The impact of dispersion in today’s networks

Since a reliable fiber foundation is critical to supporting all types of communication, including 5G, it is important that networks seamlessly evolve to support the higher speeds and major bandwidth demands of today’s data-intensive world.

An important consideration for today’s networks is dispersion, which can be the enemy of high-quality transmission. Systems running at less than 10 Gbps are not really affected. But at higher speeds, chromatic dispersion (CD) and polarization mode dispersion (PMD) can impede transmission. These rate limiting events threaten signal integrity, which in turn affects quality of service (QoS).

Let’s answer some of the most common questions about the impact of CD and PMD on today’s networks.

What are CD and PMD?

CD and PMD are two phenomena that have different impacts, but ultimately both can negatively affect performance.

CD is a natural characteristic of optical fiber and is related to the fact that wavelengths (colors) travel within the fiber at different speeds, resulting in the dispersion or propagation of pulses. When purchasing fiber, you have access to “natural” dispersion fiber, or fibers where the dispersion curve has been shifted to reduce the impact in certain wavelength ranges.

For simplicity, PMD occurs when the fiber core is not perfectly round, causing the two different axes of light (for example, vertical and horizontal) to travel down the fiber at different speeds. This disparity potentially induces bit overlap and impedes the signal. A good fiber should have almost zero PMD and otherwise bad sections should be mitigated for optimal network performance. When you buy fiber optic cable, you always expect the PMD to be as low as possible.

So would any high speed fiber optic network experience CD and PMD?

Yes, DC is a fiber characteristic that is unavoidable but can be managed by selecting the right transceivers. However, it is important to maintain a certain level of DC to prevent other non-linear effects from occurring. So even if it could be done, network operators wouldn’t want to have zero DC on their fiber.

Statistically, PMD will occur more frequently in older fiber optic cables, but even recently deployed newer cables have shown out-of-spec PMD values. These sections of fiber with high levels of PMD must be detected, isolated and replaced, since the goal is to reduce PMD to a minimum.

Are there fiber types that can reduce these dispersion factors?

G.653 Zero Dispersion Shift fibers are optimized for CD reduction, but do not support DWDM transmission and are therefore not a popular choice. There are also non-zero dispersion shifted G.655 fibers that have a lower but non-zero CD around 1550nm, so they are optimized for long distances but are more expensive. However, most of the fibers deployed in today’s networks are standard G.652 “natural dispersion” optical fiber, which is cost-effective and easy to manufacture.

PMD could be mitigated during fiber manufacturing by:

  • providing a perfect homogeneity of the dopant
  • control the cylindricity of the core
  • adding a turning step in the drawing process.

What other techniques can network operators use to reduce CD and PMD?

In the past, CD was the main limiting factor in achieving long distances, and compensators using negative chromatic dispersion were developed to avoid regenerating the signal using an optical/electrical/optical (OEO) regeneration site.

Now, instead of having to implement dispersion-shifted CD fiber, transceiver manufacturers have developed new modulation techniques and tricks like negative chirp and forward error correction (FEC) to deal with a small amount of dispersion. Short to medium range transceivers can use these techniques, and it is very important to select the right transceiver type for the fiber type characteristics of your network.

For longer distances and faster speeds like 400 Gbps, post-compensation with digital signal processing developed for coherent transceivers can accomplish more in managing CD and PMD to some extent.

When does it become critical for network operators to measure CD and PMD?

Today’s higher-speed networks carrying intensive amounts of data and mission-critical services have zero tolerance for service interruptions. It is imperative that network operators pay attention to fiber characterization to ensure QoS.

CD metering and PMD management should be a priority for network operators, and should not wait until service-impacting issues arise. Also, the speed deployed today will soon be superseded by ever faster speeds, so if you want your network to be ready for the next wave of evolution, fiber characterization is critical.

What are the best practices for managing CDs and PMDs?

Knowing the type of fiber that is deployed in the network is essential. If a network operator has a typical 80% standard G.652 fiber, they need to know what type of fiber comprises that remaining 20% ​​and where those fibers are located. Otherwise, when they switch to a higher speed stream, there might be some issues during “wake-up” or poor QoS for the end client.

Current testing technology allows network operators to determine the PMD of fiber throughout the network. And it is entirely possible to isolate specific fiber lengths that have a high PMD.

Is there something unique about CD and PMD in relation to 5G networks?

The previous point about the importance of reliability to avoid interruptions in service is even more relevant in 5G networks. Imagine mission-critical activities like using a self-driving car, remote surgeries, or world-class sports or entertainment broadcasts that are interrupted mid-stream. That is totally unacceptable and can be avoided with the diagnostics mentioned here to isolate fiber cable problems and ensure they are addressed, such as replacing faulty legs.

It’s the same as if you have a faulty connector. You don’t buy a more powerful amp to make up for the power loss due to the faulty connector, you go to the field and clean or repair the connector. The same applies to ensuring end-to-end fiber integrity within the network.

PMD is a tricky phenomenon because even if the average PMD delay does not change over time, specific wavelengths will see specific delays at specific times. So you can have different delays on the same wavelength measured at different times, or a delay spike that moves from channel to channel. This is very difficult to fix. An operator may be sure that its mobile network operating at 25 to 50 Gbps is working reliably, but an hour, day, or month after activation, Differential Group Delay (DGD) may change on a specific channel. and interrupt service. . Therefore, it is especially important to measure PMD before activating services. Don’t just plug and pray!

How are transceivers likely to evolve due to CD and PMD?

While coherent transceivers have helped advance CD and PMD management, developing next-generation transceivers that accommodate 25 and 50 Gbps mobility is a real challenge if you want to keep cost affordable. Digital signal processing can be implemented to reduce the impact of dispersion, but that can increase cost.

People sometimes assume that CD and PMD are no longer a problem, but that is not the case at all. Ignoring that fact can be costly for network operators because they have to fix incidents that could have been prevented. There is also potential exposure for QoS and service level agreements.

Won’t consistent transmission over high-speed networks make CD and PMD problems obsolete?

Coherent transmission, which essentially means using coherent transceivers with digital signal processing (DSP) to support high-speed 100G, 400G, and 800G networks, does not completely eliminate the need for CD and PMD testing.

For CD, the trigger factor is knowing the type of fiber deployed in your network. If you have a brand new network within which you have deployed a known fiber type, then selecting the correct type of transceivers to match the fiber type means that a CD test is not required.

On the other hand, deploying any transceiver on an unknown fiber type can lead to failures and errors. Many network operators have purchased fiber networks over the years where the documentation and fiber type are missing. For example, the implementation of long-haul coherent transceivers and amplifiers on dispersion shift fiber (DSF) could generate non-linear effects that degrade QoS.

In any network that uses any type of fiber, PMD still presents a risk. Remember I mentioned earlier that even new cables can have out-of-spec PMD values. To mitigate that risk, testing is a must, even if you have a high-speed network that relies on consistent transceivers.

That’s because PMD can always be present, and the only way to know if PMD is at a level that won’t prevent transmission is by testing. Not all coherent transceivers are equally tolerant to PMD, and one outage, even for one second at 800G speed, means the loss of 800 billion bits of data. That’s not a risk most network operators want to take by going back online and praying.

The bottom line: When coherent transponders came along 15 years ago, we never stopped trying CD and PMD because these effects could still affect a transmission, even with digital post compensation. CD and PMD remain real considerations that must always be taken into account as high-speed connectivity and 5G networks proliferate.

Gwennael Amito is a subject matter expert and a senior member of EXFO’s technical staff.

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