WDM VS OTN–What are the Differences?

What is Wavelength Division Multiplexing (WDM)?

Wavelength Division Multiplexing(WDM) refers to multiplexing optical signals of different wavelengths into the same fiber for transmission. In the SDH system, each fiber can only transmit one wavelength; while in the WDM system, each fiber can transmit multiple wavelengths in parallel(The rate of each wavelength is 2.5G/10G).

WDM System

WDM System

Compare a WDM system with a highway and a vehicle: in the SDH system, only one vehicle can travel on the highway. In the WDM system, multiple vehicles can travel on the highway.

The analogy between a WDM system and a highway

The analogy between a WDM system and a highway

The icons corresponding to the components of the WDM system are as follows:

Highway: Fiber Optic

Patrol Car: Monitoring Signals

Gas station: Optical relay (amplification) station

Grey Cars: Different Client Side Services

Colored cars: bearer services in different channels (wavelengths)

Lane: Light Wavelength

Characteristics of traditional WDM:

  • Significance of traditional WDM: (solving the problems of capacity and distance)

Solve the problem of insufficient SDH network capacity. The maximum bandwidth of metro WDM can support 80 × 10G, while the maximum bandwidth of the SDH network is 10G.

  • Limitations of traditional WDM: (inadequate scheduling, protection, and management functions)

Similar to the PDH system, the WDM system can only form point-to-point chains. The wavelength cannot be flexibly scheduled, and complex networks cannot be formed. Therefore, it is not suitable for the development of IP networks.

It does not support ASON intelligent features and cannot evolve into an intelligent optical network, which is not the development direction of the next-generation transport network.

What is OTN?

Traditional WDM has limitations in carrying broadband services. The problems include inflexible service scheduling, poor networking capability, imperfect protection mechanism, simple OSC, and inability to accurately manage channels…… SDH has powerful and flexible cross-connection scheduling capabilities, a variety of perfect protection mechanisms, standardized mapping, and multiplexing, multi-level embedded overhead, rich operational and manageable experience, etc. These advantages can make up for the deficiencies of WDM.

The limitations of traditional WDM in protection, management, and scheduling make it unable to adapt to the transmission requirements of large-particle broadband services. The transmission capability of WDM is combined with the electrical layer processing mechanism of SDH and the OTN (Optical Transport Network) technology comes into being.

The transmission network needs to have new features to adapt to future service requirements:


Large particles and large bandwidthThe capacity of IP core routers has reached the Tb/s level. Various high-speed interfaces such as GE/10GE, 2.5G/10G/40G POS, etc. have been widely used.


High network survivability and reliability
When the transmission network equipment fails, the network needs to realize the protection and recovery of services
High network efficiency and fast service provision
The optical transport network can adapt to the dynamic and fast requirements of services, and quickly provide the transport resources required for IP service bearer.
Low TCO (total cost)
The optical transport network should simplify the network structure and integrate the functions of the equipment, so as to make the transmission more efficient, the operation and maintenance more convenient, and the cost lower.
Flexible network expansion
The optical transport network should be able to adapt to the needs of the rapid development of IP services, and the network can be expanded quickly and easily.
operational network bandwidth
The optical transport network should be able to easily manage network resources and facilitate bandwidth operation.

Features of the OTN :

  1. The large bandwidth transmission capability of WDM
  2. The flexible networking capability of SDH
  3. ASON intelligent features can be loaded and upgraded to intelligent optical network

Development of Optical Network Technology:

Development of Optical Network Technology

Development of Optical Network Technology

  • PDH and SDH are both TDM technologies, and proper overhead processing makes the point-to-point chain into an end-to-end network;
  • The current WDM is still a point-to-point chain. To adapt to the IP-based network in the future, it must develop into an end-to-end network, and add appropriate overhead processing, which illustrates the development of OTN;
  • ASON (GMPLS) is the control layer. It cooperates with the channel-layer SDH, OTN, and AON successively during its development to control the cross-scheduling of VC4, ODUk, and wavelength in a unified way.

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