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32Gbaud. As you would expect, this has the same spectral efficiency and payload as the twelve-carrier super that is clocked at 16Gbaud. So why would we choose this implementation option? Tere are at least two use cases. First is a core

Figure 2: Super-channel spectral efficiency and reach varies by modulation type. In this example, all three super-channels shown have the same, fixed capacity

64Gbaud PM-16QAM would deliver 400Gb/s on this carrier. However, there’s a problem. While 32Gbaud

optoelectronics is widely available, the industry is approaching a bottleneck for serial data rate processing. In other words we can’t buy (or build) 64Gbaud electronics today, or even in the near term future (let’s say the next two to five years – two years being optimistic and five years pessimistic). Electronic processing power is still increasing, but it does so by mainly delivering parallel processing power. Even if we could create a 64Gbaud serial signal (which has been demonstrated in experiments using specialised test gear), you would see a similar ‘spillage’ outside the fixed 50GHz slot. ITU-T has already defined a different approach to

spectrum management, called flexible grid. In a flexible grid system we define a flexible slot with a granularity of n times 12.5GHz. Tere can be multiple optical carriers in this flexible slot – so basically flexible grid was designed with super- channels in mind.

Super-channels to the rescue Let’s now look at how a super-channel approach could help to solve these problems while also providing a lot more flexibility. Earlier I mentioned that switching from PM-QPSK to PM-BPSK halves the capacity of the super-channel line card. Instead we could create an implementation that fixes the capacity of the line card so that we can optimise the use of the switch fabric capacity while still using flexible coherent modulation to optimise the link design. Figure 2 shows a line card with 12 carriers,

because by increasing the number of carriers we can potentially use lower baud rates and still achieve the same spectral efficiency and super-channel payload as a single carrier clocked at a baud rate that is far beyond what current optoelectronics can support. Te blue super-channel at the top of Figure 2 is

14 FIBRE SYSTEMS Issue 9 • Autumn 2015

clocked at 16Gbaud, uses PM-16QAM modulation, and delivers 100Gb/s per carrier for a total capacity of 1.2Tb/s. Te typical reach for PM-16QAM in real networks would be about 1,000km (note that this number varies a lot because the reach of PM- 16QAM is very dependent on specific installations – something that is true for all vendors, despite some aggressive claims to the contrary). Te purple super-channel in the centre of Figure

2 also has twelve carriers, but is clocked at 24Gbaud, uses PM-8QAM modulation, still delivers 1.2Tb/s, and can now close links up to about 2,000km. Te red super-channel at the bottom of Figure 2 is clocked at 32Gbaud, uses PM-QPSK modulation, still delivers 1.2Tb/s, and can close links up to about 4,500km. Tese examples show that, by manipulating the Baud rate at the same time as changing modulation type we can create a ‘capacity fixed’ implementation. Now that engineers have this capability they can

choose other design options. In the upper right of Figure 2 we see a six-carrier, PM-16QAM super-channel in which each carrier is clocked at

network implementation where we want to reduce power consumption, and so can turn off half of the lasers, and more importantly the ASICs that are driving this super-channel. When we do this the higher baud rate will have a negative impact on reach, but perhaps this is an acceptable trade-off. Te second example might be a small form factor node used for short-reach applications such as data centre interconnect. In this case only six optical channels would be implemented to save both power and space, yet still achieve maximum density. A capacity-fixed approach is an essential cost

optimisation option for vendors and service providers alike because it allows the optical transport switching platforms to be loaded to their

The flexible grid system was designed with super-channels in mind

maximum capacity. We know this is an important architectural issue because one of the principle reasons that IP over WDM (IPoWDM) has failed to achieve market traction is that when we switch from grey optics in the router line card to coloured WDM optics we take a big hit in chassis density (typically halving the chassis capacity). Capacity- fixed super-channels enable us to avoid this pitfall. Some of my colleagues have authored a paper to

be presented at the 2015 ECOC conference in Valencia on this very topic (see further reading).

Figure 3: Sliceable super-channels from one line card (top), and how programmable modulation might be used in a real network to support service protection and restoration (bottom)

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