EXECUTIVE SUMMARY: Cisco exhibited its Super Channels development, demonstrating 1Tbit/s and 400Gbit/s super channels in coexistence with 100Gbit/s channels on a single fiber.

After putting its ultra-long-haul solution under test, Cisco was eager to show us what they’ve been up to on the research front. Cisco says its acquisition of Core Optics and partnership with the Polytechnic University of Turin (Politecnico di Torino) has helped it to ramp up their focus on optical systems and develop optical technologies beyond 100 Gigabit. With the knowledge that the prototype system was still undergoing the final steps of development and not quite ready for testing, we agreed to take a walk over to the research lab to see what they were up to.

The setup running was research-oriented -- only one direction of traffic was emulated, no Optical Service Channel (OSC) was used, and modulated Distributed Feedback (DFB) lasers were once again used to emulate signals. This time, however, a different modulation scheme was used and this was the focus.

Cisco has developed a way to use multi-carrier signals to create Terabit contiguous "Super Channels" using coherent transmission. The idea is to use 16 QAM and closely spaced wavelengths to redefine how a channel is built in order to carry much more data in each channel. One consequence is that the ITU-T grid, which defines a rigid 50Ghz spacing between channels, is no longer strictly followed, but the super channel is nevertheless becoming more common in the industry. Below are two images -- one depicting 100 channels within the ITU-T grid, and one depicting a super channel.

Cisco also noted, and demonstrated, that standard 100Gbit/s channels conforming to the ITU-T grid can coexist with the super channels. In the setup, the single fiber had several signals:

• Two Tbit/s channels composed of ten 100Gbit/s channels modulated over 16 QAM
• One 400Gbit/s Super Channel, composed of four 100Gbit/s channels modulated over 16 QAM
• 64 ITU-T grid-based 100Gbit/s channels -- one of which was wedged in between two of the Terabit super channels (using a neighboring wavelength), to prove they could coexist.

Cisco’s modulated signals were transmitted across five 100-kilometer rolls of standard Single Mode fiber, each attached to an Erbium Doped Fiber Amplifiers (EDFAs). At the receiving end of the signal was the prototype receiver, the different pieces laid across the workbench. The prototype was using coherent detection to receive the signal and digital signal processing (DSP) to correct the errors digitally. In their setup the DSP was done in software, but the Cisco team explained that once they are 100 percent happy with it, it will be built in hardware for production. This same system was also reporting how many errors it corrected.

As a sanity check to see that indeed the error correction was at work and the setup was as we understood, we asked to remove 100 kilometers of fiber with the expectation that the bit error rate (BER) reported would go down. Indeed, with the full 500 kilometers the system reported values from 1.19E-02 to 1.23E-02, and after removing 100 kilometers for a total of 400 kilometers distance the system reported values from 8.75E-03 to 1.04E-02.

While the product was not quite ready for testing, we were left intrigued. The 8.8Tbit/s of traffic is certainly some load for a single fiber. We can’t think of an overwhelming number of applications that need this load today, but history has shown that planning for rates of the future is worthwhile. We look forward to testing the system end to end once it's on the market.

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