JONATHAN HOMA THE SHANNON LIMIT
What the Shannon limit is telling us is that we’re operating at the edge of the spectral efficiency in terms of what fibre can handle.
development five nanometre versions of the cost/power optimised technology, and in 18 months from now what they’re doing at 400G with seven nanometres, they’ll be able to do at 800G with five nanometres.
The approach Ribbon is advocating is what’s called modular C+L band, which means buying a C-band amplifier, which has the hooks in place to add L-band amplifiers when they’re needed. That way, companies don’t have to pay for the L-band stuff until they’re ready to incorporate them, which could be a few years down the road. This is a bit trickier, however, because when you add the L-band on you want to make sure it doesn’t affect the traffic on the C-band, so you have to equalise that quickly. If you don’t do this, then you’re going to start getting traffic hits. However, we have some approaches that compensate for that, and that makes our approach viable. But the main message is that companies are coming up with various solutions to open up the L-band to increase the capacity on an existing fibre. So that is the other way of working around the Shannon limit - by extending the fibre capacity.
close as possible to the Shannon limit to maximise spectral efficiency, which is often called capacity-reach, or performance optimised solutions. What these solutions do is use the most advanced DSP technology, which, as emerging technology, is still not standardised as different companies have their own versions of it. Ribbon uses the latest version of transceiver technology from Acacia which we’re bringing to the market at a systems level. This technology brings the level of silicon integration down to five nanometres, which enables us to increase the baud rate to 140 gigabaud. This enables us to have wavelengths that run at 1.2T for the first time for short haul DCI, 800G up to several thousand kilometres for extended Metro, and 400G which you can take across the Pacific. This technology is meant for those applications where you need the most performance, usually where you will have very high density or where you need to go those extra few hundred kilometres with fewer wavelengths. The other kind of solution, which again, is widely deployed, which can satisfy a large number of the applications particularly for Metro distances, uses smaller pluggable transceivers. These are based on standards such as OpenROADM or OpenZR+ with multiple vendors building the pluggables to standard specifications. They’re not trying to get the maximum distance but rather optimise instead for lower cost and lower power. They’re typically running today at 64 gigabaud using a previous generation of silicon integration at seven nanometres. It can deliver 400G Metro wavelengths, that can take you 500 or 600 kilometres, which is good enough for many applications, or for long haul it can go up to 200G. Typically the technology cycle is that companies invest in the higher performance five nanometre technology first in larger form factors and then try to find a way to shrink it into smaller pluggables. Even now, there’s already in
What’s the other option?
PD
JH The other thing which is taking place in terms of expanding fibre capacity is going from C-band to also incorporate L-band. This is taking place more on extended long-haul routes rather than Metro routes. On Metro routes, companies tend to put a lot of fibre in the ground, and if they’re running out of capacity on one fibre, they can generally find a spare fibre pair. Because 98% of the applications use the C-band today, they would just add regular C-band equipment for the wavelengths, ROADMs and amplifiers. What’s happening in long haul, which is a relative term geographically, is there aren’t necessarily always available fibres on those routes, and given it becomes very expensive to lay a new fibre, what companies are doing now is expanding the spectrum on existing fibre by adding the L-band transmission window on to the C-band. Fibre has two and a half regions in the infrared where there is a low insertion loss that are very amenable for transmitting. The L-band which is the close cousin of the C-band, is not quite as effective, but very close. There’s also the S-band but that’s used more for Coarse DWDM, but extending from C-band to L-band, which can be used for DWDM, there’s various ways of doing it. One way to do it is to buy equipment that provides C-band plus L-band from day one on the same fibre, which run parallel with each other. There’s two sets of amplifiers, the C-band and the L-band amplifiers, but one of the challenges with that approach is you need to install them all on day one, although the L-band capacity may not be needed at that time.
Thank you.
PD
Jonathan Homa, Senior Director, IP Optical Solutions Marketing, Ribbon Communications
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ISSUE 34 | Q3 2023
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