Optical networking equipment already use binary phase-shift keying (BPSK or 2-quadrature amplitude modulation, 2-QAM) for the most demanding, longest-reach submarine spans the workhorse quadrature phase-shift keying (QPSK or 4-QAM) for 100 Gigabit-per-second (Gbps) transmission, and the 200 Gbps 16-QAM for distances up to 1,000 km. The modulation used determines the number of bits encoded on each symbol. It also implies a denser platform by doubling the speed per line card slot.Īs you try to encode more bits in a constellation, so your noise tolerance goes down - Kim Roberts "You get more bits through, and so you are sharing the cost of the electronics across more bits," says Kim Roberts, senior manager, optical signal processing at Ciena. However, increasing the symbol rate brings cost and size benefits. But the overall fibre capacity stays the same, as the signal spectrum doubles with a doubling in symbol rate. "Increasing the baud rate gives more interface speed for the transponder," says Winzer.
#Capacity crunch meaning drivers#
The speed hike requires a performance overhaul of the electronics and the optics: the analogue-to-digital and digital-to-analogue converters and the drivers through to the modulators and photo-detectors. Today that is 32 Gbaud: 25 Gbaud for the data payload and up to 7 Gbaud for forward error correction and other overhead bits.ĭoubling the symbol rate from 32 Gbaud used for 100 Gigabit coherent to 64 Gbaud is a significant challenge for the component makers. The baud rate used is dictated by the performance limits of the electronics. Multiple carriers, if needed, to carry the overall service as a super-channel.The modulation scheme which determines the number of bits carried by each symbol.Such varied requirements are met using several design levers: For metro, more data can be carried on a single carrier to improving overall capacity but at the expense of reach. Clearly, for submarine systems, maximising reach and the traffic carried by a fibre are key. There are several performance issues that engineers must consider when designing optical transmission systems.
And research results were reported on the topics of elastic optical networking and spatial division multiplexing, work designed to ensure that networking capacity continues to scale. Several companies announced 100 Gigabit coherent optics in the pluggable, compact CFP2 form factor, while Acacia detailed a flexible-rate 5x7 inch MSA capable of 200, 300 and 400 Gigabit rates. Our experiments are within a factor of two of the non-linear Shannon limit, while our products are within a factor of three to six of the Shannon limit - Peter WinzerĪt the recent OFC 2015 conference and exhibition, there was plenty of announcements pointing to industry progress. The ASIC is the digital signal processor (DSP), used for pulse shaping at the transmitter and coherent detection at the receiver. "To exhaust that last factor of two, a lot of imperfections need to be compensated and the ASIC needs to become a lot more complex," he says. The non-linear Shannon limit dictates how much information can be sent across a wavelength-division multiplexing (WDM) channel as a function of the optical signal-to-noise ratio.Ī factor of two may sound a lot, says Winzer, but it is not. "Our experiments are within a factor of two of the non-linear Shannon limit, while our products are within a factor of three to six of the Shannon limit," says Peter Winzer, head of the optical transmission systems and networks research department at Bell Laboratories, Alcatel-Lucent. Simply put, it is becoming harder to put more information on the fibre as the Shannon limit, as defined by information theory, is approached. Because optical networking engineers have been so successful in squeezing information down a fibre, their scope to send additional data in future is diminishing. Source: Ciena.īut perhaps the biggest challenge is only just looming. The blue dots closest to the green line represent the performance of the WaveLogic 3, Ciena's latest DSP-ASIC family. The chart shows how more bits can be sent in a 50 GHz channel as the optical signal to noise ratio (OSNR) is increased.
As a result, far greater flexibility is needed, with designers looking to elastic or flexible optical networking where data rates and reach can be adapted as required.įigure 1: The green line is the non-linear Shannon limit, above which transmission is not possible. There is also a fissure emerging in the equipment and components needed to address the diverging needs of long-haul and metro networks.
Improving optical transmission capacity to keep pace with the growth in IP traffic is getting trickier.Įngineers are being taxed in the design decisions they must make to support a growing list of speeds and data modulation schemes. Feature - Part 1: Capacity limits and remedies
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