100G OTN: a future trend in IDC backbone network construction

Combined with the development demand of IDC service, this paper analyzes the current situation of 100G OTN(Optical Transport Network) development, key technologies, equipment operation and maintenance and other aspects. It believes that 100G OTN network construction will be an important development trend of future broadcasting backbone network construction. 

IDC technology development trend

Today, emerging technologies such as cloud computing and the Internet of things are booming. In addition, with the continuous popularity of fixed access broadband users, the scale deployment of HD video services and the emergence of a large number of broadband applications, the bandwidth demand of the backbone transmission networks will continue to grow rapidly. Statistics from OIF show that the average annual traffic growth rate of network broadband operators is much higher than its revenue growth rate. Operators have to reduce the transmission cost per unit traffic to alleviate their revenue pressure. Improving the system transmission capacity is the most effective means to reduce TCO. After several years of effort, the 100G standard has been completed and the technology has made a breakthrough. The mainstream equipment manufacturers have released 100G products, and the 100G era has come.

Analysis of 100G OTN core technologies

• 100G OTN line modulation technology

 The long-distance transmission of OTN is mainly limited by the following physical limitations: OSNR, dispersion, nonlinear effect and PMD. These physical constraints are closely related to the modulation signal rate. The higher the modulation rate, the more obvious the influence.

QPSK (Quadrature Phase Shift Keying) is the best choice for 100G modulation mode because the bit rate of 100G signal is 112Git/s or higher. If QPSK modulation is directly adopted, very high technical requirements will be put forward for the optical/electrical components of the system. Therefore, an optical polarization multiplexed scheme is introduced. Polarization multiplexing uses two independent optical polarization states to carry 56GHz services. Each polarization state adopts QPSK modulation, which can further reduce the baud rate to 28Gbit/s. Thus, the bandwidth requirements of optical/electrical devices can be reduced, and the system power consumption and cost can be reduced.

Specifically, at the transmitting end, the 100G signal is divided into four low-speed signals. With OTN and FEC overhead, each signal is 28Gbit/s. The light emitted by the laser is decomposed into vertical and horizontal polarization states. Using two polarization states with the same frequency to carry signals can reduce the rate by half, reduce the bandwidth and adapt to more compact channel spacing. QPSK uses four transmission phases to modulate the optical signal of each polarization state. Then, the QPSK modulated optical signals of the two polarization states will be coupled and output together. Polarization multiplexing (PM) and QPSK reduce the modulation rate to 1/4, so as to reduce the cost of 100G system.

100G DWDM coherent transmission scheme

• Coherent reception and DSP Technology

Due to the random change of polarization state of PM-QPSK optical signal after long-distance transmission, there is a frequency difference and phase difference between the local optical oscillator at the receiving end and the received optical signal. The mature solution is to use high-speed electrical signal processing (DSP) technology to process the received signal. Using complex advanced electrical signal processing and compensation technology, the PMD tolerance and CD tolerance of 100G system can be better than that of 10G system, reduce the transmission cost introduced by PMD and CD, and have high optical receiving sensitivity.

Compared with NRZ direct reception, DSP compensation technology can improve OSNR tolerance to nearly 6dB. Using DSP compensation technology, the system dispersion tolerance can reach 40000-60000ps/nm and PMD tolerance can reach 25-30ps, so as to realize simpler network deployment.

• 100Gcustomer side CFP module technology

IEEE defines the rate of 100GE as 103.125Gb/s. IEEE and ITU-T define four physical layer interfaces of 100GE. Although the relevant standards of the 10x10G customer side are still under discussion, OIF has defined that the 100G CFP customer side hot-pluggable optical module has 4 wavelengths (4×25G, 100GBASE-LR4/ER4) and 10 wavelengths (10×10G，100GBASE-LR10/ER10). Among them, 100GBASE-LR4 CFP and 100GBASE-LR10 CFP have been widely used in high-speed routers, switches and clients side of WDM systems. Plus, 100G CFP client-side module adopts photonic integration technology (PIC) to realize small volume and low power consumption.

