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Differences between SDH, WDM and OTN Technology


Global communications are advancing by leaps and bounds, the role of communication networks is becoming more and more important, and the construction of communication networks needs to be continuously expanded. At present, SDH, WDM and OTN technologies are widely used in network communication construction. But what are the differences between SDH, WDM and OTN? Today we’ll talk about it.

SDH is an early optical transmission technology (but developed after PDH). It is an integrated information transmission network that integrates multiplexing, line transmission, and switching functions and is operated by a unified network management system.

Transmission rates include 155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s, 10 Gbit/s, and 40 Gbit/s.


The rate is the same as that of the optical interface.

Strong management capability (OAM management information).

The bifurcation multiplexing circuit is convenient.

Standardized frame structure, multiplexing mode, transmission rate level, and interface code type of digital signals.

Supports protection switching of multiple services.

Disadvantages: rigid pipes, exclusive resources, and fixed rate.

More about SDH.

WDM multiplexes optical signals of different wavelengths into one fiber for transmission. There are two modes: DWDM and CWDM. Currently, DWDM is the mainstream. Currently, the mainstream WDM systems are 40-wavelength and 80-wavelength systems (which are being converted to more than 96-wavelength and 120-wavelength systems) that carry more wavelengths. This is one of the future WDM evolution directions. The transmission rates of a single optical wave signal are 10 Gbps, 40 Gbps, 100 Gbps, and 200 Gbps.


One line has multiple channels, improving the capacity. 800 Gbit/s (80 wavelengths x 10 Gbit/s)

long distance

The problem of insufficient fiber resources is solved.


Lack of OAM, inflexible scheduling, and inadequate protection, leading to the emergence of OTN.

High latency and high cost.

Strong tributary-line binding, low bandwidth efficiency

More about WDM.

Based on WDM, OTN integrates some advantages of SDH, such as abundant OAM overheads, flexible service grooming, and perfect protection modes.

OTN service grooming is classified into optical-layer grooming and electrical-layer grooming.

Optical-layer grooming can be considered as WDM.

Electrical-layer grooming can be understood as SDH.


OTN = SDH + WDM, compatible with Ethernet: EoO technology is used to transmit Ethernet packets to ODUk.

Disadvantage: The ODUk granularity is too large, which cannot meet the requirements of a large number of low-speed services.

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