. We believe this is too restrictive for MRANs, and designed
our system to allow for wavelength-level add/drop, passthrough, and ultimately full reconfigurability at each node.
More specifically, to provide full flexibility, our system could leverage 3 types of nodes: (a) Full (32-wavelength)
demux/muxes enable manual non-traffic affecting reconfiguration, and pass channels through at the hubs with
minimal power penalties. (b) Basic OADMs enable lower-cost nodes that still allow individual channels to be
dropped, and then re-inserted back into the add path, without affecting any other wavelengths, but with add/drop
scalability limited to 1/2/4. (c) Finally, a remotely reconfigurable OADM (ROADM) architecture based on
established AWG technology with additional integrated 1x2 switches, enables complete service flexibility at cost
points comparable to (a), and non-substantial higher insertion loss. We strongly believe that such ROADM
architectures will play a growing role in reducing operational expenses in MRANs. While such functionality
complicates system design considerations, we have successfully implemented them in the above experiments. We
have specifically demonstrated the use of ROADMs maintaining the system performance already described. Figure
3 indicates the OSNR of 10 Gb/s channels between two hub nodes after transversing 6 intermediate ROADMs. The
OSNR of an 8 basic node path is also shown. A small (~1dB) improvement in the system penalty actually occurs
when the ROADMs are used due to improved control of channel non-uniformities. On the other hand, the small
additional loss in the ROADM nodes is sufficiently
accommodated by the optical amplifiers, allowing reach of
more than 7x25dB (12dB of OSNR with 0.5nm resolution
bandwidth) in G.652 fiber. We have further demonstrated
similar system performance when a combination of basic
and ROADM nodes is used, as well as in the presence of
power transients. We have finally evaluated our WDM
system performance also for the new generation of EML
transmitters that enable “hot-pluggable” WDM optical
modules on IP-routers/switches, and have established
OSNR performance for 13x15dB such ROADM networks.
We will discuss the system performance, and related trade-
offs in detail during our presentation.
In conclusion we present and analyze a practical 32 x 10 Gb/s WDM system design with reconfigurable
multi-node optical-add/drop that enables metro regional networks to cost-effectively scale to > 500 km with span
and installation/maintenance flexibility, exceeding the previously published related demonstrations.
References
[1] A. Saleh et al. “Architectural Principles of…”, J. Lightwave Technol., vol. 17, n 10, p. 2431-2448, Dec. 1999.
[2] Madamopoulos, et al. , "Design, transport performance study..." In OFC Technical Digest Series, 2002, Paper ThH6.
[3] D. Cavendish, “Evolution of Optical Transport Technologies…”, IEEE Comm., 38, 6
[4] Noirie, et al. , "32x10Gb/s DWDM Metro demonstration…", In OFC Technical Digest Series, 2002, Paper ThH4.
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