In 2020, China Telecom proposed the strategic transformation goal of “cloud to digital” and released the “Cloud-Network Convergence 2030 Technology White Paper”. On the one hand, the white paper is an interpretation of China Telecom’s strategic transformation of “cloud to digital transformation”. On the other hand, it comprehensively expounds the connotation, significance, needs, characteristics, vision and principles of cloud network integration. As the “bandwidth cornerstone” of cloud network integration infrastructure, how to enable cloud network integration, and the technical development and evolution direction of optical network in various scenarios of cloud network integration have attracted much industry attention.
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Requirements of cloud network convergence for optical networks
Cloud network convergence is a profound change in the “new information infrastructure”. Its connotation lies in the comprehensive and in-depth integration and innovation of cloud network technology and production organization mode. Operators adjust their business form, business model, operation and maintenance system, service model and personnel team, and transform from traditional communication service providers to intelligent digital service providers in order to lay a solid and safe foundation for social digital transformation. As for optical networks, cloud network convergence requires optical networks to provide higher speed, wider bandwidth, more ubiquitous and cheaper bandwidth services, so as to further consolidate the bandwidth foundation.
In-cloud data center internal optical network
Data centers are one of the fastest growing traffic and one of the most important drivers of optical communications technology in the last 5 years. Back in 2018, Cisco’s global cloud index research predicted that by 2021, more than 85% of global traffic will be east-west traffic within and between data centers, and less than 15% will be north-south traffic for users to access data centers.
At present, the speed and market scale of data center optical modules have surpassed the traditional telecom network market. In recent years, an important direction of the development of fiber optical transceiver in the telecom network market is to study how to learn from and reuse the data center optical module technology, and reduce the cost of optical modules in the telecom market. For example, a significant portion of the 10Gbit/s and 25Gbit/s optical modules for 4G/5G fronthaul transmission reuse data center optical module technology, and the technical requirements of 100G and future 400G optical modules for telecom equipment are increasingly converging with data center requirements. The recent hot spot in the development of optical communication inside data center is on board optical communication, including COBO and CPO (Co-packaged Optics), etc. The on board optical communication technology brings revolutionary changes to the equipment form and industrial ecology, which deserves high attention from the whole industry chain.
Inter–cloud data center interconnect (DCI) optical network
With the rapid growth of cloud computing and data center traffic, the bandwidth growth of Internet enterprises exceeds that of traditional carriers, and the scale and level of transmission network construction of Internet enterprises such as Google and Facebook are catching up with traditional telecom carriers. With the continuous growth of bandwidth demand, the technical routes of Internet enterprises and telecom operators in data center interconnection (DCI) networking have started to converge, and both have moved toward the solution based on L1 layer wavelength division multiplexing (WDM) technology. Moreover, with the rapid growth of DCI traffic, how to maximize the transmission capacity in a single fiber has become the main development direction of DCI optical network technology.
First, to improve the single-wave rate. Some countries have been fully deployed 80×100Gbit/s WDM system that based on coherent optical communication technology. In recent years, operators began to pay attention to and deploy single wavelength 200Gbit/s and 400Gbit/s WDM systems, for example, based on PM-16QAM modulation 80x200Gbit/s WDM system has been deployed in some metropolitan networks. For long-distance transmission, single wavelength 200gbit/s and 400gbit/s WDM technology has also been reported on a large number of pilot long-distance existing networks of more than 1000km, and is about to enter commercial use. On the other side, the coherent optical communication technology with single wavelength of 600gbit/s and 800gbit/s has also been put on the agenda, but at present, there are great deficiencies in transmission distance and other performance, which can only be limited to short-range applications, and there is still a big gap from commercial use.
Second, to increase the available wavelength. At present, the main system of fully deployed WDM system is C-band 80 × 50GHz(4THz). In the past few years, a small number of systems have adopted C-band 96×50GHz (4.8THz), increasing the available wavelengths by 20%. Entering the era of single wavelength 200Gbit /s, in order to achieve the goal of doubling the capacity with comparable distance, operators have proposed the demand of extended C-band WDM system of 80×75GHz (6THz spectrum), which has been matured and started commercial deployment. So far, the available spectrum in the extended C-band has been increased by 50%.
The next goal for operators is to double capacity in the single-wavelength 400Gbit/s era while continuing to maintain distance, with the expansion targeting the L-band. In the past decade, Europe and the United States have deployed a considerable scale of C + L-band WDM systems, whose total spectrum is about 9.6 THz, which is not enough to achieve the goal of doubling the total capacity. China is currently studying the technical route and options for expanding C+L, and the available spectrum is expected to be around 11THz to 12THz. Academics are also studying the full-band optical amplification technology that pulls through O, E, S, C, L and U bands, which is a more distant research goal. The available wavelength bands for single-mode optical fibers are shown schematically in the figure below.
