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More Capacity? No Problem with Hitless C+L Expansion

portrait of Geoff Bennett

September 25, 2023
By Geoff Bennett
Director, Solution Marketing

Geoff Bennett Interviews Willy George, Head of Department – Networks at Zain Omantel International (ZOI)

Infinera’s ICE6 has revolutionized capacity on many long-distance subsea routes. With this evolution comes the demand to scale terrestrial backhaul capacity to cater to the increasing traffic, whether it is  en route to a data center, a transit segment connecting multiple subsea segments, or part of a multi-way protection topology across multiple cables.

An excellent way to increase capacity in terrestrial links is to light L-band wavelengths in addition to existing C-band ones, which can potentially double the C-band-only capacity. A common misconception is that a C+L upgrade involves service disruption that would consume a significant portion of annual planned outage time for a fiber pair operator. However, Infinera’s C+L solution was designed to allow hitless in-service upgrades that avoid the use of planned outage windows, yet still deliver the additional capacity needed.

In this blog I talk to Willy George, Head of Department – Networks at Zain Omantel International (ZOI), about his experience of C+L upgrades using Infinera’s FlexILS line system. For those unfamiliar, ZOI is a recently announced joint venture between Zain Group and Omantel, providing a new level of international wholesale telecom services as the only truly pan-regional network bridging all Middle Eastern countries to a vast global network. Omantel is the leading service provider in the Sultanate of Oman – a key nexus of Asia-Europe subsea routes on the eastern side of the Arabian Peninsula. Omantel is involved in multiple subsea cables, as well as operates terrestrial backhaul links across the Middle East region. On the other hand, Zain is a leading telecommunications operator providing digital and ICT services in seven markets across the Middle East and Africa.

GB: Willy, thank you for taking part in our guest blog. I think it’s fair to say that you have a ton of expertise around the terrestrial and subsea markets in the Middle East, and you’re also involved with global cable systems like AAE-1 that run all the way from Hong Kong and Singapore to Marseille. So when it comes to expanding backhaul capacity, maybe I can start by asking you: why is there such a need for additional capacity in terrestrial backhaul links?

WG: Of course – and thank you for the invitation. As you mentioned, we’re seeing a big increase in technical capability in subsea cables – in other words, there’s so much more capacity that subsea fiber pairs are able to deliver using modern transponders. As this traffic passes onto the terrestrial backhaul, there are a couple of common scenarios we encounter. Firstly, if the terrestrial segment is a backhaul extension from the cable landing station (CLS) to the nearest point of presence (POP), it is almost always joining other traffic that’s originating from the terrestrial networks, and so we very rarely start with an empty terrestrial route.

The second use case is when a terrestrial segment is a transit segment connecting two subsea segments that are part of a larger overall system. Typically, in this scenario, a larger number of subsea fiber pairs converge to a smaller number of terrestrial ones. This is mainly seen in legacy systems where the cost and availability of the terrestrial fibers is a consideration.

So, the ability to double the capacity along an existing terrestrial route to reflect increasing subsea capacity is really vital.

GB: And what’s creating the demand for that capacity? Where’s it coming from and going to?

WG: Our cables bridge the Far East and Europe, passing through the Middle East. There’s been significant growth in traffic over the past few years, driven by factors ranging from data sharing between cloud data centers to video streaming and social media consumption. The Middle East is evolving beyond just being a transit point between Europe and Asia. The pandemic and subsequent growth of hyperscalers’ presence in the region, the likes of Google and Meta, have played a vital role. They require tremendous capacities for backbone and backhaul connectivity.

Lastly, in addition to being the gateway between the Far East and Europe, we serve as a gateway to Africa, a continent witnessing the world’s highest international bandwidth growth. As a result, so much demand today is being driven by new connections into the whole continent of Africa.

GB: That’s a great summary – thank you. Can you talk more about some of the traffic engineering challenges? I always think of traffic engineering as “putting the traffic where the network is” – so how does that process happen?

