Multi-gig PON 101: Q&A with Nokia

Fierce’s Multi-gig PON 101 Q&A series digs deep into the high-speed future, aiming to help readers build in-depth knowledge about key technologies and the overall market landscape through progressive interviews with industry experts. This collection will delve into the forces driving operators toward multi-gig PON technology; how XGS-PON and other 10G technologies are helping deliver a new breed of ultra-fast broadband; and what’s coming down the pipe with 25G and beyond.

This week's interview features Stefaan Vanhastel, CTO for Fixed Networks at Nokia. The interview transcript has been edited for length and clarity.

Fierce Telecom (FT): How long does it take to develop new PON technology? So to go from 1G to 10G or 10G to 25G, what does that process look like?

Stefaan Vanhastel (SV): I would say that there's two different timelines to look at. The first timeline is the actual time to develop a new technology. You're typically looking at specifying the new technology standards, getting chips and then components on the market, building a product and getting that to market. So that takes a couple of years.

Then you also have to look at the time it takes for operators to adopt that new generation technology and start deploying it in volumes. We’ve actually been talking about 10 gig technologies for quite some time. So, you know, XG-PON1 was launched almost 10 years ago, NG-PON2 became available around 2015, XGS-PON became available 2017, 2018, something like that. And the 10 gig deployments are now starting to take off. So, the short answer is it takes a number of years before a new generation technology really becomes mass market.

FT: How is 25G PON different from XGS-PON or other 10G PON technologies?

Stefaan Vanhastel, Nokia

SV: We started looking at 25 gig a couple of years ago. There's a couple of drivers to go beyond 10 gig. One is the need to deliver a true 10 gig service. That might seem odd because there is XGS-PON which is a 10 gig technology. So why is that not true 10 gig service? Well one reason is that a PON network is a point-to-multipoint architecture. It means that a number of subscribers share the same fiber, typically you're talking about 16 to 32. So, if you're starting to mix and match residential and business subscribers and you want a deliver a multi-gigabit or even a 10 gig service to an enterprise, you need to make sure that you actually have more than 10 gig capacity on the PON. So that's one of the drivers for speeds beyond 10 gig.

The other reason is the simply the emergence of new use cases. For example, 5G transport could be one of the killer apps on a PON network.

So what's the difference between 25 gig and previous standards, GPON or XGS? Not that much. It runs at a faster speed. It's typically going to be used for non-residential use cases, initially, business services or mobile transport.

How does it differ from what's coming next? Even though 25 gig just became available, we're already thinking and we're working on what's coming next. We are one of the leading contributors in the 50 gig standard, we've already demonstrated with Vodafone that you can do 100 gig PON. I would say the biggest difference is that 25 gig is a non-disruptive upgrade for your 10 gig network. It can be implemented on the same chips that run GPON and 10G PON services. So that means that 25 gig can actually be activated in a very easy and simple way on current networks.

FT: Is 25G technology mature yet? If not, what else needs to be done to refine it?

SV: The specification was actually published more than a year ago now, that's the 25 gig MSA. There's an ecosystem of about 34 companies now that have signed on to the MSA.

We made the decision to implement 25 gig on our multi-PON solution, [which] as the name implies is a line card that can deliver GPON, any 10 gig flavor, so XGS-PON for example, and in our case we can also run the 25 gig from the same line card. So that means we are effectively shipping 25 gig-ready ports today.

Operators are starting to trial the solution. Proximus was the first operator who trialed this on a live network. Since then, Openreach has also gone public and started to trial 25 gig. Over the next weeks or months, we'll do one trial after the other, basically.

They’re not just testing the technology, they're also exploring potential use cases, for example multi-gigabit business services for enterprises or mobile transport.

FT: What needs to be done to upgrade a fiber network to 25G?

SV: There’s usually three things that need to happen when you upgrade one generation PON technology to another. Typically, it means you have to change equipment in the central office sites. You have to change the equipment in the end user premises, the ONT. And you have to carefully check wavelength compatibility.

The first two are pretty obvious. With our 25 gig approach, you actually don't need to upgrade the central office side, the OLT, because the 10 gig equipment that’s already there is actually ready to do 25 gig. You'll still have to change the ONT, but you only have to change the ONT for those end users who are taking the new service.

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The third concern is wavelength compatibility. One of the things that we carefully considered when the 25GS-PON MSA designed the new 25 gig spec was to make sure that 25 gig is wavelength compatible with both previous and upcoming generations of PON technology. So that means that 25 gig will run on a different wavelength than GPON or XGS-PON and will even run on a different wavelength than 50 gig, for example.

It makes upgrading between generations very easy. The last thing you want to do when you introduce a new generation of PON technology is to have to stop one of the previous generations because the wavelengths are taken.

FT: How much interest is there in 25G PON at this stage compared to 10G PON?

SV: It's a little interesting to compare 25 gig with 10 gig. You know, 10 gig took some time to take off. Part of the reason was that initially GPON was really a lot of bandwidth compared to anything else out there. So, it took some time before operators started to run out of bandwidth.

The second difference, and this one is not to be underestimated, 10 gig initially was a forklift upgrade. You had deployed GPON line cards and GPON ONTs, so when you wanted to go to 10 gig you actually had to buy new 10 gig line cards, add them to your equipment or even switch out the existing line cards. The benefit of our 25 gig solution is that this runs on your existing 10 gig line cards. So, it’s relatively easy to upgrade a 10 gig port to a 25 gig port. That makes the barriers to entry extremely low. So, the fact that there are clear use cases for 25 gig, and the very low barrier to adoption is going to shorten the ramp up phase, in my opinion.

One of the interesting trends that we see in the market is that these multi-PON line cards are now by far the largest volume, at least looking at our own our own shipments.

This represents a fundamental shift in thinking by the operators. As long as operators were buying dedicated 10 gig line cards, it means that they're deploying 10 gig in a tactical way. You add the 10 gig line cards here and there, in whatever OLT you need the 10 gig service you add a 10 gig line card. You really don’t expect a lot of 10 gig if that’s your deployment model.

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Multi-PON means every port that you install in the network is actually 10 gig capable, which means that operators apparently now start to expect that they will need 10 gig on every port. And this shows that they are preparing for mass availability of 10 gig. We started to see this trend in late 2020…and by the end of this year, most of the 10 gig ports that we ship will be multi-PON.

FT: Is 25G PON really the next logical step for the network or are things more likely to jump straight to 50G PON?

SV: It's easy to think of this as the 25 gig versus 50 gig war. And for us, it's not one or the other. We are working on 50 gig, we are working on 100 gig, we have 25 gig today. We do believe that 25 gig will play a very useful role. If you look at the other fiber standards out there, almost every fiber technology has taken a 25 gig step, after 10 gig. It’s available today, there are clear use cases and it's the easiest possible step up from 10 gig. And the combination of those three just make 25 gig a very attractive proposition.