Channel monitors offer key DWDM building block
The transformation of optical networking is once again gaining momentum and moving toward more open, flexible, and unified network architectures. In the midst of this renewed excitement, systems engineers continue to be saddled with the task of selecting the technologies that will fulfill this architectural vision.
The DWDM channel monitor is emerging as a key building block because it sits at the intersection of three of the more important new applications (see figure): tracking “third-party” wavelengths, managing reconfigurable optical add/drop multiplexer (ROADM) configurations, and making multihaul system budgets work.
The Optical Transport Network (OTN) standard, a work in progress, is one of the International Telecommunication Union’s (ITU’s) more important optical networking developments in recent years. Before the OTN, the optical-layer standards boiled down to fiber-optic cable specifications, transceiver specifications, and the definition of an optical frequency grid for DWDM. The OTN can be credited with defining an entire set of standards around the optical channel (OCh) as a fundamental entity.
One of these important OTN standards is Recommendation G.697, which is dedicated to “optical monitoring (OM) for DWDM systems.” Despite the recent telecom downturn, the OTN has not relented on its original promise of an open architecture that supports multivendor interworking and defines end-to-end wavelength services.
Although standards such as the OTN do take a long time to evolve, they nevertheless offer a de facto roadmap for the future of optical networking. To gain a competitive edge, equipment vendors might begin implementing some of the core technologies and borrowing some of the key concepts ahead of a fully developed standard to differentiate their product offerings. Carriers can then benefit by deploying forward-looking services that will improve profits through increased revenue or reduced operating expenses.
Channel monitoring in DWDM networks is available today for such forward-looking services. As the OTN standard continues to define how wavelength services should be implemented, channel monitors provide a steppingstone along the way by enabling applications such as “wavelength tracking” and “third-party wavelengths” that benefit carriers immediately.
The wavelength-tracking application can generally be defined as a method by which a network management system can track the path of a given OCh from ingress to egress as it travels across the optical network. One of the key goals of the OTN is to define quality-of-service parameters for such wavelength services. This concept is especially important to the so-called carrier’s carriers who aim to provide wholesale bandwidth services. Other carriers rely on wavelength tracking as they do away with the SONET/SDH layer and transmit native Ethernet services directly over DWDM wavelengths to lower the cost of their network infrastructure. In either case, the DWDM channel monitor is a convenient option that provides channel inventory and other critical diagnostics at any node in the network.
A third-party wavelength, also known as an “alien wavelength,” is a pair of transceiver signals originating from one equipment vendor’s system that is transmitted on an ITU frequency over another vendor’s DWDM system. At the core of this application is the concept of open or “transparent” transmission systems. In the early years of DWDM, each vendor engineered a proprietary transmission design. As a result, DWDM systems were incompatible or “closed,” meaning that a DWDM signal was generated, transmitted, and terminated on the equipment of a single vendor. In this architecture, transmission begins and ends with a regenerator: a demarcation point (or interface) between the DWDM signal and a client optical signal-for example, from a SONET/SDH terminal or a router. As the number of wavelengths increase, so does the regenerator cost as a percentage of the total DWDM system cost. It is therefore no surprise that carriers are now requiring equipment vendors to devise ways to reduce the number of regenerators.
One function of the regenerator is to monitor the health of the DWDM signal at the electrical level between the back-to-back transceivers, also referred to as “OEO” (optical-toelectrical-to-optical) transponders, where the SONET overhead bytes are decoded. One well-known complication is that many equipment vendors implement proprietary signaling within their SONET/SDH line cards by encoding overhead bytes left undefined by SONET/SDH. Proprietary overhead bytes may therefore be affected by such regenerator “opaqueness.”
A simple solution for this lack of transparency is to eliminate regenerators, which therefore requires the client line cards to source long-reach DWDM channels instead of short-reach 1310-nm ones. Yet removing the regenerator removes the SONET/SDH monitoring capability from the DWDM system. The alternative that is progressively taking hold is the implementation of DWDM channel monitors simply because they offer channel visibility without affecting the transparency of DWDM transmission.
Today, the optical layer is largely a set of point-to-point connections through fiber-optic cables or “dumb pipes”; network intelligence is located at the electrical layer in switches, crossconnects, or routers. OTN activities gained momentum before the telecom downturn as several carriers forecasted that the bandwidth flowing through some of their nodes would grow to be large enough to easily justify optical switching technologies at wavelength granularities. This implied that moving network intelligence into the optical layer would not only be cost-effective but vital to keep up with the exponential growth of Internet traffic.
