Posted on Jul 12, 2023

Brief Introduction of OTN and SONET/SDH

OTN Explained

OTN(Optical Transport Network) is an optical transport network technology based on TDM(Time Division Multiplexing) and WDM(Wavelength Division Multiplexing) technology. By applying TDM and WDM to multiple optical signals, it can achieve optical transmission of high speed and large capacity. OTN technology has advantages in transmission capacity, transmission distance and reliability, so it is widely used in the construction and maintenance of optical transmission networks.

SONET/SDH Overview

SDH(Synchronous Digital Hierarchy) is commonly used in digital communication to packaging multiple digital signals (such as digital voice, digital data, and digital video signals) together for transmission over different transmission media (such as optical fiber, copper wire, etc.) through multiplexing technology. It is widely used in optical fiber communication, copper wire communication and wireless communication, and is a significant technology in digital communication.

SONET(Synchronous Optical Network) is a further development of SDH, which utilizes optical fiber transmission media to multiplex and demultiplex multiple digital signals in order to transmit signals over optical fiber networks. Unlike SDH technology, SONET uses optical signals for signal multiplexing and demultiplexing, rather than electrical signals. The prominent advantage of SONET technology is its higher transmission speed and larger transmission capacity. It is widely used in fiber optic communication networks, especially in the Internet backbone in North America, Europe and Asia.


OTN and SONET/SDH are both network standards for optical transmission, but they differ in terms of transmission rate, topology, frame structure, error handling and network management. When choosing which standard to use, it should be considered according to the specific needs and actual situation.

Stratification Structure
Two-tier architecture
Three-tier architecture
Frame Structure
A fixed number of bytes and a variable frame period
A variable number of bytes and a constant frame period
Signal Bit Rate
Even times the base rate of 51.84 Mb/s
Bit Error Detection
BIP-8 monitoring function
BIP-8 monitoring function
Transparent transmission without loss of data, overhead, and timing
Transparent transmission only when terminating the clock and overhead
Forward Error Correction (FEC)
Standard or even higher-performance FEC solutions
Non-standard configuration and limited deployment


SONET/SDH networks utilize Virtual Concatenate (VCAT) technology to enable containers of all capacities to map a variety of customer signals, including gigabit Ethernet signals, fiber channel signals and video signals. Although VCAT also defined OTN, there was no field application, so the flexible Optical Channel Data Unit(ODUflex) subsequently replaced it to build OTN containers more simply, becoming the first choice for many customers. The main difference between VCAT and ODUflex is that ODUflex is a single container of variable size, rather than a set of containers. As a result, ODUflex is easier to manage (just one load, one container, and one set of overhead) and does not require a mechanism to solve differential latency. Of course, a single container does not support the multipath routing and protection capabilities provided by VCAT.

Mapping Adjustment

The Synchronous Payload Envelope(SPE) start point is floating in the STS frame of fixed structure, so SONET/SDH uses the pointer in the signal frame at the fixed overhead position to locate the start point of the SPE. OTN then reverts to the positive/negative filling mechanism, similar to the approach adopted with PDH multiplexing. Both methods adjust the small clock rate difference between the client signal and the server layer signal by timing a byte to be filled into the SONET/SDH/PDH frame or by marking a net load byte as unused. The Asynchronous Mapping Procedure (AMP) requires that the net load bandwidth of the server signal be equivalent to that of the mapped client signal. The latest General Mapping Procedure (GMP) for OTN has been developed as an alternative mapping/adjustment method. This method inserts an unlimited number of filling bytes into any signal frame, thus mapping any smaller bandwidth client signal to a given server layer load.

Layered Multiplexing

The multiplexing of PDH signals follows a multilayer structure through which DS0 is multiplexed to DS1, DS1 to DS2, and DS2 to DS3. To prevent network units from having to undo multiple levels of multiplexing before converting or rearranging lower bandwidth signals, SONET/SDH does not use this layered multiplexing approach. OTN started with the same idea, advocating single-stage multiplexing (for example, multiplexing ODU1 signals directly to ODU3, rather than first multiplexing ODU1 to ODU2). However, the latest OTN standard has specified multilevel multiplexing for all OTN container capacities, so OTN currently supports both multilevel multiplexings like PDH and direct multiplexing like SONET/SDH.

Layered Multiplexing


OTN uses many elements of network technologies such as PDH and SONET/SDH, but it is still a significant advance in transport technology. Service providers around the world are already deploying OTN as the first choice of transport technology. Vendors are also getting in on the act, developing new devices to bring OTN into service provider networks on a large scale. While there are still a lot of products shipped with old technology (even T1/E1 products, for example), it is difficult to conclude that it is no longer viable, but the future of the transport network from SONET/SDH to OTN is clear.

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