The coexistence of different Passive Optical Network (PON) generations, such as GPON, 10G PON (XG-PON/XGS-PON), and 50G PON, on the same Optical Distribution Network (ODN) is enabled by Wavelength Division Multiplexing (WDM). Each PON technology is allocated a distinct and non-overlapping wavelength band for its upstream and downstream transmissions. This ensures that signals from different generations can travel simultaneously over the same single fiber without interfering with each other, allowing for seamless network upgrades and service overlay.

A. Different optical power: While different PON standards may have different power budgets, this is a characteristic, not the fundamental principle enabling coexistence on a single fiber.

B. Different running timeslots: Time slots are used for Time Division Multiple Access (TDMA) within a single PON system to share upstream bandwidth, not for coexistence between different PON systems.

D. Different bandwidths: Different bandwidths are a feature and outcome of the different PON technologies, not the physical layer mechanism that allows them to coexist on the same fiber.

- ITU-T Recommendation G.9804.1, "Higher speed passive optical networks - Requirements," Section 6.2 "Coexistence." This standard explicitly states that coexistence with legacy PON systems shall be achieved by operating in different wavelength bands.

- Huawei, "From 10G PON to 50G PON White Paper," Section 3.2 "Wavelength Plan." This document illustrates the specific wavelength allocations for GPON, XG(S)-PON, and 50G PON, showing how WDM enables their coexistence.