With the rapid development of technologies such as 5G, cloud computing, and AI, optical access networks are accelerating their evolution toward higher bandwidth and lower latency. 50G PON, as a next-generation PON technology, has become a focus of industry attention. Currently, operators still have a large number of EPON and 10G EPON in their existing networks. Maximizing investment protection while upgrading networks and achieving the coexistence of EPON, 10G EPON, and 50G PON has become a major challenge in this technological evolution.

1. Wavelength Conflict: The Core Challenge of EPON Evolution
The EPON upstream wavelength range is as wide as 100nm (1260nm to 1360nm). However, the ITU-T-defined 50G PON upstream and downstream wavelengths are 1286nm and 1342nm, respectively, completely overlapping with the EPON upstream band (as shown in Figure 1).

This wavelength conflict makes the coexistence of three generations of PON extremely challenging. From standardization to industry implementation, the industry has proposed three main solutions:

1. EPON network retirement, coexistence of 10G EPON and 50G PON (Gen 2)

As early as the early stages of 50G PON standardization in 2018, the industry recognized that broadband EPON was a barrier to future PON network evolution. With the gradual retirement of EPON, Gen 2 coexistence of 10G EPON and 50G PON has become the preferred solution.

Advantages:
- Standardization: The 50G PON standard is compatible with 10G EPON, and the industry chain is mature.
- Network simplification: Avoids the complexity associated with coexistence of three generations and reduces operations and maintenance costs.
- Performance improvement: After EPON is retired, network bandwidth and latency performance can be significantly optimized to meet the demands of the AI era.

Currently, operators such as China Mobile have begun promoting the retirement of EPON, gradually upgrading network capabilities to align with GPON in the region and building a more competitive access network.

2. EPON Converges to Narrowband, Enabling Three-Gen Coexistence

After the commercial launch of 10G EPON in 2017, as DFB laser costs decreased, operators adopted enterprise standards to narrow the EPON upstream wavelength to 1290nm-1330nm and the asymmetric 1G upstream wavelength of 10G EPON to 1260nm-1280nm. This adjustment laid the foundation for the coexistence of 50G PON.

In 2022, 50G PON newly defined a 1286nm upstream wavelength. With the deployment of narrowband EPON, the coexistence of EPON, 10G EPON, and 50G PON will be possible.

Advantages:
- Smooth evolution: Retains existing EPON users and avoids forced network closures.
- Standard convergence: ITU and IEEE PON wavelengths converge, resulting in a more efficient industry chain.
- However, the widespread adoption of narrowband EPON will take time, and the retirement of broadband EPON equipment requires continued investment from operators.

3. Independently Defined 50G EPON Wavelengths and Uplink Time Division Multiplexing
Recently, the industry has proposed independently defining 50G EPON wavelengths (e.g., 136x nm) and using time division multiplexing to achieve coexistence of three generations (as shown in Figure 2).

However, this solution faces multiple technical bottlenecks:
- Fiber attenuation: 136x nm is at the edge of the water peak, resulting in high and unstable attenuation.
- Dispersion limitation: High-frequency dispersion causes signal degradation, making it difficult to support 50G NRZ transmission.
- Efficiency and latency: Three generations of PON share upstream timeslots, and the long preamble (microseconds) of EPON/10G EPON significantly reduces the bandwidth efficiency of 50G PON and increases latency.
- Industry chain costs: 136x nm optical devices cannot be reused with the GPON industry chain, resulting in high costs.

Disadvantages:
- Low technical feasibility and significant performance sacrifice.
- Continuing the standardization split between IEEE PON and ITU PON prevents industry chain convergence.
- Reduced network competitiveness makes it difficult to meet future service needs.

II. Historical Experience: Lessons from Time-Division Multiplexing
In the era of coexistence between 10G EPON and EPON, uplink time-division multiplexing (TDM) has exposed significant shortcomings: high latency and low bandwidth utilization, putting IEEE PON at a disadvantage in competition with ITU PON. If TDM continues to be used in the 50G era, these problems will be further amplified, potentially rendering the network unable to support emerging services such as 4K/8K, XR, and AI.

III. Conclusion: EPON Retirement is the Optimal Evolution Path
From a long-term industry perspective, EPON retirement and the coexistence of 10G EPON and 50G PON (second-generation) are the optimal options. This path can:
- Mitigate technical risks: Avoid wavelength conflicts and the performance drawbacks of time-division multiplexing.
- Enhance network capabilities: Unify PON standards, simplify operations and maintenance, and reduce total cost of ownership.
- Enhance competitiveness: Pave the way for the future evolution of 50G PON to next-generation PON technology.

Conversely, choosing a 50G EPON time-division coexistence solution will lead to standards fragmentation, industry chain redundancy, and decreased network performance, ultimately rendering the deployed network less technologically competitive.

IV. Industry Call: Collaborate to Promote EPON Retirement
We recommend that operators, equipment vendors, and standards organizations collaborate to accelerate EPON retirement:
- Develop a migration plan: Promote user migration to 10G EPON through policy incentives or technology replacement.
- Converge technical standards: Promote further collaboration between the IEEE and ITU on wavelength planning.
- Optimize the industry chain: Focus on the development of optical modules and chips that enable the coexistence of 10G EPON and 50G PON to reduce costs.

In the era of AI and computing power, access network bandwidth and latency capabilities will become the core competitive advantage of critical infrastructure. Choosing the right evolution path will help operators succeed in the next decade.