Big Bear Networks has introduced Photronic Signal Processing (PSP), a new technology that overcomes signal-degrading fiber impairments, enabling telecom carriers to deploy longer and faster fiber-optic links with minimum capital and operational expenditure.

Applicable to next-generation 10- and 40-gigabit-per-second optical system products, PSP combines low-cost electronics and digital signal processing (DSP) to automatically correct for common fiber impairments -- a task which previously required expensive components and equipment as well as time-intensive manual intervention by carriers. By improving equipment vendors' system design margins, PSP has the potential to significantly reduce carrier costs in both the metro/regional and multi-wavelength long-haul markets.

PSP is the foundation for a line of highly integrated electrical-optical interface solutions that Big Bear will bring to market starting later this year in the form of 10-gigabit subassemblies and transponders for long-haul applications; 40-gigabit transponders for cross-office, metro core/inter-office facility and regional networks; and 40-gigabit subassemblies optimized for long-haul transmission. The products will be deployed in equipment such as DWDM transmission systems, IP/MPLS routers and switches, SONET/SDH cross-connects and add/drop multiplexers, and optical cross-connects.

Addressing the Fiber Impairments that Degrade Signal Quality

PSP technology, for which Big Bear has submitted numerous patent applications, effectively addresses the fiber impairments that degrade signal quality. Two common impairments that become more damaging as bit rates rise are chromatic dispersion and polarization mode dispersion (PMD), in which different wavelengths or polarizations of a light pulse travel at different speeds, spreading and distorting the pulse and causing signal-detection errors at the receiver. PSP also addresses other inter-symbol interference (ISI) effects, such as transmitter and receiver patterning and self-phase modulation, further boosting system margin.

"As systems move toward higher speeds and greater transmission distances, they become far more vulnerable to fiber impairments," said Laura Adams, Big Bear co-founder and vice president of product management. "The effects of chromatic dispersion are 256 times more severe at 40G than at 2.5G, and the effects of PMD are 16 times more severe. With PMD in particular, the older the fiber, the worse the dispersion characteristics. This raises a barrier to deployment of high-speed systems on a significant portion of the installed fiber base."

In the past, chromatic dispersion has been addressed with costly dispersion-compensator fiber modules that require manual installation by highly trained personnel and allocation of system margin by the network designer for the remaining residual dispersion penalty. The expense of correcting for PMD effects is even more dramatic; carriers typically must design network links with closely spaced electro-optic regenerators which can convert and retransmit signals but which drive up system cost considerably. The potential use of emerging optical compensators to mitigate PMD effects will fall short of carrier expectations in terms of cost, power and solution footprint, limiting the applicability of such products.

Embedded Intelligence Leads to Improved System Design Margin, Carrier Profitability

Big Bear's PSP technology applies the signal processing and conditioning techniques commonly used in wireline applications to the electrical/optical interface, traditionally considered a "dumb" element of the system. Embedded intelligence in the interface provides continuous active monitoring of the data stream, measuring the quality of received data and using a DSP-controlled feedback system to provide dynamic compensation. Through the application of specifically targeted silicon gates, PSP intelligently optimizes the complex performance interactions between electronics, optics and the fiber-optic channel. Because impairments are corrected for automatically, network providers need not perform the costly characterization of their fiber infrastructure typically required to assess penalties and impairments of their fiber links.

In the long haul, this impairment compensation permits increased span lengths and eliminates costly line regenerators. In the metro network, it provides additional link margin; obviating the need for operator intervention to meticulously test and deploy optical links, and thus helping providers maintain competitive service rates. In both environments, it provides the ability to dynamically and automatically control the dispersion compensation elements.

"In a 10G long-haul network," said Adams, "PSP's adaptive compensation intelligence eliminates regeneration stages and automatically recovers system margin normally lost to coarse external line conditioners. This translates into significantly reduced operational and capital expenses for carriers. Looking out to 40G, adaptive compensation technology becomes essential to enable system vendors to economically achieve such speeds beyond the central office."

Big Bear Networks, 1591 McCarthy Blvd., Milpitas, CA 95035. Tel: 408-434-3400. Web: www.bigbearnetworks.com .

Big Bear says their customers are the optical system manufacturers who make the various communications platforms that go in the network infrastructure. The company is developing a series of integrated, high-speed, electrical-optical interface solutions optical networks. Their solutions can be in different form factors such as transponders, modules and sub assemblies for a particular application. They blend a number of skills and disciplines such as high-speed IC design, DSP capabilities, as well as optical systems and components, and packaging and interconnects.