FiberMall CFP 100G LR4 module

In 100G WDM/OTN system, CFP encapsulated customer side modules are widely used to access 100GE or OTU4 services. 100GBASE-LR4 and 100GBASE-LR10 are the most widely used. However, 100GBASE-ER4 and 100GBASE-ER10 technologies are not mature and need to be further developed.

The advantages of 100G OTN technology

From the rapid development of the 100G OTN industry chain and the mature application of the technology, the time is ripe to lay out the backbone network of 100G OTN in the broadcasting system.

-100G OTN technology realizes the fine operation of large pipelines

The key to the efficient utilization of 100G pipeline is the fine and complete OAM capability of the pipeline itself. SDH/SONET network can only provide level 1 connection monitoring capability (TCM), while WDM network does not have channel level monitoring and management capability, so network manageability and service security are poor. And OTN provides rich overhead bytes to sink the monitoring alarm to the wavelength/sub-wavelength, so as to realize the active perception of the wavelength channel to the service. Besides, OTN can provide level 6 connection monitoring function, and can realize hierarchical and segmented channel level fine management for application scenarios of multiple operators, multiple equipment manufacturers and multiple subnets.

–100G OTN technology has a perfect protection mechanism

100G OTN technology has complete optical layer and electrical layer protection, which can greatly improve 100G reliability. OTN standard defines a variety of network-level protection mechanisms, such as optical line protection, optical channel protection, SNCP protection, ODUK1+1 protection, ODUk ring network protection, etc.

In terms of optical layer protection, optical channel 1+1 protection is commonly used. The concurrent selection function of the OCP single disk is used to concurrent customer signals to different wavelength channels to avoid service interruption caused by a single OTU failure.

In terms of electric layer protection, 100G OTN commonly uses ODUK 1+1 protection to realize concurrent selective reception through electric layer crossing, and the protection switching time is less than 50ms.

-100G OTN can realize efficient bearing of multiple services

Since its development, OTN has fully possessed wide bore bearing characteristics. Packet enhanced OTN products have processing capabilities such as ODUk crossover, packet switching, VC crossover and och crossover, and can realize unified transmission of TDM and packet services.

OTN has the unified carrying capacity of SDH, OTUK, Ethernet service, FC service, CPRI service and PON service, as well as various customer side interfaces supporting the above services. Therefore, the industry generally believes that 100G OTN is the best technology to match various service ports and solve multi-service carrying.

To sum up, the benefits of 100G OTN to network construction and operation and maintenance mainly include:

1. The design of optical dispersion compensation and polarization demultiplexing on the transmission line is simplified, and the line design is simpler;
2. It eliminates the dependence on low PMD optical fiber and is suitable for transmission optical fiber of various specifications to facilitate the upgrade of optical fiber line rate;
3. Eliminate the influence of DCF optical fiber nonlinear effect of transmission line, reduce the number of line amplifiers and the influence of ASE noise, reduce the line cost and improve the long-distance transmission capacity of the system;

100G OTN Dual Transponder

4. The line transmission delay is reduced. According to the delay calculation of 1km optical fiber 5us, the delay reduction caused by eliminating DCF optical fiber is very considerable, which is of great significance to the delay-sensitive application environment;
5. The protection recovery time is less than 50ms, (different from 40Gsystem) 100G digital signal processing adaptive dispersion compensation algorithm converges rapidly and fully meets the carrier level recovery delay requirements.

Fiber Mall‘s 100G OTN helps the development of IDC Network

In 2022, the procurement scale of 100G OTN for IDC operators will become larger and larger, and the 100G OTN industrial chain will gradually mature. Compared with 10G/40G, the construction cost will become increasingly obvious. In addition, the maturity of 100G core technology, the outstanding advantages of network management and the convenient maintenance of the system in the later stage will make 100G OTN network construction an important development trend of radio and television backbone network construction.