Next Generation ROADM

White Paper

The Need for Next-Generation ROADM Networks

Prepared by

Sterling Perrin Senior Analyst, Heavy Reading

www.heavyreading.com

On behalf of

September 2010

http://www.heavyreading.com/�

© HEAVY READING | SEPTEMBER 2010 | WHITE PAPER | THE NEED FOR NEXT-GEN ROADM NETWORKS 2

TABLE OF CONTENTS

I. INTRODUCTION ................................................................................................... 3

II. FIRST-GENERATION ROADMS BENEFITS & LIMITATIONS ........................... 4

2.1 Operational Benefits of ROADM Networks ........................................................... 5 2.2 Major Limitations of Current ROADM Networks .................................................... 5

III. FUNCTIONALITY REQUIREMENTS OF NG ROADMS ...................................... 6

3.1 Colorless ROADMs ............................................................................................... 6 3.2 Directionless ROADMs .......................................................................................... 6 3.3 Contentionless ROADMs ...................................................................................... 6 3.4 Gridless ROADMs or Flexible Grid ........................................................................ 7 3.5 Optical Control Plane Innovation ........................................................................... 7

IV. BENEFITS OF NG ROADM NETWORKS FOR OPERATORS ........................... 8

4.1 Greater Automation at the Optical Layer ............................................................... 8 4.2 Reducing Opex & Capex by Moving More Functionality to Optical Layer ............. 8 4.3 Potential New Applications .................................................................................... 9 4.4 Tighter Integration Between Optical & Electrical Layers ....................................... 9

V. TECHNOLOGY BUILDING BLOCKS FOR NG ROADMS ................................ 10

5.1 WSS Components ............................................................................................... 10 5.2 Optical Space Switches (3D MEMS) ................................................................... 11

VI. TESTING IN ROADM & NG ROADM NETWORKS ........................................... 13

6.1 Requirements for Testing Optical Noise .............................................................. 13 6.2 Line-Side Testing in High-Speed Networks ......................................................... 14

VII. FUTURE DIRECTIONS & CONCLUSIONS ....................................................... 15

LIST OF FIGURES

SECTION I

Figure 2.1 4-Degree ROADM Node Architecture ....................................................... 4 Figure 4.1 Switching Costs Relative to OSI Switching Layers .................................... 8 Figure 5.1 CDC ROADM Node Application .............................................................. 10 Figure 5.2 CD Application Incorporating WSS & 3D MEMS ..................................... 11 Figure 5.3 CDC Application Incorporating WSS & Large-Port-Count 3D MEMS ..... 12 Figure 6.1 Traditional OSNR Measurement Challenges in NG ROADM Networks .. 13


I. Introduction Operators and their suppliers have been under enormous pressure to lower the costs per bit for transport for some time. This is not new, but over the past couple of years we have seen a much greater emphasis by operators on lower opex costs together with capex costs. In other words, operators view cost per bit as having both a capex component and an opex component, and that opex component is now very important to them. In addition, operators are focusing on driving more functionality from the electrical layer to the optical layer when possible, as a means of saving on both opex and capex. Why? As a rule, transport is less costly – in both opex and capex – the lower you go in the Open Systems Inter-connection (OSI) stack. Reconfigurable optical add-drop multiplexers (ROADMs) have been beneficiaries of these major operator trends and thus it is no surprise that these devices (and the systems that include them) are well-entrenched in operator networks around the world. Yet, despite the benefits that ROADM networks have delivered to operators, first-generation technology has imposed constraints in reconfigurability and automation, particularly at add/drop nodes. There is still a great deal of costly manual intervention required. Network operators want more from their ROADM networks. Fortunately, a new generation of technologies is coming to-gether to make full optical layer reconfigurability and automation both technically and economical-ly feasible. These next-generation innovations include:

• Color independent, or "colorless," functionality

• Direction independent, or "directionless," functionality

• Wavelength contention-free, or "contentionless," functionality

• Flexible ITU grid, or "gridless," functionality

• An optical control plane to work across elements and OSI layers Drawing on extensive interviews with leading operators, including AT&T, Verizon, and Comcast, this white paper discusses the characteristics of the next-generation (NG) ROADM networks and the significant benefits to network operators in moving to an NG ROADM network. The paper out-lines the building block technologies and major network testing requirements.


