07.11.2025
498 Optical networks have long been the standard for backbone and corporate data transmission. But providers and engineers often face a practical question: how to extend a DWDM link for a distance of 80-100 km without using regenerators?
Regeneration is expensive – it requires the installation of active equipment, additional power supply, space for placement and maintenance. This means that if you can set up a link without intermediate regeneration, you can significantly reduce the project budget and operating costs.
Let’s take a closer look at what to pay attention to, what technologies to use and what pitfalls you may encounter.
Possibilities for DWDM signal transmission without regeneration
DWDM stands for Dense Wavelength Division Multiplexing. This technology allows dozens or even hundreds of channels to be transmitted over a single fibre optic cable. The length of each link can vary, allowing for maximum efficiency when using a single cable.
Typically, large data centres and backbone operators connect sites 80–100 km apart. This presents a choice: install a regenerator or do without one.
It is possible to transmit signals over such distances without regeneration. However, this requires the correct selection of DWDM equipment and consideration of certain conditions:
Use modern single-mode fibre with low attenuation. The indicator will be approximately 0.2 dB/km in the 1550 nm range.
Install a minimum number of connectors and avoid splices.
Use EDFA or RAMAN amplifiers where necessary.
Use high-quality SFP+/QSFP optical modules with sufficient power budget.
Minimise dispersion and noise so that they are within acceptable limits.
If all these rules are followed, the DWDM link will operate stably at 80–100 km. Sometimes it can even reach 120 km without regeneration.
What factors affect the range of an optical channel
The signal transmission range depends on many factors. Let’s consider the most basic ones that really limit us in terms of length:
Attenuation on the fibre. This is the main parameter. Typically, at a wavelength of 1550 nm, the attenuation coefficient is ~0.2 dB/km. This means that the approximate loss over 8 km is 18 dB, and over 100 km is 20 dB. This value alone brings us close to the limits of most modules.
Connections and splices. Each additional connection results in losses. Each connector adds 0.3-0.5 dB. Each splice adds 0.05-0.1 dB. Individually, these are small values. But if there are dozens of connections on the route, the total losses can easily grow to 2-3 dB.
Optical module characteristics. SFP+/QSFP have different transmission power and receiver sensitivity. The same interface (e.g., 10G DWDM) can have different power budgets, ranging from 22 to 28 dB. This determines whether it can pull the link without an amplifier.
Dispersion. The higher the transmission speed, the more noticeable this indicator is. At distances of about 80–100 km, the signal begins to stretch and blur. This impairs its readability. Therefore, compensation and the installation of amplifiers are important.
Noise and nonlinear effects. When amplifiers are installed, noise (ASE noise) appears. And if the fibre has high power, nonlinear distortions occur. The most common effect is FWM (four-wave mixing).
If all these factors are taken into account in advance, a link of 80–100 km can be built without problems and costly regeneration.
Choosing the right optical modules (SFP+/QSFP)
Optical modules affect the signal transmission range. Two types are commonly used: SFP+ and QSFP.
Important! Don’t just check the stated distance. Also pay attention to the receiver sensitivity. If the module has a sensitivity of 23 dB and the total loss is 25 dB, the link will not start.
SFP+
It is used at speeds up to 10G. There are several options here:
LR (10 km) – not suitable for DWDM.
ER (40 km) – rarely used, low budget.
ZR (80 km) – the best option, but just barely. In reality, they more often operate at 60–70 km without amplifiers.
DWDM SFP+ 80 km – can reach up to 100 km with an ideal line.
Modules with 80 km support – ZR – are most often used. But to ensure a stable channel, it is better to allow for a margin. This is important because many manufacturers specify the maximum transmission distance under ideal conditions. They do not take into account the presence of connections and additional losses.
QSFP
This module is suitable for speeds of 100G/400G:
Classic modules are suitable for 100G. For example, 100G DWDM PAM4 or with external transponders.
For 400G, it is necessary to compensate for dispersion and use EDFA amplifiers. Raman amplifiers are installed less frequently.
Also, the fibre optic cable must be of higher quality than for SFP+. Additionally, external amplifiers and compensators will be required.
The role of multiplexers and demultiplexers in the link
Multiplexers (MUX) combine dozens of channels into a single fibre. Demultiplexers (DEMUX) perform the opposite action. They separate these channels.