This vision of the intelligent optical layer quickly became the mantra of optical equipment vendors as well as their challenge to router vendors. These router vendors would rather see the intelligence remain at the service layer, powered by increasingly large routers. In this race to control network intelligence, optical networking vendors accelerated the development of large all-optical switches with router-like network management protocols known as the “optical control plane.” This contest of the giant optical crossconnect against the supersized router yielded no winners as the telecom market collapsed. Still, the concept of the all-optical crossconnect survived, in a downsized version now well-known as the ROADM.
The optical networking industry has kept a sustained interest in ROADM technologies over the past several years. This interest is gaining even more momentum as significant telecom and cable carriers around the world are now sending out RFPs with the ROADM as a centerpiece. Carriers are attracted to ROADMs primarily because they provide the kind of optical switching functionality that they have grown accustomed to in the electrical domain: The ROADM can switch any wavelength at any port just like a SONET add/drop multiplexer can switch any STS-1 to any port. In addition, to simplify network engineering, ROADM systems are built with variable optical attenuators (VOAs) as a means to automate the process of balancing the add/drop channel powers with the express channel powers.
Given the application, the DWDM channel monitor becomes a critical element of the ROADM architecture. In a real-world ROADM system, it is incumbent upon the DWDM channel monitor to provide an inventory of the incoming wavelengths to avoid “wavelength collision” with added channels, as well as an inventory of the outgoing channels when the ROADM is used for ring-to-ring switching applications. The DWDM channel monitor is also relied on to provide channel-power information to the VOA control electronics so that the added channels can be equalized with the pass-through channels.
Since the Telecom Act of 1996, the telecom and cable industry has been abuzz with the term “convergence.” Convergence universally implies consolidation or the shedding of networking layers, and therefore the reduction of capital and operating cost. In practice, convergence is an ongoing evolution on multiple fronts. At the carrier level, it is where transport-layer and service-layer operations might merge. At the equipment level, it is the integration of any combination of transport- and service-layer functions into one product, which in some extreme cases is also referred to as the “God Box.”
Today it seems that the industry has reverted back to a more practical reality in which transport and services are fundamentally two different sets of competencies that are better managed separately. In the context of network services, convergence now refers to the “triple play” of voice, data, and video services all available from one carrier. In the context of network transport, convergence now refers to the merger of metro, regional, and long-haul systems into so called multihaul systems. The multihaul trend is the answer to the industry’s shift in focus over the past few years: away from maximizing DWDM transmission capacity on any given span and toward minimizing the cost of ownership across the network.
Equipment vendors seem to be taking one of three approaches for their multihaul offering. Some vendors are consolidating their optical networking products into a common shelf design. Others go as far as eliminating backplanes in favor of a modular “pizza box” design. A few are beginning to look at the emerging Advanced TCA architecture backed by Intel. But the common thread is that the shift to multihaul leads to a redesign of the DWDM system hardware.
As the name implies, these new multihaul systems must accommodate greater span ranges using the simplest building blocks. Such flexibility is convenient for carriers but very challenging to systems margins for parameters such as signal-to-noise ratio and chromatic-dispersion compensation, to name two. In most cases, these requirements influence system architects to select more dynamic components than ever, such as variable attenuators and electronic dispersion compensators. Designers rely on these dynamic components for automatic span configuration at setup and self-adjustment over time to compensate for changing environmental conditions, thus enabling the designers to maintain ever tighter system margins.
But many of these new dynamic components will not function effectively without a feedback control loop of some sort. To implement this critical function, new multihaul system designs now routinely call for DWDM channel monitors, which effectively build more intelligence in the optical node. The resulting design is a self-managed optical node that can adjust for changing environmental and input conditions, which makes it much easier to test and regression-test in network configurations. This will naturally lead to network architectures with increasingly distributed intelligence that will drive more embedded DWDM channel monitoring, not only in every optical node, but eventually in every optical submodule as well.
The growing demand for networking services is leading the optical layer towards a more cost-effective and easy-to-use optical network. This in turn is challenging traditional optical transmission design and driving the industry toward transparent, flexible, and intelligent optical network elements where DWDM channel monitors play a critical role in the feedback loop (see table). This gradual proliferation of distributed intelligence throughout the optical network calls for new optical monitoring technologies that are able to keep up with the economies of scale and form factors that such new applications will demand.
MarkLourie is director of product management at Aegis Semi conductor Inc. (Woburn, MA). He may be reached by e-mail at firstname.lastname@example.org.