WDM Principle
Wavelength Division Multiplexing: Multiplexing optical signals of different wavelengths into the same optical fiber for transmission, this method is called wavelength division multiplexing (WDM). Features: Multi-wavelength multiplexing/high-speed long-distance transmission/optical layer monitoring and management.

Coarse wavelength division multiplexing (CWDM): The wavelength interval is 20nm. With the development of technology, more and more optical signals can be multiplexed, and dense wavelength division multiplexing technology has emerged.

Dense wavelength-division multiplexing (DWDM): Wavelength interval is less than 0.8nm.

OTN - Optical Transport Network
Optical Transport Network (OTN) is defined by recommendation G.709, provides a network-wide framework that adds SONET/SDH-like features to WDM equipment (also known as Wavelength Switched Optical Network equipment, or WSON equipment). It creates a transparent, hierarchical network designed for use on both WDM/WSON devices and TDM devices. It provides an efficient way to transport, switch, and multiplex different services onto high-capacity wavelengths across the optical network. 

Features
- WDM technology is adopted on the line
- Multiple client signal encapsulation and transparent transmission
- Large-grained bandwidth multiplexing, crossover, and configuration
- Strong overhead and maintenance management capabilities
- Enhanced networking and protection capabilities

Differences and Relevance Between WDM and OTN

OTN has electrical crossover units. N-channel wavelength multiplexing system includes the following functional units:
- Optical Transform Unit (OTU)
- Crosspoint Control Unit (XCU)
- Optical Multiplexing and Optical Demultiplexing (OM/OD)
- Optical Amplifier (OA)
- Optical Supervisory/Channel Electrical Supervisory Channel(OSC/ESC)

DWDM is equivalent to expressway (large capacity/long distance/high speed), OTN is equivalent to overpass expressway, and intelligent control plane is equivalent to traffic light and traffic management system.

WDMs Applications
- Metropolitan Area Transmission Network
- Secondary Backbone Network
- Trunk Line Network

Networking Element Type of WDM products
- Optical Terminal Equipment (OTM)
- Optical Line Amplification Equipment(OLA)
- Optical Add Drop Multiplexing(OADM)
- Optical Terminal Equipment(OTM)

Common Test Networking Methods
- Back to Back Network
- Point to Point Network
- Chain Network
- Star Network
- Multipoint Ring
- Multipoint MESH Network
- Ring Network Tangential Network

Network Layer
- Customer Information Layer
- Optical Channel Payload Unit
- Optical Channel Data Unit
- Optical Channel Transmission Unit
- Optical Channel Layer
- OMS Layer
- Optical Transmission Segment Layer

Typical Boards
- Business Boards (Business Processing and Cross Board)
- Optical Line(Amplification, Mux/Demux, Protection, Light Regulation, POADM)
- Monitor Module
- Electromechanical Management (Power Supply and Fan)