Evolution to enhanced reconfigurable optical add/drop multiplexers

The enhanced ROADM architecture leverages the simplicity, precision, and flexibility of micro-electromechanical systems (MEMS) technology to enable unrestricted add/drop and redirection of any wavelength at any port.

By Fady Masoud, Nortel

The fast adoption of business and residential broadband services requires the implementation of emerging technologies and multiple network topologies to meet subscribers' expectations for 'on demand' content at any time and from anywhere. Many of these new services, including video-on-demand (VoD) and voice-over-IP (VoIP), are difficult to forecast in terms of traffic routing patterns and bandwidth demand. Moreover, such applications have stringent quality-of-service requirements.

As a result, traffic patterns are becoming more diversified and unpredictable, challenging network operators to forecast bandwidth requirements at numerous sites spread across the network. Providing high-bandwidth connectivity to every single site requires a major capital investment and results in a significant increase in network complexity. Service providers and cable operators, therefore, are looking for a flexible and cost-effective means to provide "on the fly" high-bandwidth connectivity to any network site without interrupting the existing services or re-engineering the network.

Thanks to the fast pace of technology evolution in photonics, many equipment vendors have developed reconfigurable optical add/drop multiplexers (ROADM) that allow service providers and cable operators to terminate or insert a specific wavelength at a certain site.

First- and second-generation ROADMs

Most available ROADMs are based on either wavelength blocker (WB) or planar lightwave circuit (PLC) architectures.

ROADMs based on WBs are considered the first generation of these network elements, where all wavelengths are sent or "broadcasted" to the site at which only some wavelengths need to be terminated. The WB, in conjunction with a set of optical filters, blocks the selected wavelengths from passing through the site and directs them to the filters for extraction. Similarly, wavelengths can be inserted, pass through the WB, and, using optical combiners, can be added to the line.

WB-based architecture provides many advantages, such as cost effectiveness, remote configurability, 100% add/drop capacity (access to all wavelengths), and good optical performance. However, WB-based ROADMs require the use of an extensive set of components, including filters and splitters, which increases the site's cabling complexity and reduces its reliability (They are too many potential sources of failure).

Moreover, adding a new optical path to the site (branching) is a very costly and complex task; service providers must re-engineer the site and, in some cases, interrupt existing service. This limitation significantly reduces network flexibility, which, in turn, reduces the service providers' ability to dynamically meet traffic demands and expand service into new territories in a cost-effective and timely manner.

Second-generation ROADMs feature a PLC-based OADM that enables service providers to remotely insert/terminate (add/drop) or route any particular wavelength with fewer components through wavelength directors. While this approach is less expensive than first-generation WB-based ROADMs, optical performance is affected by polarization dependent loss (PDL), and the system is limited to 100-GHz channel spacing. In addition, optical branching still is a complex task for PLC-based ROADMs.

Many vendors have packaged second-generation ROADM capabilities and wavelength multiplexers/demultiplexers into their existing DWDM platforms in the form a circuit pack, allowing service providers to leverage their installed base. However, equipping a DWDM platform with all the circuit packs required for the ROADM functionality quickly exhausts the platform's real estate. The ROADM module, for example, requires two slots, the demultiplexer module requires one slot, and a fiber breakout panel must be installed due to the number of connectors required to terminate, etc.

Furthermore, the ports on PLC-based ROADMs have a predetermined wavelength value, as they are hardwired to a specific frequency. As a result, service providers cannot remotely configure wavelength connectivity and must send technicians to perform the reconfiguration manually, even if tunable lasers are used.

Evolution to enhanced ROADMs

Second-generation ROADMs afford service providers slightly better control over the adding, dropping, or routing of any wavelength at a specific site. However, such devices still fall short in the area of optical branching, a key functionality for video-on-demand deployment. Moreover, second-generation ROADMs integrated into existing DWDM platforms require a higher upfront cost; service providers must install hardware that covers the entire wavelength spectrum from day one in order to meet their future bandwidth needs.

A new ROADM architecture, dubbed "enhanced ROADM" (eROADM), leverages the simplicity, precision, and flexibility of micro-electromechanical systems (MEMS) technology to enable unrestricted add/drop and redirection of any wavelength at any port. Using eROADMs, service providers can perform "on the fly" wavelength insertion, termination, or switching at any site or add a new optical path or branch without re-engineering the network or interrupting existing services.

Figure 1: Unlike first- and second-generation ROADMs, which required OEO regeneration to connect three or more network nodes, "enhanced" ROADMs provide any-to-any wavelength connectivity entirely in the optical domain.

