Got High-loss Fiber or Long Spans? ICE6 Can Help with That

For many network operators, fiber is one of their most valuable assets. Laying new fiber is an expensive endeavor. One 2022 article put the cost per mile at between $60,000 and $80,000 USD. Furthermore, as data on the cost per mile of roadside fiber installation from the U.S. Department of Transportation indicates, the cost per mile can actually be a lot higher. Deploying new fiber can also be a slow process. Delays can be related to planning, materials, rights of way, and installation. For this reason, many network operators must make do with poor-quality fiber – either because that is the fiber that they have, or it may be the only available option, or at least the only cost-effective option, where the fiber is leased.

High-loss Fiber

But what is poor-quality fiber and why does it matter? When we talk about poor-quality fiber, we are primarily talking about the attenuation, or loss, per kilometer. All fiber has some loss caused by the absorption of energy as light travels through the fiber and the scattering that occurs when the light hits particles in the fiber. Factors that can increase the per-kilometer attenuation of fiber include:

• Splices for installation – due to the limited length of fiber reels used for terrestrial deployment, splices are often required every few kilometers
• Splices for repair – where a fiber has been cut and then repaired, a poor-quality splice can significantly increase attenuation
• Fiber additions for repair – additional fiber may need to be spliced in during the repair process
• Connectors – dirty or poor connectors at the location of the DWDM hardware and the optical distribution frames can increase attenuation
• Bending – micro and macro fiber bending, again most commonly in the same physical location as the DWDM hardware and the optical distribution frames, can also increase attenuation

Poor-quality fiber will suffer from one or more of these factors, resulting in high per-kilometer attenuation. Newer G.652 fiber typically has attenuation in the 0.18 to 0.20 dB/km range, while older fiber is typically closer to 0.25 dB/km. However, attenuation significantly in excess of 0.25 dB/km is possible. Losses of up to 1 dB/km have been seen in geographies with loosely regulated construction leading to the accumulation of fiber repair splices. Furthermore, per-kilometer attenuation can increase over time due to repairs, changes in temperature, the weight of aerial fiber, and high point pressures from earthquakes.

Long Spans

Figure 1: Span length distribution in long-haul networks

The span length is the distance between sites, for example, add/drop or terminal sites with ROADMs or in-line amplifier (ILA) sites, as shown in Figure 2. While the ideal distance between sites in a long-haul network is typically 60 to 80 km, for a variety of reasons many operators have longer spans, as shown in Figure 1, which is based on a previous Infinera study. One reason might be the operational cost savings enabled by a reduction in the number of sites that must be leased, cooled, and powered. Another factor driving the use of longer spans is the availability of sites for ILAs. If these sites do not exist and it is cost-prohibitive to build them, then the operator may have no choice but to build its network with long spans.

High-loss Spans

High-loss fiber and/or long spans typically result in high loss-spans. High-loss spans are often found in long-haul terrestrial networks and can even be found in some metro networks where loosely regulated construction has resulted in high per-kilometer attenuation due to frequent repairs. The problem with these high-loss spans is that they force the amplifiers to operate outside their sweet spots, resulting in excess amplifier noise. This noise, known as amplified spontaneous emission (ASE) noise, accumulates as the signal traverses the network, resulting in a low optical signal-to-noise ratio (OSNR) that makes the signal hard to decode at the receive end, as shown in Figure 1. This in turn can limit wavelength capacity-reach and spectral efficiency, pushing up cost and power consumption while limiting total fiber capacity.

Figure 2: Noise accumulates along the wavelength path, making the signal harder to decode

ICE6 Can Help with That

Infinera’s ICE6 optical engine has been able to deliver stellar results in terrestrial networks with favorable conditions, including 60- to 80-km fiber spans and the latest low-loss G.652 fiber. However, ICE6 also includes multiple features, shown in Figure 3, that enable it to deliver high performance under less favorable conditions, such as high-loss spans resulting from some combination of high-loss fiber and long span distances. These features include long-codeword probabilistic constellation shaping (LC-PCS), which provides superior noise tolerance; a super-Gaussian probability distribution with superior PCS performance at high power levels; lower-order modulation formats, including QPSK and hybrid 4/3QAM; a high-gain 33% overhead FEC option; and Infinera’s unique SD-FEC gain sharing. Bandwidth virtualization over the two wavelengths can also be useful for delivering 400 GbE transport even where the underlying wavelengths are not able to operate at 400 Gb/s or above.

Figure 3: Key ICE6 Features for networks with high-loss spans

Together these features enable network operators to boost spectral efficiency and fiber capacity while reducing cost per bit, footprint, and power consumption, even on terrestrial networks with high-loss spans. To learn more about how ICE6 and ICE6 Turbo can address challenging terrestrial conditions, which also include G.655 fiber with low chromatic dispersion, high ROADM cascades, and aerial fiber in regions with frequent lightning strikes, download the new Infinera application note, ICE6 for Challenging Terrestrial Conditions.