A Variable Optical Attenuator (VOA) is a component designed to reduce the power level of an optical signal by a controllable amount. Within a Wavelength Selective Switch (WSS) board, an array of VOAs is integrated, with one VOA dedicated to each wavelength path. This architecture allows for the independent adjustment of optical power for each specific channel. By selectively attenuating channels with higher power, the system can equalize the power levels across all wavelengths, achieving a flat power spectrum. This process, known as flatness adjustment or channel equalization, is critical for maintaining consistent signal quality and signal-to-noise ratio (SNR) across all channels in a WDM system.

1. Official Vendor Documentation: Huawei Technologies Co.

Ltd. (2019). OptiX OSN 8800/6800/3800 V100R019C10 Product Description. Section 3.4.2

"Optical Layer Grooming Boards". The documentation for WSS boards like the WSMD9 explicitly states that the integrated VOA is used for "per-wavelength power adjustment" to achieve "automatic power equilibrium of optical channels."

2. Peer-reviewed Academic Publication: Strasser

T. A.

& Wagener

J. L. (2010). Wavelength-selective switches for ROADM applications. IEEE Journal of Selected Topics in Quantum Electronics

16(5)

1150-1157. DOI: https://doi.org/10.1109/JSTQE.2010.2049520. Section II-A

"WSS Functionality

" describes how a WSS incorporates VOAs to "provide channel-power equalization by attenuating each wavelength channel by a programmable amount."

3. Peer-reviewed Academic Publication: He

G. S.

& Cartwright

A. N. (2014). Optical WDM Networks. McGraw-Hill Education. Chapter 5

"Optical Network Components

" Section 5.7

"Variable Optical Attenuators (VOAs)." This section details that VOAs are used in WDM systems

particularly within ROADMs (which use WSSs)

for "channel power equalization" by adjusting the power of individual wavelength signals.

In optical networks, “insertion loss” is the standard term defined by ITU-T G.671 and industry textbooks for the reduction in optical power that occurs when a passive component (e.g., connector, splitter, filter) is placed in the light path. It quantifies the difference between input and output power levels, expressed in dB, and is the accepted metric for passive-component attenuation.

1. ITU-T Recommendation G.671

“Transmission characteristics of passive optical components

” §3.4 Insertion loss

pp. 4–5.

2. Senior

J. M.

Optical Fiber Communications: Principles and Practice

3rd ed.

Pearson

2009

§10.1

pp. 394–395.

3. Keiser

G.

Optical Fiber Communications

5th ed.

McGraw-Hill

2021

§6.2.2

pp. 212–213.

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).

These terms refer to Subnetwork Connection Protection (SNCP) monitoring methods defined in Optical Transport Network (OTN) and SDH standards:

• SNC/I (Inherent Monitoring): Protection switching is triggered by defects detected at the server layer (the physical or lower transport layer), such as Loss of Signal (LOS) or Alarm Indication Signal (AIS). It uses information "inherent" to the server signal rather than checking the specific connection's data integrity.
• SNC/N (Non-intrusive Monitoring): Uses a non-intrusive monitor to "listen" to the signal's Path Overhead (POH) without terminating the connection. This allows the detection of Signal Degrade (SD) or bit errors on the specific path.
• SNC/S (Sub-layer Monitoring): Monitors a specific sub-layer or segment of the path, typically using Tandem Connection Monitoring (TCM) overhead. This is used to protect a specific operator's domain or segment within a longer end-to-end path.

ITU-T Recommendation G.873.1 (10/2017), Optical Transport Network (OTN): Linear protection.

See Section 3 (Definitions), specifically definitions for SNC/I (Subnetwork Connection with Inherent monitoring), SNC/N (Subnetwork Connection with Non-intrusive monitoring), and SNC/S (Subnetwork Connection with Sub-layer monitoring).

ITU-T Recommendation G.841 (10/1998), Types and characteristics of SDH network protection architectures.

See Section 7.2 (SNC protection with inherent monitoring), Section 7.3 (SNC protection with non-intrusive monitoring), and Section 7.4 (SNC protection with sublayer monitoring).

ITU-T Recommendation G.805 (03/2000), Generic functional architecture of transport networks.

Provides the foundational architectural definitions for Subnetwork Connections (SNC) and monitoring types.

The question asks to identify statements that are not features of WDM network expansion. WDM technology is designed for rapid and efficient capacity upgrades.

Key advantages (features) of WDM expansion include adding new channels without disrupting existing traffic (B) and leveraging the existing fiber plant to increase capacity, thereby protecting the initial investment (C).

Conversely, adding more wavelengths, nodes, and equipment inherently increases the operational and management complexity of the network (A), which is a consequence or drawback, not a desirable feature. Furthermore, a primary benefit of WDM is the speed of expansion compared to laying new fiber; therefore, the statement that it "takes a long time" (D) is factually incorrect.

B. No impact on existing services during the expansion: This is a core advantage of WDM. New wavelengths are independent and can be added without affecting traffic on existing channels.

C. Maximized protection of existing investment: WDM increases the capacity of existing fiber optic cables, avoiding the high cost of installing new ones, thus protecting prior investments.

1. Huawei Certified ICT Associate - Transmission V2.0 Training Material (H31-341). Chapter 2

"WDM System Basics." This material explicitly lists the advantages of WDM technology

including "Smooth Capacity Expansion" and "Protection of Original Investment." It highlights that expansion can be achieved by adding wavelengths to existing fiber

which is a fast process that does not affect online services. This confirms B and C are features and D is false.

2. Huawei OptiX OSN 1800 V100R007C00 Product Description. Section 1.2

"Benefits." The document states that the platform supports "smooth network expansion" and "protects initial investments

" allowing for flexible

on-demand service addition. This contradicts option D and supports option C.

3. Ramaswami

R.

& Sivarajan

K. N. (2002). Optical Networks: A Practical Perspective. Morgan Kaufmann. Chapter 1

Section 1.3

"The Optical Layer

" pp. 12-14. The text describes WDM as a means for "graceful growth in capacity" on existing fiber infrastructure. It also introduces the challenges of managing a multi-wavelength network

noting the increased complexity in fault management and performance monitoring

which aligns with option A being a consequence rather than a feature.