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CDC ROADM Applications and

Cost Comparison

FUJITSU NETWORK COMMUNICATIONS INC.

2801 Telecom Parkway, Richardson, Texas 75082-3515

Telephone: (888) 362-7763

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1

Introduction

Colorless Directionless Contentionless (CD/C) ROADM architectures have recently generated considerable interest in the optical transport

industry. CD/C ROADM is a relatively new type of architecture designed to offer additional exibility and operational simplicity. Industry technical

conferences are populated with papers focused on the architectural implementations and underlying component technologies used to construct

CD/C ROADM nodes. While the technology and architectural benets are appealing, a review and analysis of the fundamental applications and

benets of CD/C ROADM networks is missing from the discussion, along with a cost analysis of the different CD and CDC ROADM node architectures.

CDC ROADM Applications and Benets

CD/C ROADMs offer additional architectural exibility and operational savings, but questions remain whether the benets and new network

applications justify the additional cost and complexity of CD/C ROADM nodes. The three primary applications enabled by CD/C ROADM architectures

are bandwidth pre-positioning, bandwidth on demand, and optical layer re-optimization/restoration.

Bandwidth Pre-Positioning

With bandwidth pre-positioning applications, carriers deploy “pools” of transponders and regenerators at major network locations, pre-connected

to CD/C ROADM nodes. As additional bandwidth and services are required, an operator simply establishes A-Z wavelength connections across the

network, using these pre-positioned transponders. New services can be established in minutes, as opposed to the months required under normal

processes to engineer, order, receive, deploy, install, and provision new equipment. Service velocity and minimizing technician involvement

are the key advantages of this application. Issues with bandwidth pre-positioning include the cost of “pooled” transponders and technician

involvement with client-side service activation. If growth patterns are not predictable, pools of unused transponders or regenerators remain

scattered throughout the network. These unused assets would be a stranded network expense. Over time, these stranded network assets may be

utilized by ongoing network growth. While a CD/C ROADM enables pre-connection of the network side of a transponder, it still requires a technician

to connect and activate the client-side optical interface. Given the technician involvement in client-side connections and provisioning, there may

be little operational savings from pre-connection of the network interface.

Bandwidth On Demand

Bandwidth on demand is another commonly referenced application used as a rationale for implementing CD/C ROADM architectures. Most

telecom services are provided on a xed basis between two or more customer locations and remain operational 24 x 7 x 365. Over the years, the

telecom industry has debated the feasibility of selling transport services “on demand” for fractional periods of time. For example, an enterprise

customer may want a 10G wavelength service between primary and secondary data centers, but only require the service between midnight and

3 am to perform nightly backups and offsite archiving. Wavelength on demand services come with a whole host of their own issues, including

whether a viable market exists for these types of services, and whether they generate a positive ROI for carriers. In addition to the increased OSS/

billing complexity, it’s unclear whether selling wavelength services for fractional periods improves nancial performance for carriers, or simply

lowers revenues as enterprise users shift to lower cost, fractional-use models compared to purchasing carrier bandwidth to accommodate peak

demand periods.

Optical Layer Protection Switching and Re-Optimization

The most frequently cited application for CD/C ROADM networks is to enable optical layer protection switching and optical layer re-optimization.

With optical layer protection switching, carriers can choose among several protection methods, including 1:1 protection with <50 ms switching for

mission critical services, 1:n share optical protection, and dynamic mesh restoration. In particular, 1:n protection allows carriers to protect their

optical layer services, but without having to reserve 50% of their network capacity for protection, as required with 1+1 protection. Mesh restoration

is the ability to dynamically calculate and signal new routes at the time of a failure. Mesh restoration is typically used as a backup with 1:n optical

protection, in case of secondary failures.

The optical layer poses some unique challenges to optical protection switching. The optical layer consists of physical bers, lasers, and modulated

signals running over those bers. A host of optical impairments, such as optical loss, chromatic dispersion, polarization mode dispersion, and

OSNR must be measured and managed on each optical span and route. At the physical layers, these optical impairments vary by ber type, by

modulation, by span distance, and by overall optical path distance (i.e. OSNR). ROADMs incorporate techniques to compensate for these optical

impairments on the primary optical path. When a failure occurs, optical reach analysis must be performed on all potential protection paths. A path

computation engine (PCE) could calculate the viability of each optical protection path in real time, but a simpler approach relies on optical reach

tables.

CDC ROADM Applications and

Cost Comparison

FUJITSU NETWORK COMMUNICATIONS INC.

