Design software follows optical network trends

By Meghan Fuller Hanna

Overview

With networks becoming increasingly complex, the use of software tools for network design and simulation has grown. Today’s software mirrors future networking trends, including support for high-speed transmission, the convergence of the IP and optical layers, and the integration of optics and electronics.

Like their counterparts in the test & measurement field, the developers of photonic design software must always be several steps ahead of the overall market; the software they develop today is used to design, model, and simulate the components, systems, and networks that will populate the optical communications landscape in the future. Based on currently available design software, we can surmise that the future will be marked by higher-speed transmission, the convergence of the optical and IP layers, and the integration of optics and electronics.

A quick word about component design: It is very much a viable business, and the design software manufacturers continue to develop new capabilities and features in this arena. RSoft Design Group (www.rsoftdesign.com), for example, recently introduced the ability to model a reflective semiconductor optical amplifier (RSOA), which is critical for the development of bidirectional WDM-PON systems. But the real growth in the design software market seems to be at the system and network level.

High-speed transmission

Enrico Ghillino, director of optical systems and networks at RSoft Design Group, believes the biggest trend in photonic design at the system level is “the rush to 100G systems. In particular,” he says, “people want to be able to transmit 100 Gbps on a single wavelength.”

To achieve 100 Gbps on a single wavelength, equipment vendors must use a new and advanced modulation format. Among those under discussion include orthogonal frequency-division multiplexing (OFDM), differential quadrature phase-shift keying (DQPSK), duobinary, and the perceived frontrunner, PM-QPSK or polarization multiplexed QPSK.

“This is a coherent modulation,” explains Ghillino. “At the receiver section, there is a local oscillator and there is a DSP section that is taking care of the coding information that is needed and also compensating for the transmission impairments. Having a DSP increases the complexity of the whole receiver section a lot.”

What this means for software developers like RSoft is increased complexity within the software itself. The DSP section, in particular, creates challenges; direct counting of errors at the receiver, which requires the simulation of millions of bits for pre-FEC bit-error-rate (BER) estimates, is the only way to achieve an accurate measurement. “For a bit-error rate of 10-4, let’s say, you need to run around 1 million bits,” Ghillino explains. “This typically can be a problem for most of the optical system simulation tools out there, but OptSim has a specific technique that’s called split-step time domain that allows it to simulate very, very long bit sequences.”

OptSim 5.1 also includes blind and training-sequence-based equalization algorithms, fixed and adaptive electronic and optical dispersion compensation, and Viterbi-Viterbi phase estimation, all of which make the new software a necessity and not merely “something nice to have,” as it was viewed in the past, says Ghillino.

Of course, 100G is not yet and may never be a ubiquitous pursuit. Marc Berry, team lead in OPNET’s Optical Transport Network Solutions division (www.opnet.com), reports that many of OPNET’s customers are still mulling over the deployment of 40-Gbps wavelengths. “It really depends on the service provider and the current state of their network, basically the amount of capacity left in their network, whether or not they are even considering upgrading to a next-generation 40-gig system.” He reports that 100G “hasn’t been a dominant request.”

IP and optical convergence

VPIsystems (www.vpisystems.com) believes the biggest opportunities lie in simulation software for the network level, which chief technology officer Yufei Wang credits, in part, to the maturation of WDM as a network technology. “Ten years ago, when people talked about WDM, it was only a transmission technology [that] provided a bandwidth pipe from point A to point B,” he recalls. “There were a lot of photonic layer constraints.”

But those constraints began to disappear with the advent of reconfigurable optical add/drop multiplexing (ROADM) technology, which provides longer and colorless wavelengths. “Today’s WDM networks behave more and more like traditional SONET/SDH networks,” notes Wang, “so that gives us a good opportunity to provide optimization in planning a WDM network. If you have too many constraints, if WDM is essentially only a transmission technology, you really can’t do much in terms of planning or optimizing the WDM network.”

