PON Evolution Path

With its shared point-to-multipoint network architecture, and the advantages brought by the passive infrastructure, PON technology is the primary FTTx solution for network operators. At present, there are more than 700 million users worldwide enjoying high-speed broadband access services delivered over PON networks. Due to the effectively infinite bandwidth of optical fiber and long reach, coupled with low cost and low energy consumption, the rapid replacement of copper access by fiber is the biggest revolution in access network for a century. PON has become the most important solution to meet the demand for ultra-broadband access.

Since 2004, when IEEE completed the EPON standard and ITU-T SG15 Q2 completed the GPON standard, there has been a global upsurge in FTTx construction based on PON technology. Gigabit Passive Optical Network (GPON) and Ethernet Passive Optical Network (EPON) dominated early PON deployments to provide users with hundreds of megabits of broadband access. EPON began mass deployment since around 2006, while GPON began mass deployment around 2008. After more than ten years of large-scale construction, the deployment of EPON has essentially ended, and the construction of new GPON networks has peaked.

With the continuous growth of user demand for bandwidth, network operators have started to upgrade existing EPON and GPON network to 10G PON systems since 2016. Using PON technologies such as XG(S)-PON and 10G-EPON network operators can provide users with gigabit broadband services using their already deployed fiber infrastructure. By the end of 2021, nearly 10 million 10G PON OLT ports have been deployed worldwide, providing gigabit network services for tens of millions of users. In 2021, the port shipments of 10G PON OLTs surpassed GPON for the first time, making 10G PON the clear mainstream choice for PON deployments today. Due to the deployment volumes in the tens of millions, the cost of 10G PON has dropped rapidly in recent years, and the economics are now better than GPON. Analysts predict that, in the next few years, there will be an ever-increasing upgrade rate of existing GPON/EPON deployments to 10G PON. All in all, 10G PON has become the logical choice for current PON deployments.

With the large-scale application of PON networks, and the rapid development of full-service offerings, operators have increasingly higher expectations for PON system capabilities. These include higher bandwidth requirements, new service support capabilities, access node equipment and supporting equipment performance. The future-oriented evolution direction for PON systems has attracted a lot of attention from the industry.

After the 10G PON standards were completed, the International Telecommunication Union (ITU-T) started to study PON technology beyond 10Gb/s in 2016. After a two-year technical feasibility study period, it was agreed in 2018 to start an ITU recommendation on Higher Speed PON operating at 50Gb/s line rate. In September 2021, the first version of the 50G-PON standard was officially published by the ITU-T, including technical specifications that support asymmetric rates (50G/12.5G, 50G/25G) and two generations of PON coexistence (i.e. coexistence of 50G with 10G PON or coexistence with GPON). In September 2022, ITU-T consented the first amendment of the 50G-PON standard, which added the technical specifications of the symmetric 50G-PON (50Gb/s in downstream and 50Gb/s in upstream), and that also supports the coexistence of three PON generations (i.e. 50G-PON, 10G-PON and GPON coexist on the same fiber network). These two new features open up new service opportunities for operators and give more flexibility to their PON network upgrades.

In the PON world, there have been two PON standards existing in the market at the same time i.e. from the IEEE and ITU bodies. The existence of two standards divides the industry chain, causes repeated investment, reduces volume scaling effects, and is not conducive to the long-term health of the PON ecosystem. Therefore, after 10G PON era, there has been a strong call for the convergence for these two standards tracks in the next-generation PON. Considering that the ITU PONs have become the mainstream choice, the world’s largest IEEE PON deployers such as China Telecom and China Unicom believe in PON convergence by a unified evolution of 10G-EPON to ITU-T 50G-PON. The ITU-T 50G-PON standard has accommodated this long term evolution of 10G-EPON and it supports the migration from 10G-EPON to ITU-T 50G-PON.

Therefore, we believe that, after 10G PON (i.e. XGS-PON and 10G-EPON), the next mass market PON generation will be ITU-T 50G-PON, and the hoped for PON convergence will be achieved in this 50G era. The primary reasons for this belief are:

50G-PON meets the natural cadence and market timing requirements of PON network evolution.

Experience has shown that network operators typically upgrade to next generation of PON every 8 to 10 years or so, and the bandwidth of each generation increases by at least 4 to 5 times. 50G-PON perfectly meets this rhythm and demand considering the start of 10G PON deployments in 2016.

50G-PON is the next generation of PON selected and standardized by ITU-T.