2801 Telecom Parkway, Richardson, Texas 75082-3515

Telephone: (888) 362-7763

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2

Optical reach tables dene all reachable paths for every A-Z node combination in the network. Optical path reach tables can be calculated ofine,

utilizing vendor DWDM modeling tools, and periodically uploaded to the PCE. When a network failure occurs, the PCE selects a protection path

from one of the available routes dened in the optical reach table. In addition to optical layer protection, many carriers are interested in the

optical layer control plane for periodic re-optimization of the optical network, sometimes referred to as “optical defragmentation.” Optical layer

re-optimization enables carriers to periodically clean their network routes, recovering up to 20% of additional capacity. Over time, service churn

results in fragments of stranded bandwidth scattered throughout a network. By re-optimizing their network connections, for example once every

four months, the network can be “de-fragmented” and the stranded capacity recovered.[1,2]

CD

ROADM

CD

ROADM

CD

ROADM

CD

ROADM

CD

ROADM

CD

ROADM

Figure 1: Optical Layer Protection Switching and Restoration

Comparing Classic, CDG and CDCG Architectures

A ROADM consists of optical ampliers, optical switching, multiplexer/demultiplexer, transponder, and muxponder cards, enabling a complete,

exible, optical transport node, as shown in Figure 2. The mux/demux provides the connection point between the composite WDM layer and

the individual channels or wavelengths, which are implemented with transponder and muxponder units. In classic ROADMs, the mux/demux is

a passive device implemented with AWG technology, essentially a prism that separates each wavelength into individual input and output ports.

AWG technology is widely utilized, reliable, and cost-competitive, but each wavelength is xed to a specic physical port. Moving a transponder to

a different wavelength, or to a different degree, requires manual technician involvement to unplug the transponder from one port and re-insert it

into a different mux/demux port.

CDC ROADM Applications and

Cost Comparison

FUJITSU NETWORK COMMUNICATIONS INC.

2801 Telecom Parkway, Richardson, Texas 75082-3515

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Amp Amp

WSS WSS

AWG

Mux/Demux

1x9

Spl

1x9

Spl

1x9

WSS

1x9

WSS

Demux

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

Mux MuxDemux

Figure 2: Classic ROADM Node Architecture

CD/C ROADMs allow wavelength reassignment without the need for manual intervention, but with the tradeoff of higher node complexity and

costs. The two primary architectures under consideration are CD and CDC. Both architectures use a common optical core, but differ in how the drop

side is implemented. Both CD and CDC ROADMs support exible grid channel spacing, also known as “gridless” and sometimes labeled with a

subtending “G,” as CDG or CDCG ROADM.

At the optical core, CD/C ROADMs replace the broadcast and select architecture used on “classic ROADMs” with a route-and-select architecture

based on 1x20 twin WSS modules, as shown in Figure 3. In addition, the xed port AWG mux/demux is replaced with exible drop-side

architectures that allow any transponder or muxponder to be assigned to any wavelength and sent to any WDM degree.

Recently, the optical transport industry has shown more interest in the CDCG ROADM architecture, due to the additional “contentionless” feature.

One limitation with CD ROADMs is wavelength contention. Wavelength contention occurs when wavelengths of the same frequency (i.e. color)

terminate from different WDM directions (i.e. East, West, North, South). Since CD ROADMs don’t allow “contention” on the drop side, careful

network planning is required to ensure that wavelengths dropped at a given node are assigned unique frequencies. For many carriers, avoiding

this additional planning and operational issue, required by CD ROADMs, is the justication for migrating to a CDC ROADM.

CDG ROADM CDCG ROADM

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

Amp Amp

xN

Amp Array

4-Channel

8x16

Multicast

Switch xN

WSS

To other

degrees

To other

degrees

WSS

1x20

WSS

1x20

WSS

1x20

WSS

1x20

WSS

8x16

MCS

8x16

MCS

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

TRPN

Amp Amp

••• x8

••• •••

••• x8

x8

WSS

To other

degrees

To other

degrees

WSS

1x20

WSS

1x20

WSS

1x20

WSS

1x20

WSS

MxN

WSS

MxN

WSS

1x16

Splitter

1x16

Coupler

Figure 3: CDG versus CDCG ROADM

CDC ROADM Applications and

Cost Comparison

FUJITSU NETWORK COMMUNICATIONS INC.

2801 Telecom Parkway, Richardson, Texas 75082-3515

Telephone: (888) 362-7763

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With a CDCG ROADM, the drop side is implemented using 8x16 MCS modules, enabling any wavelength from any degree to be dropped to any

client port (contentionless). The high insertion losses of the 8x16 MCS requires additional ampliers for both add and drop directions, for each

degree. Colorless capability is implemented by utilizing the inherent “tuning” feature of coherent receivers. Each coherent transponder “tunes” its

optical receiver to the provisioned channel, essentially blocking or ltering all other channels present at the optical port. An important concern

with CDCG architectures is the need for large numbers of ampliers, or amplier arrays, on the drop side as channel counts and node sizes

increase.

CD/C ROADM Cost Comparison

To get a comparison between classic, CDG, and CDCG ROADMs, a cost analysis was performed based on a two degree (2D) ROADM (East/West)

conguration, 88-channel drop capacity, without transponders. Since the transponders are common to all models, they were excluded from the

pricing comparison, allowing an analysis of just the ROADM architectural differences. Many carriers refer to this as the “rst cost,” meaning the

cost of the ROADM network prior to deploying any services (i.e. transponders). Figure 4 lists the primary optical components incorporated in the

classic, CD, and CDC ROADM models. Figure 5 provides the corresponding normalized cost comparison of classic, CDG, CDCG ROADM nodes.

