The three main types or layers of Optical Transport Networks (OTNs) are the ODU layer, the OCh layer, and the WDM layer, each serving distinct roles in signal transport, multiplexing, and wavelength management.

1. ODU Layer (Optical Data Unit)

The ODU layer is responsible for encapsulating client signals such as Ethernet, SDH/SONET, MPLS/IP, or Fiber Channel into standardized OTN frames. This layer provides end-to-end transport, multiplexing, and monitoring, ensuring that multiple client signals can share the same optical channel while maintaining integrity and error detection. ODU layers also support flexible sub-rate clients through ODUflex, allowing efficient bandwidth utilization for varying traffic demands .

2. OCh Layer (Optical Channel)

The OCh layer manages the optical channels themselves, including wavelength assignment and routing across the network. It handles transport and switching of optical signals using devices like ROADMs (Reconfigurable Optical Add-Drop Multiplexers) and optical wavelength switches. This layer ensures that signals traverse the optical network efficiently, maintaining low latency and high reliability while supporting survivability mechanisms such as protection and restoration .

3. WDM Layer (Wavelength Division Multiplexing)

The WDM layer operates at the physical media level, combining multiple optical channels onto a single fiber using different wavelengths. This layer enables high-capacity transmission, allowing OTNs to scale from 2.5G up to 400G per channel and beyond. WDM also supports multiplexing of multiple OCh signals, providing the backbone for long-haul and metro-scale optical networks .

Summary

Together, these three layers—ODU, OCh, and WDM—form a hierarchical OTN architecture that allows for transparent transport of diverse client signals, efficient bandwidth utilization, and robust network management. OTNs also include management and adaptation functions, fault detection, and performance monitoring to ensure reliable operation across optical networks . These layers are standardized under ITU-T G.709, ensuring interoperability and scalability for modern high-speed networks, including Ethernet, storage, and legacy telecom traffic .