The ROADM to Smarter Optical Networking
by George Lawton
Optical networking is the key to today's high-capacity, high-speed networks, including the Internet. And as network traffic keeps getting heavier and less predictable, the switching must keep getting smarter.
For optical networking to continue to advance, service providers must implement improved intelligent switching. Providers are thus beginning to adopt reconfigurable optical add-drop multiplexers in their networks.
Although not a well-known technology, ROADMs — sold by vendors such as Alcatel-Lucent, Adva Optical Networking, and Ciena — are crucial to high-speed networking's future.
The technology simplifies the process of adding multiple optical signals to and dropping signals from a single fiber, a critical part of optical switching. This makes building and managing wavelength-division-multiplexing (WDM) networks easier.
ROADMs can also switch data via optical circuits without having to convert signals into an electronic format and then back into an optical format.
"This saves a tremendous amount of money on optoelectronic-conversion equipment," said Andrew Schmitt, directing analyst at Infonetics Research, a market analysis firm.
ROADMs can boost a network's performance, scalability, and remote configurability; and let service providers more quickly change topologies, roll out new services, adjust network configurations, and add communications channels.
ROADMs are not new, but owing to technology improvements, bandwidth demand, and lower costs, they now represent the fastest-growing segment of the optical-equipment market, Schmitt noted. He said the market is now growing at about 20 percent per year.
Information Gatekeepers Inc. (IGI) predicts the worldwide ROADM market will grow from US$2.35 billion this year to $3.4 billion by 2012.
Demand is being driven by an increase in Internet traffic and in bandwidth-intensive applications such as high-definition video, noted Current Analysis analyst Jason Marcheck.
Nonetheless, ROADM technology still faces technical and marketplace challenges.
Backgrounder
WDM optical networks increase capacity by simultaneously carrying multiple signals — each on its own wavelength — on a single fiber. This increased capacity makes them ideal for long-haul networks.
The networks switch signals by adding them to and dropping them from the various cables in the system.
Traditional OADMs used hardwired components to add and drop only single specific wavelengths on single fibers at a time when switching. ROADMs, on the other hand, can add or drop multiple wavelengths at a time.
History
In 2002, vendors such as JDS Uniphase announced the commercial availability of ROADM components.
The first generation used wavelength blockers, said JDSU chief technology officer Brandon Collings. Wavelength blockers use filters to let specific wavelengths travel out of fibers, while blocking the others.
However, these ROADMs still required some manual reconfiguration of optical circuit paths, noted Marcheck. In addition, they required manual adjustments of optical-signal power levels when changing the path a particular wavelength traveled.
Planar lightwave circuit (PLC) ROADMs, introduced in 2004, were smaller than early ROADMs, but still could add or drop optical signals on only a fixed wavelength at a time.
The wavelength selective switching (WSS) ROADM emerged in 2005, said Collings. A single component can simultaneously add or drop up to 96 signals on up to 23 fibers, thereby offering higher performance than PLC ROADMs.
The ability to automatically add or drop multiple optical signals, thereby supporting more connections, makes ROADMs more cost effective and gives carriers more flexibility as they design and configure their networks, Collings said.
ROADM and WDM
ROADMs are part of the evolution of WDM. Early WDM networks used hardwired lasers, multiplexers, demultiplexers, and receivers. They thus had to be manually reconfigured by switching out components to make network-infrastructure changes.
Today's WSS-based ROADMs let carriers avoid manual reconfiguration, said IGI analyst Clifford R. Holliday.
Also, the processing involved in switching data between fibers required early WDM systems to convert signals from optical to electronic and then back again.
However, this process consumes considerable energy and slows performance, said Michael Ritter, Adva's vice president for technical marketing. Moreover, optoelectronic switches must be upgraded every time network bandwidth increases.
A Closer Look
A ROADM switch sits at each hub in a regional network. A typical WSS ROADM consists of elements such as electrically tunable mirrors, microelectromechanical systems, or liquid-crystal-on-silicon components to selectively guide multiple light signals from the rest of the wavelengths on a fiber and switch them to other fibers. Control-plane software provides management functionality.
