400G OTN Technologies: Single-Carrier, Dual-Carrier and Quad-Carrier


In order to achieve 400G long-haul (LH) transmission, three 400G Optical Transport Network (OTN) technologies come into being to meet the needs: single-carrier 400G, dual-carrier 400G, and quad-carrier 400G. They differ from each other mainly in the number of wavelengths used for transmission. This post will reveal what they are and their respective pros and cons.

Single-Carrier for 400G OTN

Single-carrier 400G, or single-wavelength 400G, means there is 400G capacity on a single wavelength. The single-carrier 400G adopts high-order modulation formats such as PM-16QAM, PM-32QAM and PM-64QAM. Normally, a single-carrier for 400G optical transport network is used only in network access, metro, or DCI (Data Center Interconnection) transmission.

Single-Carrier for 400G OTN

Figure 1: Single-Carrier for 400G OTN

Take PM-16QAM (Polarization-Multiplexed-16 Quadrature Amplitude Modulation) as an example. PM refers to a process where the 400G (448Gbit/s) optical signal is separated into two signals and modulated to transmit in two polarization directions – X and Y, which can cut the original signal rate in half (224Gbit/s). QAM is a process of separating the signals in X and Y to further reduce the rate. 16 stands for 4 bits, which means the signal in X and Y is respectively divided into 4 signals and the rate will accordingly decrease to 1/4 on the basis of the previous 224Gbit/s. By using PM-16QAM, the signal rate at this moment becomes 56G Baud (the rate of electrical processing).

Note: Because in current circuit technology, 100Gbit/s has approached the limit of the electronic bottleneck. If the Baud continues to increase, problems like signal loss, power dissipation, and electromagnetic interference will remain a hassle, which will, even if solved, require tremendous costs.


Figure 2: PM-16QAM

Pros of Single-Carrier for 400G Optical Transport Network

  • Compared with the multi-carriers scheme, single-carrier 400G is an easier wavelength allocation solution with simpler structure and smaller size that can provide easy network management and low power consumption.
  • With higher-order QAM, single-carrier for 400G OTN network can increase signal rates and spectrum efficiency, which will significantly expand network capacity and increase the number of users to support.
  • Also, with high system integration, it can connect the separate subsystems into a complete one and make them work in coordination with each other and achieve the best overall performance.

Cons of Single-Carrier for 400G Optical Transport Network

Since single-carrier for 400G OTN network adopts more advanced QAM, it requires a higher OSNR (Optical Signal Noise Ratio) and greatly reduces transmission distance (less than 200km). Also, single-carrier is more susceptible to laser phase noise and fiber nonlinear effects. Therefore, it is the best solution only for some specific applications that don’t require ultra long-haul transmission distance, but need large bandwidth capacity.

Dual-Carrier for 400G OTN

Dual-carrier 400G, also named dual-wavelength 400G, offers 400G capacity via two 200G wavelengths. The dual-carrier 400G system based on the 2× 200G super-channel scheme adopts lower-order modulation formats like PM-QPSK (Quadrature Phase Shift Keying, a symbol represents two bits, which means the rate is reduced to 1/2), PM-8QAM or PM-16QAM. Dual-carrier for 400G optical transport network is applied in more complex metro networks to achieve 400G long-haul transmission.

Dual-Carrier for 400G OTN

Figure 3: Dual-Carrier for 400G OTN

Pros of Dual-Carrier for 400G Optical Transport Network

  • The spectrum efficiency of dual-carrier 400G has increased by more than 165%, with relatively high system integration, small size, low power consumption. Dual-carrier 400G is regarded as the most commonly-used technology for 400G OTN.
  • The span of dual-carrier 400G is longer than single-carrier 400G, which can reach up to 500km for commercial use. When deployed with low-attenuation fiber optic cable and EDFA (Erbium Doped Fiber Amplifiers), dual-carrier for 400G OTN network can cover more than 1000km, which can basically satisfy the 400G long-haul transmission application.

Cons of Dual-Carrier for 400G Optical Transport Network

Even with low-attenuation fiber optic cable and EDFA, dual-carrier 400G still fails to reach as long as quad-carrier 400G does, not suitable for ultra long-haul (ULH) transmission over 2000km.

Quad-Carrier for 400G OTN

Quad-carrier 400G refers to a solution that offers 400G capacity through four 100G wavelengths. It is achieved by constructing a 400G super-channel based on 100G PM-QPSK with four carriers, suitable for ultra long-haul (ULH) transmission over 2000km.