When installing such equipment in DWDM networks, this must be taken into account. When a signal is transmitted over 80–100 km, the use of MUX/DEMUX introduces losses of 1 to 3 dB. If there are two devices in the circuit – one on the transmit side and one on the receive side – the attenuation is up to 2–6 dB. These are already serious figures. They are particularly noticeable at a distance of 100 km.
To minimise risks:
take the contribution of MUX/DEMUX into account when calculating the power budget;
use passive multiplexers with low losses;
add amplifiers after MUX/DEMUX, especially under heavy loads.
Please note! The attenuation level depends on the type of optical devices. When using high-quality models, the loss is lower – up to 1 dB. Standard MUX/DEMUX add 1.5–3 dB on average.
Use of optical amplifiers (EDFA, Raman)
Signal transmission over 80–100 km without amplifiers is practically impossible. The EDFA (Erbium-Doped Fibre Amplifier) model is most commonly used. This amplifier operates in the 1550 nm range, which perfectly matches DWDM. It amplifies the signal in optical form and does not convert it to electrical. It adds 20–30 dB of amplification. Using this model is much cheaper than regeneration.
Raman amplifiers are used in more complex projects. They distribute amplification across the entire line and reduce noise levels. They are suitable for long routes of 150–200 km. However, Raman models are more expensive and more complex.
For 80–100 km, the optimal scheme is as follows. Use one EDFA amplifier on the transmitting side. If the case is complex, then install two. To do this, simply add an EDFA to the receiver.
Calculating the power budget for a distance of 80–100 km
To understand whether the link will start, you need to calculate the power budget. The formula is simple:
Module budget ≥ Total line losses
Example:
Fibre attenuation: 0.2 dB/km × 100 km = 20 dB.
2 MUX/DEMUX = 4 dB.
10 splices at 0.1 dB = 1 dB.
Total: 25 dB.
If the module has a budget of 28 dB, the link will work, but without a margin. Therefore, it is better to install an EDFA amplifier. It compensates for 5–10 dB and guarantees stability.
Always leave a margin of 3–5 dB. Fibre ages, connectors get dirty, and modules degrade. All of this leads to signal loss. Without a reserve, your link will start to ‘fall apart’ in a couple of years.
DWDM channel configuration and testing
Once installation is complete, testing must be carried out. To do this, perform the following:
OTDR measurements. These show exactly where losses are occurring in the line. This also allows you to determine the approximate number and location of splices and connectors.
Power meter or power measurement at the output and input. This helps to ensure that the signal at the receiver does not ‘drop’ below sensitivity.
BER test. This is bit error monitoring. It is used to check the actual level of malfunctions.
Monitoring via DWDM equipment. Most devices allow you to see OSNR. They also determine channel power and other parameters.
Testing should always be done under load. Sometimes the link ‘lights up’ and indicates that the empty channel is working. But under 100G traffic, errors begin to creep in. Therefore, testing in real conditions checks the stability of the line. This way, you take less risk.
Limitations when working without regenerators
It is quite possible to assemble a DWDM link for 80–100 km. However, without regeneration, there will be some limitations. These include:
Speed. There are almost no problems for 10G. For 100G, dispersion becomes critical. For 400G, compensation and amplifiers are essential.
Number of channels. The more channels there are, the higher the total power. This can cause non-linear distortions.
Reliability. In the event of a failure (welding added, patch cords replaced), the power reserve may be lost.
Equipment ageing. Over time, both the fibre and the modules lose their characteristics. Therefore, it is necessary to either build in a power reserve in advance or replace the equipment. Otherwise, in the long term, the stability of the link will deteriorate.
Therefore, you need to be very careful when designing such links. You should always have a backup plan in mind.
Conclusions
It is quite possible to assemble a DWDM channel for 80–100 km without regeneration. To do this, you need to:
select DWDM equipment;
use SFP+/QSFP modules;
take into account all losses in the fibre, splices and MUX/DEMUX;
install EDFA amplifiers to compensate for attenuation;
calculate the power budget and perform testing;
build in a reserve.
Such projects allow for significant savings. There is no longer any need to build intermediate nodes and rent sites. However, such networks require a careful approach and experienced engineers.
Interestingly, similar principles apply to PON networks. There, it is also important to take into account attenuation, the number of splitters, and build in a margin. Only in PON are we talking about tens of kilometres and hundreds of subscribers. In DWDM, we are talking about hundreds of kilometres and terabytes of backbone traffic.
The main thing is competent design, calculation and testing. Then the link will work stably. This will save on unnecessary equipment.
