Internet traffic keeps growing at a mind-boggling rate, and in order to keep up, the industry is pushing fiber optics in terms of both transmission rates and reach, to the point where optical technology is elbowing out copper in some surprising places.

The reach of fiber has extended far, far beyond the network level, and photonics are now critical in interconnect with routers, switchers, and similar network equipment. The industry is also investigating ways to incorporate photonics not only on printed circuit boards, but even beyond that all the way down to the chip level.

The communications industry won’t become anywhere near copper-free any time soon, but when communications service providers (CSPs) start investing in companies that are integrating ICs with optical interconnect, you know a major technological transformation in networking technology is already in progress.

Long haul communications lines were taken over entirely by fiber optics years ago. In recent months, CSPs including Verizon, CenturyLink, and Cable & Wireless, among others, have begun to use 100G transmission in metro area networks (MANs), using transmission equipment from vendors such as Ciena and Fujitsu.

Meanwhile, Gigabit transport is becoming a viable alternative for local area networks (LANs), typically in enterprise and SMB applications. Advocates include Arris, CommScope, and the Association for Passive Optical LAN, whose members include Corning, IBM, SAIC, TE Connectivity, Tellabs, Zhone, and 3M. Given current traffic trends, it is not unreasonable to expect to eventually see 10G traffic in LANs.

The transition from 10G to 100G transport is exacerbating an already vexing problem. The amount of power consumed and the associated thermal dissipation that come with faster data processing speeds are already making it difficult for data centers and similar facilities to draw off the waste heat. That problem is naturally compounded as routing gear gets more dense.

The difficulties associated with cooling data centers, such as those belonging to Facebook and Google, are well known, as are the lengths to which many CSPs have to go to cool the largest video headends and central offices. As data throughput increases, the power/thermal problems in facilities such as these threaten to become debilitating.

One way to respond is to transition from copper to photonics. Every major interconnect supplier has developed, or is developing, optical interconnect. And though every manufacturer of routing and switching equipment is using optical interconnect, there is yet no clear best technological approach for optical interconnect.

Bill Gartner, the vice president and general manager of Cisco’s converged optical and routing business unit, noted that every time data throughput gets accelerated, that means routing equipment has to get denser. A router today might have 10 10G ports per linecard, and 8 to 10 linecards per chassis, for a total throughput that could reach 1 Tbps.

“We’ve effectively taken 10 linecards’ worth of power and are running it through a single linecard,” Gartner noted. Now put 10 of those routers in a single facility? “Customers are telling us ‘we can’t cool the room’,” Gartner said.

For decades the only options for photonic circuitry were III-V combination materials such as indium phosphide or gallium arsenide.

“The classic III-Vs work. They can switch photons,” Gartner said. “The challenge is that they’re finicky.” They’re also specialty items to make and buy, he noted. As important, if not more so, it’s hard to squeeze costs out of III-Vs in the same way you can reduce costs through integration when you use silicon. 

That’s one of the reasons why in 2012 Cisco bought Lightwire, a company that specializes in the new class of silicon photonics. One year later – one year ago – Cisco introduced its proprietary CPAK transceivers, based on silicon technology the company picked up with the Lightwire acquisition.

Another group of companies is pursuing a similar technology called CFP (they include Avago Technologies, Finisair, Fujitsu, JDSU, Oclaro, and Sumitomo Electric).

Anyone running big routers and operating a data center has three key factors that have to be weighed against each other: density, power, and cost, Gartner said. Reducing power consumption is the key path to reducing cost.

Silicon photonics is likely to be “only one option to help with power, density, and cost – today, and scaling for the future,” Gartner said.

One of those options is to find ways to integrate photonics directly on printed circuit boards (PCB). TE Connectivity, for example, is bringing a product line called Coolbit to market, that includes a set of transceivers that would be used just as any other transceiver would, but run cooler, and also scale up to 400 Gbps. But the Coolbit line also includes an on-board module that would pave the way for an entire new way to build a router; it would pull the point of interconnect away from the faceplate which, in large part because of power/thermal issues, is becoming a design chokepoint.

[See Q&A with TE Data Communications technologist, systems architecture, Nathan Tracy]

Europe has just initiated a project to explore photonics as a means of addressing the power/thermal issues now bedeviling data centers. The project, called PhoxTroT, will examine unconventional approaches to the problem, including on-board approaches like TE’s. The three initial goals of the project are to develop 1) optical transmission on printed circuit boards, 2) a 2 Tbps optical backplane, and 3) a rack-to rack optical cable. (See video, below.)

Another option is being proposed by Compass-EOS, a startup that has developed what it claims is the world’s first optical chip-to-chip interconnect, designed to handle 100G transmission rates in-system. The company is building routers based on the chips. Cisco has invested, as has Comcast Ventures. NTT is already using Compass-EOS routers. (Compass-EOS claims other Tier 1 customers, but says it can not yet identify them.)

“If you have a 100G link on a PCB – if it’s copper? After two- to three centimeters you lose so much energy you have to amplify the signal. So you have to design things so the signal doesn’t have to go more than three centimeters,” Somekh explained.

So Compass-EOS designed a product that avoids long copper traces on PCBs. “We have what looks like a chip, but it has a window open over lasers and diodes, and it can push out 1.34 terabits a second,” said Asaf Somekh, Compass-EOS’s VP of marketing and business development. “The chips communicate with other chips.”

Moreover, multiple chips within a router form a full mesh network. Full mesh supports multicast content delivery better than a switching fabric (“I can talk for hours about this,” Somekh said), which is one of the reasons a company like Comcast might be interested. Cable operators’ business is based on multicast transmission.

By replacing copper with optical, Compass-EOS estimates it can reduce the electronics in a router by as much as one-third, of a switcher by as much as a half. The power consumed is slashed, the amount of heat generated is concomitantly reduced, the amount of environmental cooling required is diminished, “and just think of the reduction of MBTF,” (mean time between failure) Somekh said. 



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