The new eROADMs also provide remote wavelength reconfigurability, eliminating the need for expensive truck rolls. Unlike previous generation ROADMs in which each port is assigned a particular frequency or wavelength, eROADMs are "colorless." Ports are not assigned to any particular wavelength, enabling service providers to perform remote and unrestricted wavelength rerouting for true network agility.

Unlike second-generation ROADMs, which are offered only as integrated circuit packs on DWDM line platforms (i.e. chassis-based), eROADMs are offered as both integrated circuit packs as well as standalone modules (i.e. chassis-less).

The following table summarizes the differences between first- and second-generation ROADMs and enhanced ROADMs.

ROADM technology	First-generation (WB-based)	Second-generation (PLC-based)	eROADMs (MEMS-based)
Availability	2002-2004	Late 2004	Late 2005
Pay-as-you-grow	Must install all wavelengths from day one	Must install all wavelengths from day one	Modular add-as-you-grow
Optical branching	No	Complex	Yes
Colorless ports	No	No	Yes
No. of wavelengths	Up to 72	Up to 36	Up to 72
Spectral efficiency	100 GHz	100 GHz	50 GHz and 100 GHz
Patchcord count	Moderate	High	Low-moderate
Packaging	Good	Integrated in DWDM platforms; 3 service slots	Good--modular or integrated

eROADM in action

Say a service provider is planning to offer new broadband and triple-play services in a two-phased rollout. In the first phase, services will be rolled out to highly populated areas where higher take rates are expected. During phase two, the service providers will extend the new service offering to existing service areas as well as into new territories served by its competitors.

The service providers' challenge is formidable--to offer broadband and triple-play services while addressing each of the following business and operational requirements:

Ensure minimum capital expenditure for faster return on investment (ROI).

Maintain service integrity; triple-play services, including IPTV and voice-over-IP (VoIP), have stringent quality requirements, such as low jitter and latency, etc.

Avoid increasing network complexity.

Extend the services in Phase Two to new territories at minimal cost without interrupting existing services.

Ensure seamless network and service scalability.

The limited and cumbersome implementation of optical branching with first- and second-generation ROADMs prohibits their use in this application, forcing the service provider to deploy numerous OEO platforms and extensive cabling in junction sites to meet the service expansion requirements of Phase Two. This will require an extensive capital investment and significant increase in operational expenditures and network complexity, possibly invalidating the service provider's business case.

The eROADM, by contrast, was designed to address the dynamic changes in the network "on the fly"-- a key capability for broadband and triple-play deployments, which often are characterized by unpredictable traffic patterns.

Figure 2: In the above example, a service provider deploys eROADMs at junction sites to facilitate the quick setup of optical branches during the service expansion phase of its rollout, eliminating the need for expensive truck rolls.

In this case, the service provider can deploy an eROADM at junction sites, providing unrestricted any-wavelength-to-any-port connectivity between all five ports. In addition, eROAMDs facilitate the quick and simple setup of new optical branches in Phase Two of the rollout. The colorless ports on eROADMs eliminate truck rolls while providing true network agility, giving the service provider an edge over its competitors.

The added value of eROADM rests in its ability to address traffic changes, network expansion, and reconfiguration "on the fly" and at minimal cost. Such systems:

Enable wavelength agility throughout the network to dynamically address network demands without re-engineering or service interruption;

Extend the service offering to new territories quickly and without truck rolls with optical branching (up to five branches per eROADM);

Reduce overall network latency and increase network robustness by eliminating OEO conversions at junction sites;

Ensure seamless interoperability with other standards-compliant vendors through the support of foreign wavelengths; and

Provide scalability from 1 to 72 wavelengths, supporting both 50- and 100-GHz channel spacing.

While first- and second-generation ROADMs enabled service providers to remotely insert, terminate, and redirect wavelengths across the network, they fell short in the area of optical branching, which is critical for the deployment of triple-play services. Enhanced ROADMs fulfill this requirement with a higher level of flexibility and efficiency, while providing low initial first costs, a significant reduction in operational expenditures, successful interoperability, and seamless service evolution. eROADMs were designed and built around broadband service requirements, and they are a key factor in the successful deployment and growth of such services.

Fady Masoud is manager of optical products and solutions marketing at Nortel (Montreal, Quebec). His area of expertise focuses around the definition, architecture, and requirements of next-generation optical platforms. Masoud holds a Bachelor in Electrical Engineering from Laval University (Quebec City, Canada) and a Master's Degree in Systems Technology (Simulation of Optical Networks) from the Superior School of Technology (Montreal, Canada). He may be reached at [email protected].