II. First-Generation ROADMs Benefits & Limitations A ROADM is an all-optical subsystem that enables remote configuration of wavelengths at any node. It is software-provisionable so that a network operator can choose whether a wavelength is added, dropped, or passed through the node. The technologies used include wavelength block-ing, planar lightwave circuit (PLC), and wavelength selective switching (WSS) – though the WSS has become the dominant technology. A ROADM system is a metro/regional WDM or long-haul DWDM system that includes a ROADM subsystem. ROADMs are often talked about in terms of degrees of switching, ranging from a minimum of two to as many as eight degrees, and occasionally more than eight degrees. A degree is another term for a switching direction and is generally associated with a transmission fiber pair. A two-degree ROADM switches in two directions, typically called East and West. A four-degree ROADM switches in four directions, typically called North, South, East, and West. In a WSS-based ROADM network, each degree requires an additional WSS switching element. So, as the direc-tions switched at a node increase, the node's cost increases linearly. Figure 2.1 illustrates a four-degree ROADM switching node. Figure 2.1: 4-Degree ROADM Node Architecture

Source: JDSU, 2009 Today, ROADMs are widely deployed worldwide. JDSU, a leading supplier of ROADMs, has shipped more than 45,000 ROADMs worldwide since inception. Verizon, reports that it has more than 2,000 ROADM nodes running in its network today.


2.1 Operational Benefits of ROADM Networks The key operator benefits of ROADMs can be summarized as follows:

• Simplified planning: reduces the impact of inaccurate growth forecasting.

• Better bandwidth utilization: no capacity is stranded due to wavelength banding.

• Simplified and reliable network engineering: ROADM systems typically provide per-channel power control distributed through the network to manage new channel introduc-tion and adjust powers on an ongoing basis.

• Reduced truck rolls: wavelength path setup can be done remotely, avoiding sending technicians to intermediate nodes.

2.2 Major Limitations of Current ROADM Networks To date, WSS-based ROADMs have been deployed widely in networks throughout the world for adding, dropping, and express-routing traffic though network nodes. These WSS ROADM ele-ments are relatively low cost. However, these WSS architectures are not as flexible as required in order to move to the next phase of optical switching that operators envision. These first-generation ROADM architectures, which include a single WSS element for each direction (de-gree) at the node, are limited by:

• Fixed wavelength assignments to specific ports

• Fixed direction assignments for multiplexers (i.e., North only, South only, etc.)

• Partitioned add/drop structures due to wavelength contention conflicts Intermediate nodes along a route benefit from the reconfigurable functionality of ROADMs, but the WSS end-points of a connection must be physically wired and rewired, which greatly con-strains the amount of reconfiguring that can be done without physical rewiring by a technician "We called [first generation ROADMs] reconfigurable but the only thing reconfigurable was the middle," said Verizon Director of Backbone Network Design, Glenn Wellbrock. "What we're really trying to do is move that to the ends of the network, to the access and the egress, to give them the same capability." Such activity is manpower intensive, which translates to higher costs and longer times to provi-sion. In addition, every physical rewiring presents the opportunity for errors. According to Wellbrock, "Today, you have to remove that connector and move it to a completely different WSS switch – facing, say, East instead of West or North. And that is, again, not only manpower intensive but also opens up the opportunity for a lot of mistakes to be made or a dirty connector – simple things that happen any time you move traffic, that we'd rather avoid. If you can automate that functionality then you don't necessarily have the opportunity for mistakes. And it also reduces the time and effort and makes it more flexible, as well."