WG: Absolutely. In a simple sense, traffic that enters or leaves a subsea cable has to come from somewhere and must be going somewhere. Terrestrial links act as the on-ramps to subsea cables, and it’s important to understand that there’s a long-running trend to move away from digital termination of subsea services in the CLS and toward a sort of optical express, where the analog optical path is carried on over a terrestrial link to the nearest convenient data center or POP. This is a major boost for the economics of these very high-capacity subsea cables.

As mentioned before in the case where terrestrial backhauls are used to transit between subsea segments, there are fewer terrestrial fiber pairs in relation to the subsea fiber pairs, and the need for increased spectrum per fiber pair is extremely important.

We also use backhaul as part of a resilience strategy – especially in the Middle East, where there’s an estimate that 17% of the world’s internet traffic passes through the region. Having strategies to counter potential cable cuts or unplanned outages is vital. Increasing the capacity in terrestrial backhaul provides flexibility in planning these strategies.

GB: It’s fascinating how this region is playing such a pivotal role. Let’s dive a bit deeper – no pun intended. Why is an L-band upgrade on existing fiber pairs so attractive? I know some operators would prefer to just light up another fiber pair with C-band waves.

WG: And that might be right for them – particularly if they own dark fiber. If you’re leasing a fiber pair, that’s a fixed cost. If you can carry twice as much revenue-generating service capacity across that same fiber, then the economics become favorable. This is what C+L upgrades deliver – about twice the capacity over the existing fiber pair.

GB: That’s a really clear advantage. We’re seeing most terrestrial operators designing C+L into new routes or route overlay upgrades, but what we’re talking about in the ZOI links is an in-service upgrade on a fiber pair that’s carrying existing services. Which leads us to the title of this blog – Hitless C+L Expansion. Perhaps you can define for me what hitless means in this context.

WG: That’s a great point because it does mean different things to different people. Some consider fault resolutions within 50 milliseconds as hitless. However, what we achieved on each occasion with the Infinera C+L expansion is a scenario where the signal quality never drops below the forward error correction (FEC) threshold, and so the network equipment never even sees post-FEC bit errors – not even for periods of shorter than 50 milliseconds. One of the reasons we chose FlexILS is because it has this “true hitless” upgrade capability built in.

Timeline of C+L insertion processFigure 1: Timeline of C+L insertion process

In Figure 1 you can see how this process happens. The chart shows a timeline on the horizontal axis, and we have two parameters measured. The left-hand vertical axis in green displays the signal quality or Q value. In this case I’ve just shown a couple of channels, but in a real plot we’d see Q being monitored for each transponder on the fiber pair. Q has to stay above a certain threshold or customers will start to see data errors – and that’s not an acceptable outcome. In this case you can see Q stays pretty flat – although there is a slight change after the forced protection switch, and there is a very small permanent drop that’s completely normal after the L-band waves are added.

GB: Why is there a drop after the L-band insertion?

WG: When you run a C+L system, there is energy transfer from higher frequencies (C-band) to lower frequencies (L-band) caused by a phenomenon called stimulated Raman scattering (SRS). While the FlexILS platform has very good tilt control, there is a small effect. This drop is quite marginal – less than a tenth of a dB – and it’s built into the link modeling.

GB: And the right-hand vertical axis in blue shows the errors, right?

WG: Correct. Just before T minus 2.5 hours, we planned a protection switch to incorporate a monitoring channel on the C+L link. The chart suggests that the errors lasted for a while, but that’s only because we’re sampling at 15-minute intervals, making it appear broader than its actual span. The actual protection switch takes less than 20 milliseconds, which is well within accepted definitions of “hitless.” This minor outage is only for the monitoring channel; you can see at T=0 when the L-band was inserted, there are no errors at all.

GB: And as soon as the L-band capacity is added you can start provisioning services over it?

WG: We usually do a soak test, just to make sure everything is working as we expect. Yet, up to this point, the results have always aligned with our expectations.

GB: Willy, thank you so much for the explanation of the demand drivers, and that detailed look at how the process works from an operational point of view. I know we’ll be seeing each other in Singapore at Submarine Networks World very soon – so safe travels!

WG: Thank you Geoff – see you in Singapore!