Moreover, network engineers now have introduced subwavelength technology, using protocols like OTN. VPIsystems’ software enables users to conduct wavelength planning across both WDM and OTN, and Wang confirms that customers are now asking for packet capabilities such as Carrier Ethernet, PBB-TE, and MPLS-TP as well. “Now they talk about three layers, either Ethernet or MPLS-TP on top of OTN, and that OTN, in turn, is on top of WDM,” he explains. “They build three layers, three technologies into one box, one physical piece of equipment called the ‘god box.’”

Peter Arjis, principal marketing product manager at OPNET, agrees that network convergence is a key trend in today’s market. “When we talk to a service provider nowadays about optical network planning, there’s always someone sitting at the table that is responsible for the IP/MPLS network,” he reports. “And when we talk about the IP/MPLS network, there’s always someone at the table that is responsible for the optical network because they are so intertwined these days. They really need to be managed and planned together in a much more service-aware way.”

In fact, Arjis continues, the service providers themselves are even starting to merge teams within their own organizations. There is not always a clear distinction now between the team that handles the IP network and the one that covers the optical network, and that merger has led to increased demand for software tools. In the past, when service providers’ organizations were “more siloed,” they often managed without tools or with internal tools, “spreadsheets they could get by with,” says Arjis. “Now with all the interdependency, it has become very hard to do any decent design work without tools that can capture the different layers and how services are being provisioned over those.”

OPNET offers two software tools, the SP Guru Transport Planner for optical networks and the SP Guru Network Planner for IP/MPLS networks. They can be used independently or simultaneously to facilitate converged IP and optical network planning.

VPIsystems’ Wang agrees that manually designing these converged networks has become prohibitively complex. Equipment vendors and carriers alike will require design software to accomplish this feat—and they’ll need it sooner rather than later, he notes. He says he has already seen some customers release alpha and beta versions of their god boxes, and they are looking for planning tools they can roll out with this equipment. “That’s why we have already started preparing that now,” he confirms.

Optoelectronic integration

The folks at Optiwave Systems Inc. (www.optiwave.com) hope to capitalize on the burgeoning market they see at the intersection between optics and electronics.

“There is so much interest in putting optics and electronics together; we saw the need for a tool that can do the whole thing in one package,” reports Jackson Klein, director of optical systems at Optiwave. “System tools work really well with optics only or electronics only. But if you want to do a circuit around a VCSEL laser or in your transimpedance amplifier, you can’t do that [with existing software tools],” he says. “You have to use a SPICE-type of software and a system software, two independent tools.”

Borrowed from the more mature electronic design automation (EDA) industry, SPICE is an acronym for Simulation Program with Integrated Circuit Emphasis. Dubbed ‘OptiSPICE,’ Optiwave’s new circuit design software suite analyzes the interactions between optical and electronic components and can be used to design a variety of electrical components, including diodes, transistors, BJTs, and MOSFETS, as well as optical components, including laser diodes, optical fibers, and photodiodes.

The folks at RSoft also recently integrated an embedded SPICE engine, which they call the Custom Component for SPICE (CCS) model, into their OptSim 5.1 to enable the co-simulation of custom electrical circuits.

Optiwave’s Klein believes the ability to simulate optical and electronic components simultaneously will be attractive to both optical engineers who need expertise in electronics and electrical engineers who suddenly find themselves dealing with optics. “When we were at DAC 2009 [Design Automation Conference] last month, which is an EDA show, we were the only ones in optics there,” he reports. “A lot of people came to us and said, ‘It’s very interesting that we can finally see people trying to introduce new tools to the market.’ Because [the] EDA [industry] is also very saturated; there’s nothing new there. So that’s something we’re trying to get into as well.”

At the end of the day, developers of design and simulation software must anticipate the technologies of the future, be they in the near term, like high-speed networking, IP and optical convergence, and the development of more complex optoelectronic circuits, or longer term.

“Things that people are using our tools for now, they may be in reality a few years ahead,” muses Klein. “It is very challenging because we are always making tools for the future. Right now, for example, we have to deliver laser modules so people can try to design something that will not be deployed in the next five years. We have to do R&D on something they are just starting to think about.”