0.00

Normalized Cost

Number of Dropped Channels

ROADM Cost Comparison

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

8 16 24 32 40 48 56 64 72 80 88

Classic ROADM

CDC ROADM

CDCG ROADM

Standard

ROADM CD ROADM CDC

ROADM

1x9 WSS 2 – –

1x8 Splitter 2 –

44-Channel mux/demux 4 – –

1x20 WSS – 42

MxN WSS – 2 –

1x16 Splitter – 6 –

1x16 Coupler – 6 –

Ampliers, drop side – 12 –

Amp Array, 4-channel ––6

8x16 MCS switch – – 12

Figure5: ROADM Cost Comparison

Figure 4: Optical Components

The twin 1x20 WSS route and select architecture, combined with the 8x16 MCS, provides the exibility to support colorless, directionless,

contentionless, and gridless WSS network architectures. However, the additional optical components and ampliers result in higher initial node

costs.

For the purposes of the cost analysis, the classic ROADM conguration and pricing was normalized to 1.0, providing a baseline for comparison of

the CD and CDC architectures. The results of the cost analysis show approximately 2.5x difference between similarly congured classic, CDG, and

CDCG ROADM nodes, with the increase being primarily due to the use of twin 1x20 WSS modules in the core and 8x16 MCS in the drop banks. The

CDCG ROADM is slightly less expensive than CDG ROADMs at low channel counts (<44), but increases at higher drop channels, primarily due to the

increasing number of amplier arrays required as channel counts increase.

While the initial cost difference between the ROADM nodes is signicant, when compared to the true network costs with deployed 100G channels,

any price differences are minimal. As 100G transponders or muxponders are added to the network, the cost of these devices begin to dominate

the overall network costs. The relative cost differences between the classic, CDG, and CDCG ROADM nodes drop to 6–10%, a relatively small

difference given the much more exible and capable optical networks enabled by CDCG ROADM architectures. Figure 6 shows the previous model

with 100G transponders.

CDC ROADM Applications and

Cost Comparison

FUJITSU NETWORK COMMUNICATIONS INC.

2801 Telecom Parkway, Richardson, Texas 75082-3515

Telephone: (888) 362-7763

us.fujitsu.com/telecom

5

0.00

Normalized Cost

ROADM w/100G Transponder Comparison

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

8 16 24 32 40 48 56 64 72 80 88

Classic ROADM

CDC ROADM

CDCG ROADM

Figure 6: ROADM Comparison with 100G Transponders

Summary

CDCG ROADMs offer additional network exibility by enabling wavelength re-assignment and re-routing, optical layer 1:n protection switching,

mesh restoration, and optical layer defragmentation, all without the need for manual technician involvement. However, this additional exibility

comes with increased node complexity and cost. The increased cost is primarily due to the additional WSS modules, ampliers, couplers, splitters,

and MCS modules required to implement CDC ROADMs. The cost difference between the CD/C architectures, with 100G channels, is only 6–10% in

comparison with “classic” ROADM nodes. Between CD and CDC ROADM architectures, the industry appears to be favoring the CDC ROADM due the

“contentionless” capability. In addition, CDC ROADM networks result in slightly lower cost for nodes with less than 4 Tb of drop capacity (i.e. ~40

channels of 100G). When the additional cost of 100G units is included in the analysis, the cost differences between all three network options is

within 6–10%.

References

[1] Xi Wang, Qiong Zhang, Inwoong Kim, Paparao Palacharla and Motoyoshi Sekiya, “Utilization Entropy for Assessing Resource Fragmentation in

Optical Networks” OFC NFOEC 2012, March 2012, Los Angeles, CA

[2] Xi Wang, Inwoong Kim, Qiong Zhang, Paparao Palacharla, and Motoyoshi Sekiya, “A Hitless Defragmentation Method for Self-optimizing

Flexible Grid Optical Networks” ECOC 2012, September 2012, Amsterdam, The Netherlands

CDC ROADM Applications and

Cost Comparison

FUJITSU NETWORK COMMUNICATIONS INC.

2801 Telecom Parkway, Richardson, Texas 75082-3515

Telephone: (888) 362-7763

us.fujitsu.com/telecom

6

Acronyms

Acronym Denition

AWG Arrayed Wave Guide

CD Colorless Contentionless (Gridless)

CDG Colorless Contentionless Gridless

CDC Colorless Directionless Contentionless (Gridless)

CD/C Colorless Directionless and/or Contentionless

CDCG Colorless Directionless Contentionless Gridless

DWDM Dense Wavelength Division Multiplexing

EDFA Erbium Doped Fiber Amplier

MCS Multicast Switch

OSNR Optical Signal-to-Noise Ratio

OSS Operational Support System

PCE Path Computation Engine

ROADM Recongurable Optical Add/Drop Multiplexer

ROI Return on Investment

Tb Terabit

WDM Wavelength Division Multiplexing

WSS Wavelength Selective Switch

CDC ROADM Applications and

Cost Comparison

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