III. Functionality Requirements of NG ROADMs "Next-generation ROADMs" is a moniker for new-generation ROADM elements coming to market with a host of new capabilities and functionalities that clearly separate these ROADMs from their predecessors. These key new ROADM capabilities are:




• Flexible ITU grid, or "gridless," functionality Along with these new hardware functionalities, a software control plane will be required to auto-mate the optical functions across nodes and also across OSI layers (known as the multi-layer control plane). Below, we describe each of these key NG ROADM functions in detail.

3.1 Colorless ROADMs Current-generation ROADMs are limited by fixed add/drop transceiver and wavelength assign-ments. With existing ROADM deployments, when a wavelength is selected, the transceiver must be manually connected to the correct mux/demux port at the add/drop site. Again, while express nodes benefit from the presence of the ROADMs, the add/drop sites must be physically wired and rewired whenever a change is made. New "colorless" ROADM node architectures provide the means for building ROADMs that auto-mate the assignment of add/drop wavelength functionality. There are several variations for build-ing colorless ROADMs, but they typically involve using additional WSSs or 3D micro-electro-mechanical systems (MEMS) switches in place of different multiplexers and demultiplexers in the ROADM subsystem. (We will discuss architecture options later in this paper.) Regardless of ar-chitecture approach, the end result is that any wavelength (color) can be assigned to any port at the add/drop site, completely by software control and without a technician on site.

3.2 Directionless ROADMs Directionless and colorless ROADMs are increasingly being discussed together as "must haves" for true optical layer flexibility. As described above, existing ROADMs are directionally dependent, meaning that add/drop port pairs, and the transponders connected to them, are fixed to an out-going direction (i.e., North only, South only, etc.). Changing the direction of a particular trans-ponder requires physical rewiring by a technician. Directionless ROADMs, by contrast, allow any wavelength to be routed to any direction served by the node, by software control, and without physical rewiring.

3.3 Contentionless ROADMs Colorless and directionless ROADMs have been under discussion within the industry for some time. Newer is the concept of contentionless ROADMs networks. Driving the operator require-ment for contentionless ROADMs is the fact that, even with colorless and directionless functio-nality, a ROADM network is still has limitations that could require manual intervention in some cases. In other words, the colorless/directionless network is still not completely flexible. The problem is that wavelength blocking can concur when two wavelengths of the same color converge at the same WSS structure at the same time. This causes network contention. Opera-


tors must avoid this potential blocking/contention situation by partitioning the add/drop structures so that different colored wavelengths are associated with different structures – thus eliminating the possibility for two red wavelengths to converge on the same add/drop structure simultaneous-ly. While this level of engineering does resolve wavelength contention potential from a provi-sioned perspective, it means that operators sacrifice a level of dynamic flexibility and may require additional add/drop structures to accommodate particular wavelength channels. A contentionless architecture, by contrast, allows multiple copies of the same wavelength on a single add/drop structure (without any partitioning restrictions). A colorless/directionless architecture combined with true contentionless functionality is the end goal of any network operator that has deployed – or is planning to deploy – a ROADM network. Such architectures, known as CDC, give them the ultimate level of flexibility at the optical layer.

3.4 Gridless ROADMs or Flexible Grid A fourth key concept in NG ROADM architectures is the concept of the flexible wavelength chan-nel grid. Thus far, the flexible grid concept has been championed by Verizon as a way to future proof a network that will ultimately need to contend with transport speeds beyond 100-Gbit/s transport. For speeds beyond 100 Gbit/s – i.e., 400 Gbit/s or 1 Tbit/s – more than 50 GHz of spectrum will likely be required. Network operators would also like to be able accommodate those future speeds on the same 40G and 100G ROADM networks. Verizon's proposed solution is a more granular version of the International Telecommunication Union (ITU) grid that breaks spectrum down to 25GHz granularities. ROADM nodes supporting a flexible grid could operate at any speed that is based on increments of 25GHz spacing, such as 75GHz spacing, or 125GHz spacing, etc. Wellbrock stresses that the plan is not to get rid of the ITU grid. "We believe that would be a nightmare," he said. Rather, it's a more granular use of the spectrum.