Quad-Carrier for 400G OTN

Figure 4: Quad-Carrier for 400G OTN

Pros of Quad-Carrier for 400G Optical Transport Network

  • Quad-carrier for 400G OTN network adopts the mature 100G transmission technology that has been widely-used for commercial purpose.
  • It can achieve ultra long-haul transmission of more than 2000km at relatively low cost.

Cons of Quad-Carrier for 400G Optical Transport Network

Quad-carrier 400G system makes sense only when spectrum compression technology is introduced to improve spectrum efficiency, and the 100G chip is upgraded to solve the problems of integration and power consumption. Otherwise, a 400G system built on the current 100G chip is essentially a 100G system.


In all, 400G long-haul transmission is mainly realized by single-carrier, dual-carrier and quad-carrier. Single-carrier for 400G OTN network can only cover a distance of less than 200km; dual-carrier for 400G OTN network is the ideal solution for MAN transmission (with PM-16QAM) and medium long-haul transmission (with PM-QPSK); quad-carrier for 400G OTN network has the same transmission distance as 100G and is appropriate for ULH transmission. As global data traffic keeps climbing, there is no end to bandwidth demands. While it may take time to transit to 400G, you can learn about What’s the Current and Future Trend of 400G Ethernet? to make preparations first.

Original Source: 400G OTN Technologies: Single-Carrier, Dual-Carrier and Quad-Carrier

Why Is OTN Becoming More And More Important?

In an era of various services operating, telecom operators will be transformed into the comprehensive service providers of information and communications technology (ICT).The abundant services drive the higher demand for broadband services, directly manifesting the requirement for the ability and performance of the transport network. Owing to meet the needs of all kinds of new services, the Optical Transport Network (OTN) technology comes into being, playing a leading role in the transport network.

OTN, the next-generation backbone transmission network based on WDM techniques, lies in the optical layer network. Standardized by a series of ITU-T advice covering G.872, G.709 and G.798, it is the new generation of digital transmission network and optical transmission network. OTN is based on a fixed frame size with 3 key sections: Overhead, Payload, and Forward Error Correction (FEC). These OTN frames are routed across the network in a connection-oriented way. Similar to a synchronous digital hierarchy/ synchronous optical network (SDH/SONET) frame, the overhead carries the information required to identify, control and manage the payload to maintain the deterministic quality. The payload is simply the data transported across the network, while the FEC corrects errors when they arrive at the receiver. The number of correctable errors depends on the FEC type. The most common is GFEC described in the G.709 standard, which can identify 16 symbol errors and correct 8 symbol errors per frame. As is shown in Figure 1, the OTN concretely operates as follows. What OTN successfully deals with are the traditional WDM net working issues of missing wavelength, weak scheduling ability of subwavelength services, bad networking ability and poor protection ability. Combining the strength of optical-field handling with that of electrical-field treatment, it is the optimal technology for transmitting the large-particles broadband services, providing the huge transmission capacity, the completely transparent connection of end-to-end wavelength and subwavelength as well as the various protection at telecommunication level.

Figure 1:The operation of an OTN network

The core of most current operator networks is SDH/SONET, which has always offered good fault management, performance monitoring, predictable latency, a protection mechanism and, of course, synchronization. This very stable network has become the expected minimum in performance objectives for network operators today and is often described as having “five 9s” (and higher) performance, meaning at least 99.999% up time. On the basis of the current SDH/SONET managerial functions, ONT provides not only the complete transparency of communication protocols, but also the ability of end-to-end connection and networking for WDM, whose technology inherited the dual advantages of SDH and WDM. It solves the SDH issues that the cross particles based on VC-12/VC4 are too small to meet the transmission requirements of large particles services,causing the complicated schedule. At the same time, it also partly settles the WDM problems of the positioning difficulty due to a system fault, weak networking and poor ability and means to provide the network survivability. OTN reduces transport costs and delivers enhanced network and performance management functions. Forward error correction (FEC) algorithms improve the reach of the transmission links, helping to reduce regenerators and optimize the spectral efficiency. Additionally, an OTN “digital wrapper” includes many layers and components known from SDH/SONET but at enhanced performance levels.

The ever-increasing demand of broadband services has significantly contributed to the application of OTN, which is simpler and better than SDH/SONET and increases the scalability of WDM systems. OTN technology comes into being, not only following the development of the communication technology, but also impelling the transmission network to a better stage. What’s more, the transmission requirements of IP services and the adapter IP services OTN facing has become an important issue that the optical communication further develops. As the optimal choice for developing the transport network, OTN is becoming more and more important, and will be extensively applied to play a dominant role in the transmission network in the future!