This diagram illustrates the standard OTN electrical-layer grooming process:

1. Client-side device (1): The signal flow begins with external client equipment (e.g., a router or switch) transmitting a client signal (labeled 100GE) to the Tributary Board (T401).
2. Cross-connect board (2): The Tributary Board maps the client signal into an ODUk container. This electrical signal is then sent to the Cross-connect board, which switches the ODU4/ODUflex signal to the appropriate Line Board. This is the central "hub" shown as the vertical bar between T401 and N401.
3. Multiplexer/demultiplexer board (3): The Line Board (N401) converts the electrical ODUk signal into a colored optical signal (OTU4). This optical signal is then transmitted to the Multiplexer/demultiplexer board (optical layer), which combines multiple wavelengths onto a single fiber for long-haul transmission.

Huawei Technologies Co., Ltd., OptiX OSN 8800 Hardware Description, "Signal Flow and Board Functions," V100R011. (Describes the T-board as tributary and N-board as line units connected via the cross-connect unit).

ITU-T Recommendation G.798 (12/2012), Characteristics of optical transport network hierarchy equipment functional blocks, Section 3. (Defines the functional architecture of OTN, including the roles of adaptation (tributary), connection functions (cross-connect), and termination (line/mux)).

Thunder-link.com, Huawei TNV5T401Q01 Datasheet, Product Description. (Confirms the T401 is a "100G Tributary Service Processing Board" that supports backplane cross-connections).

Huawei Carrier Support, WDM/OTN Signal Flow Analysis, HCIP-Transmission Courseware. (Standard teaching material illustrating the Client → Tributary → Cross-Connect → Line → Mux flow).