3.5 Optical Control Plane Innovation The points discussed above are hardware-related, but optical control plane innovation is critical to the success of all of the hardware innovations and must also be included as part of the NG ROADM evolution. For example, in the case of the flexible grid, as described above, WSS ele-ments coming to market in the near future will support 25GHz granularity. The real burden here will be on the optical control plane software to be able to manage these changes in the network. Similarly, optical control plane software is required for the point-and-click provisioning to be achieved in colorless and directionless ROADM networks. The software enables the "remotely configurable" functions of the ROADM hardware elements. Moving forward, the control plane-based automation will extend from the intermediate nodes of today to the endpoints, as well. This should not be particularly difficult given today's technology. A bigger challenge is to extend the optical control plane to operate across adjacent layers of the OSI stack. For example, in the case of a fiber failure, the electrical layer (such as the Sonet/SDH or, increa-singly, the Optical Transport Network [OTN] layer) would perform rapid protection switching to route traffic onto an available path through the network. But when the optical layer on that broken path is up and running again, it would signal to the electrical layer, and the electrical layer would reroute its traffic back to the original path. As another example of a multilayer control plane in ac-tion, an Ethernet element receiving an Ethernet services request would be able to generate ser-vices requests in the Sonet/SDH or OTN layer, which would then set up the network connection for the Ethernet services. The Optical Internetworking Forum (OIF) is leading the multi-layer interoperability work, though it is in the early development phase currently.


IV. Benefits of NG ROADM Networks for Operators 4.1 Greater Automation at the Optical Layer NG ROADMs extend the benefits to operators delivered by first-generation ROADMs, specifically optical layer automation. Instead of automating just the intermediate nodes across a link, the en-tire link – including the endpoints – will be automated in an NG ROADM network. According to Jim King, Executive Director at AT&T Labs, end-to-end network automation using ROADMs was the original goal of moving to ROADMs in the first place. "We had this vision of a full photonic mesh backbone with ROADMs everywhere and with direc-tionless/colorless ROADMs, we've had that vision since the early part of the decade. We have been beating on the drum for almost 10 year, and we are finally getting vendors saying 'I can make this.' And I am thrilled!" said King. Automating the provisioning of optical capacity in the network saves directly on operational costs, the time required to provision bandwidth (a potential differentiator versus competitors) and the potential for human error in making changes. In addition to automating provisioning, operators also view NG ROADMs serving an important role in optical layer protection and restoration. When there is a failure at the optical layer, optical switching can be used to reroute connections away from the failed fiber, node, or even away from an entire domain. When the original link is back on line, traffic can be routed back to the original link. Restoration can be done either via point-and-click operations or dynamically, depending on the operator's preference. Here again the presence of a distributed optical control plane, coupled with NG ROADM elements, is required.

4.2 Reducing Opex & Capex by Moving More Functionality to Optical Layer Beyond the benefits of automation, there are additional cost savings to be had in driving more network functionality to the optical layer. As a rule, the higher the layer at which traffic is switched in the network, the higher the cost per bit for switching that traffic. The inverted pyramid concept, presented frequently by Verizon and shown in Figure 4.1, illustrates this point nicely. Figure 4.1: Switching Costs Relative to OSI Switching Layers

Source: Verizon


The conclusion is that traffic should always be switched at the lowest effective OSI layer possible. Such thinking is at the heart of the networking adage of the 1990s, "switch where you can, route where you must." NG ROADM switching technologies bring a new adage to networking. "Switch optically where you can, switch electrically where you must." Such thinking is also a prime driver behind Comcast's push to increase the role of all-optical switching in its next-generation backbone network. Comcast's Shamim Akthar, Director of Engi-neering for Optical Transport, explained:

"In many locations in our backbone we are out of space and power to accommodate rou-ters, which occupy a much higher carbon footprint as well as the rack space… Unless we provide an innovation on the optical front to be able to provide flexibility to alleviate some of the pressure, what we have in the conversion of optical signals to electrical every time will very soon become alarming for us. So any type of connectionless, directionless, con-tentionless [network] in combination with coherent technology, we are constantly trying to see how big a role we can offer these technologies to be able to hang these routing de-vices off of, as opposed to giving routers a much bigger role to play."

