Comprehensive and Detailed Explanation From Nokia Optical Networking Fundamentals:

In the context of WDM (Wavelength Division Multiplexing) node architecture, anexpress channel interface(often associated with OADMs or ROADMs) is specifically designed to handle "through" traffic. In a multi-node optical network, not every wavelength (channel) needs to be processed or terminated at every site it passes. To maintain signal integrity and reduce latency, these wavelengths are kept in the optical domain.

The express interface allows these optical channels—thosenot terminatedor "dropped" at the local node—to bypass the local transponders and multiplexers, flowing directly to thedownstream node. This photonic bypass avoids unnecessary O-E-O (Optical-Electrical-Optical) conversions, which would otherwise require expensive hardware and increase power consumption. By utilizing express paths, the Nokia 1830 PSS can scale to support massive core network capacities while ensuring that only the relevant traffic is diverted to the local client-facing ports.