Standardization has been, and remains, an essential factor for the successful commercialization of any PON technology. ITU-T standards have always led the evolution path of the PON industry thanks to the strong direction given by the biggest deployers of PON. After years of research and discussion, ITU-T finally chose 50G-PON as the higher speed PON technology for beyond 10G. With a considerable joint effort from the world’s largest network operators and the leading equipment vendors, the 50G-PON standard has been developed. The publication of this standard has given a clear steer to the global PON ecosystem that the major network operators are going in the 50G-PON direction and vendors have initiated the necessary technology developments.

50G-PON is favored by mainstream operators globally

During the formulation of the ITU-T 50G-PON standard, it has received support from top operators around the world (including North America, Europe,  Asia …), especially China Telecom, China Mobile and China Unicom, which account for nearly two-thirds of the global PON market , they indicate it very clear that they will choose 50G-PON as the next generation.

50G-PON is the most cost-effective solution for the next generation

50G-PON provides the necessary bandwidth jump to meet operator capacity growth requirements that are driven by competition, new applications and user demand. The key to the cost-effectiveness of 50G-PON is that it uses digital signal processing (DSP). Through DSP, the 50G-PON system can relax the requirements for optical components and still achieve the required optical link budget. In this way, 50G-PON systems can provide these higher speeds at more economical costs – in high volume, achieving a lower cost per bit compared to lower line rate PON.

During the ITU study into the next generation of PON after 10G, other PON solutions have also been proposed and rigorously debated. Among these was a 25G PON but we do not see that a 25G PON will be the mainstream next generation PON after 10G PON. The main reasons are:

Compared with 10G PON, the bandwidth improvement of a 25G PON is too small and it is not cost effective.

Usually, operators need each generation of PON to last for 8 to 10 years in order to recover the investment. The bandwidth multiple for a 25G PON versus 10G PON is just too small to see mainstream application as the next PON generation. It simply doesn’t fit the operator bandwidth growth and upgrade cadence. Compared with GPON, a 25G PON arguably has too large a bandwidth increment and, considering 10G PON has been deployed in tens of millions of ports over recent years, the cost of 10G PON is very hard to beat and this makes a 25G PON uneconomical beyond niche applications.

There is no recognized official standard for a 25G PON.

During the research and discussion phase in ITU-T concerning beyond 10G PON, operators strongly objected to the development of a 25G PON because the bandwidth step was too small. The massive growth of 10G as the next generation PON means that, for major network operators, 25G will not meet their demand for a 4-5 times capacity growth per generation.

The time window for a 25G PON is too short before 50G-PON becomes available.

10G PON has been widely deployed and has become mainstream for new deployments. Meanwhile, 50G-PON will mature soon, which makes it difficult for a 25G PON to have a long enough deployment life. It is stuck between the highly successful, and cost reduced, 10G PON and the coming 50G-PON supported by the vast majority of major network operators.

With the publication of the 50G-PON standard behind us, the ITU-T has now started to study the next step for PON after 50G. In September 2022, ITU-T SG15 Q2 has agreed to start a project to study PON technologies for beyond 50G. This may be a 100G PON, a 200G PON or some new ideas driven by coming requirements from network operators.

With the natural PON generation cadence, we can expect the beyond 50G era to be oriented to after 2030. By then we can expect three-dimensional and ultra-wide intelligent connections will be ubiquitous, and business scenarios will be more abundant. The world of Metaverse, XR, naked-eye 3D, and digital twins is booming, and bandwidth demand will increase significantly. Individuals, families, and enterprises are expected to enter the 10-gigabit service era, and the demand for extreme low latency will become an important consideration. One network carries multiple services, and the very high-speed PON network will be not only used for home broadband access, but can also be used in enterprises, business parks, mobile xhaul, private line access, intelligent manufacturing and other vertical industries. To improve the service experience for home broadband, and to provide service guarantee for vertical industries, network slicing, network autonomous driving, green low-carbon and higher energy efficiency will all become key requirements and key features of the network. At the same time, in order to be compatible with legacy passive ODN network infrastructure, in the transmission physical layer, the NRZ direct modulation and direct detection seen in GPON, 10G-PON, 50G-PON, and also the TDMA time division multiplexing approach itself may face greater challenges beyond 50G. The business requirements and technologies for beyond 50G are still under research and exploration, and we still have time before mature commercial use.

In summary, we expect that the mainstream evolution path of PON networks is as shown in the figure below. From G/EPON to 10G PON, and the next step will be ITU-T 50G-PON. While for the PON system beyond 50G, the industry has just started to study this and there are many exciting possibilities driven by new services that are probably yet to be imagined.