Note Comcast's focus on the opex savings (i.e., space and power) in driving down to the optical layer, so the move is not simply about lowering capex, by any means. The qualifier "effective layer" is critical here, because the optical layer is not always the most ef-fective layer for switching and neither operators nor Heavy Reading is advocating the elimination of electrical layer switching altogether. Sub-wavelength level switching (i.e., Gigabit Ethernet) is just one important example of where electrical layer switching will always be required in the net-work. In fact, Heavy Reading sees co-existence of all-optical and electrical layer switching within the same element as an important trend and a key component of packet-optical transport sys-tems (P-OTS).

4.3 Potential New Applications New functionality in the network leads to new applications, and operators already see one poten-tially exciting application on the horizon: "network defragmentation." In our interviews, both AT&T and Verizon cited this network defrag as an interesting new application enabled by NG ROADM technologies. The concept is analogous to the defragmentation application for PCs, according to AT&T's King. Networks grow and evolve over years, and the path for a connection determined on day one of operation may be a very inefficient path two years down the road. Network defrag then allows operators to "reset" the optical network in an automated fashion, moving traffic to the most effi-cient available paths and freeing up stranded bandwidth throughout the network.

4.4 Tighter Integration Between Optical & Electrical Layers Increased switching functionality at the optical layer does not eliminate the need for electrical layer switching. In fact, the trend we see at the systems level is the integration of electrical layer switching and optical layer switching (ROADM) in a single network element, as P-OTS. The Tel-labs 7100 OTS is one example of a P-OTS element with this converged functionality. The move toward a multi-layer control plane will only tighten this integration between the optical and elec-trical layers, whether the optical layer and electrical layer functions are contained within the same network element (as with P-OTS) or remain in separate physical network elements.


V. Technology Building Blocks for NG ROADMs 5.1 WSS Components The wavelength selective switch is at the heart of current-generation ROADM networks and, simi-larly, has a central role to play in NG ROADM networks. The benefits of WSS switches are that the technology is widely deployed and well understood by network operators around the world. Today, there are several variants of WSSs commercially available for different applications from edge to metro to core, including 1x2, 1x4, and 1x9 WSS variants. New WSS modules are coming to market for next-generation networks and CDC applications. In colorless/directionless applications, a WSS port is used for every wavelength that is added/ dropped (independent of which degree of the node it is associated). Therefore, higher port count WSSs are advantageous as they improve the cost per port, as well as increase the number of add/drop ports supported by a WSS module. Also, with the total number of colorless and direc-tionless add/drop ports serving a node determined by the number of high port count WSS mod-ules, the total number of ports can be incrementally increased over time as the traffic require-ments increase simply by adding additional modules. This allows operators to flexibly scale both node equipment and costs according to the unpredictable network growth with lower upfront costs. Finally, the modularity of add/drop ports allows for cost efficient architectures with separate modules to support the isolation of working and protect wavelength pairs. Thus, we see a trend toward WSSs with port counts beyond nine and toward 20 or above. Long envisioned for NG colorless and directionless ROADMs is an MxN WSS capable of switch-ing any wavelength from any of a multitude of input ports to any of several output ports. Such a technology-challenging device, if available, would enable a highly efficient NG ROADM node ar-chitecture by integrating all the functionality required for colorless/directionless and contentionless multiplexing/demultiplexing into a single device. Figure 5.1 illustrates a CDC application. Figure 5.1: CDC ROADM Node Application

Source: JDSU, 2010


5.2 Optical Space Switches (3D MEMS) MEMS is a technique in which arrays of tiny tilted mirrors send light in various directions. Two basic types of MEMS arrays are used today:

• 2D MEMS, in which the multiple mirrors are arrayed on a single level, or plane, in an opt-ical component

• 3D MEMS, in which the mirrors are arrayed on two or more planes, allowing light to be shaped in a broader range of ways.