Reducing the size and increasing the precision of these components have let manufacturers squeeze more of them into a ROADM switch, thereby enabling the device to add or drop signals on up to 23 fibers at a time.
Reducing the spacing between signals on the fiber and passing through the ROADM from 100 to 50 GHz has increased the number of signals a ROADM can switch simultaneously from 44 to 96.
ROADMs can be used on Ethernet, Sonet (synchronous optical network), and other types of networks because they are protocol-neutral.
ROADM networks are scalable and easy to roll out because operators can add or upgrade transmitters and receivers without touching the networking equipment in between.
Also, ROADMs enable remote network changes via components that users can direct to electronically adjust various elements.
Newer ROADMs can automatically maintain signal strengths at levels high enough to reach receivers without being strong enough to damage them. ROADM equipment makers like Adva and Ciena combine raw optical components, such as filters, with control-plane software that measures the optical-signal-to-noise ratio and makes adjustments as required.
Increased flexibility
The increase in the number of signals and fibers ROADMs can work with adds flexibility to network operations.
For example, today's ROADMs can easily add more capacity between nodes and can enable more nodes to connect as necessary, either as end or intermediate points. This also adds routing options.
Managing traffic
All mission-critical networks need a backup connection in case a fiber is cut. In a traditional Sonet network, an entire fiber must be dedicated to back up each link. Because ROADMs allow multiple fibers to be connected, more complicated network meshes are possible, which would provide redundancy using fewer fibers.
ROADMs use two primary approaches to managing optical switching.
Adva works with generalized multiprotocol label switching, which automatically sets up traffic paths through a switched optical network, using packet labels rather than a routing table. This moves traffic faster and makes it easier to manage.
Ciena and some other vendors back the International Telecommunication Union’s family of Automatically Switched Optical Networking Protocols. They enable intelligent, automated, dynamic, policy-driven control of an optical network based on user needs in areas such as bandwidth and quality-of-service.
ROAD-M-Blocks
One of ROADM adoption's biggest roadblocks has been cost. "I don't think anyone argues that they don't pay for themselves [in the long run]," IGI's Holliday explained. But operators have to pay for the entire network up front first, noted Infonetics' Schmitt.
Because of the high up-front costs, Ritter said, ROADMs are not cost-efficient for low-capacity services.
And at some point, network systems must convert optical signals into electrical ones for routing because, Ritter pointed out, there are no optical routers. This limits network performance and energy efficiency.
ROADMs lack the granularity of traffic control necessary to route individual packets in ways that dynamically optimize network routes on the fly.
And ROADM systems will need more automated management capabilities to enable automatic network reconfiguration, thereby avoiding manual intervention.
The ROADM Ahead
ROADMs are growing more popular because their equipment and operation are becoming less expensive, because of higher production volumes, less-expensive components, and more power efficiency, said Holliday.
Initially, most vendors anticipated ROADMs would be used primarily in long-haul, high-volume networks between cities, which require greater the ability to add or drop greater numbers of signals.
However, such networks represent only 20 to 30 percent of the ROADM market, according to Vinny Morrin, Ciena's senior director of transport portfolio management.
Market adoption has been mostly in smaller, shorter-haul, lower-volume, metro and regional networks, said Morrin. This has occurred as operators have realized that ROADMs could let them expand and operate their networks more cost effectively.
Because ROADMs for short-haul networks only need to add or drop up to four signals at a time, they can use less expensive lasers and receivers. Thus, Holliday noted, they cost 50 to 75 percent as much as high-end ROADMs.
Therefore, Morrin predicted, vendors will focus on building smaller, less expensive ROADMs.
For high-end products, vendors are trying to increase the simultaneous number of signals ROADMs can add or drop, as well as the number of fibers they can work with. This could improve optical networks’ performance, reconfigurability, and adaptability.
ROADMs' role in improving optical-networking automation will be limited until component makers develop better technology for measuring and compensating for changes in characteristics such as signal dispersion and attenuation, according to Current Analysis' Marcheck.
Ritter predicted most optical networks will use ROADMs as a standard component within five years and will integrate more with control-plane software, which will improve management, enhance reconfigurability, and use network resources more efficiently.
George Lawton is a freelance technology writer based in Monte Rio, California. Contact him at glawton@glawton.com.