On-cloud optical network with high quality
The needs of enterprise users for cloud services are diverse, and the requirements for service quality are very different, so it is difficult to use a single technical solution to solve all users’ needs for cloud access.
First, enhance the quality of services into the cloud. At present, enterprises’ demands for cloud network convergence products and services are rising, especially for high-value industry users such as government, finance, medical, industrial park, etc. Besides, the security of the cloud, as well as the bandwidth, latency, reliability, security and self-management of the network have put forward clear index requirements, which poses a serious challenge to the operators’ existing cloud network convergence products and services. OTN (Optical Transport Network) technology is undoubtedly the best solution for the demand of high-quality enterprise cloud services. OTN technology can provide high-quality cloud network integration services that meet the requirements of five dimensions: security, reliability, convenience, responsiveness and perception, and fully meet the needs of users in different industries in the cloud of office system, production system and core system.
Second, increase the flexibility of cloud services. OTN technology has appeared and developed for more than 20 years, but its extensive bandwidth granularity in Gbit/s has greatly improved the access threshold and blocked a large number of small and medium-sized enterprise users. For this reason, China Telecom, together with the industry chain, has introduced the concept of metro-optimized OTN (M-OTN) and introduced optical service unit (OSU) technology with a minimum granularity of 2Mbit/s through technological innovation, realizing a refined service of “one service, one pipe”.
Cloud edge collaborative metropolitan optical network
With the continuous development of Internet of Things and other technologies and the continuous increase of data, cloud based Internet of Things solutions can not meet people’s growing needs. Increasingly enterprises begin to turn their attention to edge computing
The bandwidth demand of edge optical network is not as large as that of backbone optical network. At present, it is basically below 100G. At the same time, it is very sensitive to the cost. The high-speed coherent optical communication technology popularized in backbone optical network is difficult to meet the cost expectation in the near future. Therefore, in the field of cloud side cooperative metro optical network, we should focus on low-cost incoherent technology solutions with single wavelength of 100Gbit/s and below.
O-band is in the low dispersion and low loss region in single-mode fiber, traditionally used as the working window for customer-side optical signals. The main reason for choosing O-band for 25Gbit/s and 50Gbit/s signals is the small dispersion cost, which can guarantee 10km or longer transmission distance for 25Gbit/s NRZ and 50Gbit/s PAM4 signals without dispersion compensation. But as the customer-side service rate increases, the industry has also started to introduce WDM technology, such as the IEEE-defined QSFP28 100G LR4 and 100G ER4 interfaces using O-band 800GHz interval LWDM (Lan WDM).
With the maturity of DSP technology, the industry also has a scheme to upgrade the single wavelength of O-band WDM system to 100Gbit/s. For example, the 400G-LR4 scheme being studied by the 100G Lambda MSA organization plans to use 4-wavelength, 800GHz spacing O-band WDM technology to realize 400GE service interface through 4×100Gbit/s PAM4 signals. Therefore, for cloud-side collaboration, O-band WDM technology is a cost-effective solution of great interest in the access and metro-edge layers of optical networks.
Conclusion and outlook
With the popularity of the concept of “new infrastructure” and the in-depth promotion of the digital transformation of thousands of lines and industries, cloud network integration has become the consensus of telecom operators. As the “bandwidth cornerstone” of cloud convergence infrastructure, optical network will be duty bound to provide higher speed, wider bandwidth, more ubiquitous and cheaper bandwidth services for the development of cloud convergence services.
For the internal optical network of data centers within the cloud, data centers have become the first driving force for the development of the optical module market and technological advancement. Besides, 400G optical transceiver is now becoming popular, and 800Gbit/s optical modules have started to appear. In the future, On-Board Optics and CPO (Co-packaged Optics) are the next development hotspot of optical network inside data center, which deserves high attention of the whole industry. For high quality optical network into the cloud, OTN technology is the best solution for the demand of high quality enterprise into the cloud service. In addition, for cloud-edge collaborative edge optical network, the massive bandwidth demand brought by the rise of 5G fronthaul transmission and edge computing has introduced WDM technology to the edge layer of the network for the first time. Based on the cost-performance advantage, single-wavelength 100Gbit/s and below rate non-coherent optical communication technology will be the mainstream technology form of edge optical network now and for quite a long time in the future. Finally, 0-band WDM technology is an emerging technology that deserves great attention, and we hope that the industry chain will collaborate to choose a reasonable technical route to complete the development of G.owdm standard in ITU-T as soon as possible.