Generally, 2D MEMS are lower cost and easier to make than 3D MEMS, but they support a smaller switch port count. WSS elements have used 2D MEMS, as well as other technologies, such as liquid crystals. Large port count photonic cross connects, using MEMS on multiple planes (i.e., 3D MEMS), re-ceived large amounts of venture funding at the beginning of this decade. For these photonic switches, with configurations like 1,000 x 1,000 ports, 3D MEMS technology was required in order match the hundreds of input ports to the hundreds of output ports. Despite the lack of applicability in early- and current-generation ROADMs, 3D MEMS technolo-gies and companies survived, reliably serving applications in submarine networks, research ap-plications, and network and equipment testing labs, among others. While 3D MEMS did not play a role in early-generation ROADM networks, we are seeing a resurgence of interest in applying 3D MEMS technologies and products in NG ROADM networks for CD and CDC applications.

Lower-Port-Count 3D MEMS Switches

On the one hand, we are seeing a scaled down version of 3D MEMS-based switches emerge, with port counts to match the number of wavelengths per DWDM system, at 80x80 or 96x96 ports, for example. These 3D MEMS switches could be used in the add/drop portion of ROADM nodes to provide a switching port for each wavelength on a DWDM that required an add/drop. In this application, the WSS elements would continue to handle the express traffic through the node, so the ROADM node consists of both WSS and 3D MEMS technology. Figure 5.2 shows WSS and lower-port-count 3D MEMS in a colorless/directionless application. Figure 5.2: CD Application Incorporating WSS & 3D MEMS

Source: Calient, 2010


High-Port-Count 3D MEMS Switches

High-port-count 3D MEMS switches are targeting applications that require colorless/directionless and contentionless functionality, in which access at the node from every wavelength to every port is required without partitioning add/drop switching elements. Figure 5.3 shows a CDC application in which 96 wavelength node with 10 degrees requires 100 percent add/drop capability. In this case, a 960x960 3D MEMS photonic switch connects every wavelength at the node. Note again, however, that WSS elements are still required for the express traffic (in this example, serving 10 degrees/directions). Figure 5.3: CDC Application Incorporating WSS & Large-Port-Count 3D MEMS

Source: Calient, 2010


VI. Testing in ROADM & NG ROADM Networks The move to ROADM-based networking has important implications for network testing, which we will explore a bit in this section.

6.1 Requirements for Testing Optical Noise In traditional point-to-point or ring topology networks, all wavelengths reaching a certain destina-tion came from the same origin, and therefore used the same transmission path, with minimal spectral filtering to disturb the optical noise. If problems were found on a single DWDM wave-length, a technician could assume that a particular transponder was the probable cause. A bro-ken fiber or malfunctioning amplifier, by contrast, would cause problems on all of the wavelengths on that system at once. The advent of ROADMs changed this simple reality. In a ROADM network, every wavelength reaching a certain location can originate from a different location and every wavelength could tra-vel different optical paths along the way. As a result, the optical noise of a particular wavelength may be totally different from the neighboring wavelength. Thus, a single receiver site indicating an optical problem cannot be quickly traced to the corresponding transponder. Since all wavelengths shared the same history and optical noise in traditional DWDM networks, the noise floor in-between channels was representative of the noise within the channel, making this the simplest and most common way to measure the optical signal to noise ratio (OSNR). In ROADM networks, with different channels traveling different paths, and the ROADMs spectrally filtering the optical noise between the channels as part of the wavelength routing process, mea-suring the optical noise between two adjacent wavelengths can no longer be used to estimate the channel's OSNR. Using the traditional IEC interpolation method of OSNR testing gives operators a false sense of security on the performance of their optical network. The migration of transmission speeds from 10G to 40G transmission also poses OSNR measur-ing challenges. The spectral shape of 40G is much broader and more complex than that of 10G and 2.5G, yet these wider 40G signals must be placed within the existing 50GHz channel spacing so operators can make use of existing networks (a critical operator requirement). This means that optical signals are closely spaced and may overlap. As a result, traditional optical spectrum ana-lyzers cannot accurately measure the noise level between two large signals. In the case of 40G, the traditional IEC interpolation method of OSNR testing leads operators to believe they have worse OSNR than what they have in reality. Figure 6.1 illustrates to two OSNR challenges described above. Figure 6.1: Traditional OSNR Measurement Challenges in NG ROADM Networks