When carriers deploy 10 Gb/s optical cable it isn't until they get to long reach distances of 80 km or more that impairments start to take effect. That's where Big Bear says their DSP compensation techniques are applicable and their electronics technique can correct for optical impairments. For carriers deploying 40 Gb/s cable, the proper installation technique becomes critical. For example, at 40 Gb/s there are significant high-speed broadband issues for which you have to compensate. It really requires a close coupling of electronics, the optics and the interconnect to get the right cost solution for the link. As soon as optical cable gets beyond the central office, chromatic dispersion hits very quickly and then after several tens of kilometers polarization mode dispersion becomes a factor. These impairments, while significant for 10 Gb/s optical cable in a long haul environment, are critical for 40 Gb/s cable and a cost-effective way to deal with them is required.

Chromatic dispersion is the concept that each light pulse consists of multiple wavelengths and those different wavelengths are traveling at different speeds as the pulse goes down the fiber. Therefore, the light pulses begin to spread in time and when adjacent pulses overlap it leads to errors in detection at the receiver. The polarization mode dispersion (PMD) is a different physical mechanism but shows itself in a similar way. Different polarization components of the light travel at different speeds, which leads to pulse spreading and detection errors. It becomes more significant as the data rates increase. For example, the chromatic dispersion at 40 Gb/s is 16 times more severe than at 10 Gb/s, and 256 times more severe than it was at 2.5 Gb/s.

Correcting for these impairments using current methods could force you to look for a venture capitalist to fund the solution - as much as 1 million dollars. Big Bear solves these problems from an electronics basis, at a cost they say is in the noise - about 40 thousand dollars.

How are these system distortions contended with today? Chromatic dispersion is typically addressed by adding fixed compensator modules at a predetermined distance apart, which is essentially dispersion compensating fiber. This must be done manually at installation and it requires significant operator expense. For 10 Gb long-haul systems it can take a couple weeks to bring on line.

Photronic signal processing, a term coined by Big Bear, is the technique they use to deal with the optical impairments. It is a term they use to describe the DSP capability with the electronic and photonics technology. It's really the application of this DSP function and conditioning technique that has been used in wireline applications for a number of years for 1 Gb Ethernet on twisted pair and the DSL environment. Big Bear expanded the idea and applied it to optical communications. Photronic signal processing requires a tight coupling of the electronics with the optical channel and devices that make up the opto-electronic solution.

The photronic signal processing is used for 10 Gb/s subassemblies and transponders in the long-reach and long-haul environment; and for the 40 Gb/s solutions it's used for cross office, connecting the switches and routers and the client-side interfaces of that transmission equipment.

The various optical systems that Big Bear solutions are appropriate for include the DWDM transmission equipment, both the client side that looks inward toward the routers as well as the line-side interface that looks out toward the long-haul core. It's also used for the big IP boxes, SONET/SDH cross-connects and groomers as well as the optical cross connects.

The company says that their technology can be used with existing 10 Gb/s vendor platforms, and also at 40 Gb/s across all the application segments. Big Bear says that their solution doesn't require a new vendor platform to be qualified in order for their technology to be incorporated. The company states that the solution can be implemented as a new line card in an existing platform. Big Bear also says that the solution is mostly hardware but includes software for the self-contained signal processing operations. For polarization mode dispersion, Big Bear corrects it purely in electronics. For example, when an optical signal comes into your receiver and gets converted into an electronic signal, the Big Bear electronics within that receiver chain takes out some of the distortion that the PMD has introduced. So it's a combination of electronics and signal processing which enables it to adapt and correct the impairment on the line. And since PMD can change dynamically, their solution has to track those changes over time.

Typical example

A typical long-haul deployment for 10 Gb/s has no compensation for PMD. It incorporates electro-optics regenerators and once PMD has degraded the optical signal a regenerator must be inserted which then changes the signal into electronics, cleans it up and retransmits it optically. How frequently you do this and how many regenerators you have to use depends on the PMD of the particular fiber.

Another solution incorporates optical PMD compensators. This allows you to roughly double the spacing between the regenerators. However, deployment of the optical PMD compensator requires an additional card for each wavelength. You will need about four per wavelength.

Polarization Mode Dispersion assumes 20 wavelengths at 10 Gb/s, and a regenerator that handles all these wavelengths, for a typical cost of about $1M each. So for 1500 km it would need 3 regenerators and cost a total of $3.3M.

If you used PMD Optical compensators for the same 1500 km it would cost about $12k for each compensator and you would need four for each wavelength and there are 20 wavelengths so it would cost about $960k plus the cost of 1 regenerator at $1.1M for a total of about $2M.

The Big Bear solution also uses one regenerator and 80 electrical compensators at about $500 each for $40k, which is really in the noise compared to the regenerator, and that totals about $1.140M.

No data sheet was available at the time of this review.