IEC OSNR MEASUREMENT FAILS WHEN… IMPACT FOR OPERATORS …ROADMs are present along the optical path Noise is not flat anymore:

it is filtered Using the IEC interpolation method will lead to

underestimation of the noise level, leading to a situation in which performance is assessed as better than it is in reality, thus creating a false

sense of security.


IEC OSNR MEASUREMENT FAILS WHEN… IMPACT FOR OPERATORS …40-Gbit/s signals are transmitted Signal does not go back to noise level:

signals too large Using the interpolation method will lead to

overestimation of the noise level, leading to a situation in which performance is assessed as

poorer than it is in reality, thus creating a false sense of a problem.

Source: EXFO, 2010 To overcome these testing challenges posed by ROADMs and next-generation transmission speeds, optical spectrum analyzers, such as those offered by testing specialist EXFO and by JDSU, have evolved accordingly. In-band analysis is now performed, meaning analyzing the noise present within a channel is measured directly, even if a wavelength signal is present. The two most popular methods of performing these measurements are:

• Polarization nulling, which is limited in case of polarization mode dispersion (PMD) (depo-larizing the signal), or when the OSNR levels are greater than the extinction ratio of the polarizer used for discrimination

• Hybrid approaches, combining polarization diversity with spectral content diversity, which benefits from the power of polarization nulling without suffering from its limitation

6.2 Line-Side Testing in High-Speed Networks A key benefit of ROADM networks is the ability for traffic to remain in the optical domain for greater distances without passing up into the more costly electrical domain. A key requirement is that these wavelengths do not require electrical regeneration along the way. In other words, per-forming an optical-to-electrical-to-optical (OEO) conversion only because a wavelength has reached its optical transmission distance limit greatly diminishes the benefits promised by the ROADM network. This is particularly an issue in 40G (and 100G) transmission. Differential phase-shift keying (DPSK) and other phase modulation technologies have been developed for 40G and 100G to ensure much higher performance levels at these higher rates when compared to traditional on/off keying approaches used for 2.5G and 10G. Coupled with forward error correction (FEC) abilities of OTN, these 40G networks of today and the 100G networks of the future will have essentially the same performance levels – including transmission distances without regeneration – as exist-ing 10G networks. Again, these performance levels will be critical for operators to realize the benefits of ROADM and NG ROADM networks as next-generation optical transmission speeds are deployed. DPSK tunable laser and OTN inclusive through-mode testing will be required for qualifying and monitor-ing selected channels at the ROADMs. These testing capabilities will be required in 40G networks that are being deployed today and for the 100G networks that will be deployed in the future.


VII. Future Directions & Conclusions "Next-generation ROADMs" is a moniker for new-generation ROADM elements coming to market with a host of new capabilities and functionalities that clearly separate these ROADMs from their predecessors. These key new ROADM capabilities are:




• Flexible ITU grid, or "gridless," functionality

• Software control plane to automate functions across elements and across OSI layers In addition, network testing has evolved to enable precise testing in multi-point ROADM networks. Operators are pleased with the new ROADM advancements because they will enable truly recon-figurable and automated optical layer networks, as originally envisioned. Once proven in on their capex and opex savings merits, NG ROADM networks will open new application opportunities, with "network defrag" being just one example of the possibilities. Especially pleasing to operators is that the technologies make full reconfigurability and automation not just technically feasible, but also economically feasible. With operator requirements and technology availability aligning now, a new era of ROADMs is just beginning.

Next Generation ROADM

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