Comprehensive Guide of Transport Network

Posted on Jul 11, 2023 by

The transport network is an important part of the communication network. Without it, data communication is not possible between different devices on the network.

Definition of the Transport Network

The transport network is the infrastructure that provides the transmission means of business information for various service networks. It is the pipeline connecting communication equipment and terminals. The transport network is a basic network in the entire communication network, which plays the role of transmitting the signals of various service networks.

Transport Network

The transmission medium of the transport network is mainly optical fiber, which can be simply understood as: Transport Network = Transmission Equipment + Transmission Medium (optical fiber).

Key Features of the Transport Network

  • Large Capacity: The bandwidth of optical fiber is almost infinite relative to the equipment, the transport network capacity is huge, and can carry a large number of service signals. At present, it has the transmission capacity of absorbing 48T single fiber.

  • Long Distance: The transmission medium of the transport network is optical fiber with low transmission loss and can be used for long-distance transmission of service signals. Coherent light and other technologies can also be used to achieve electroless relay transmission for thousands of kilometers.

  • Multi-Service Access: The transmission network supports multiple service interfaces, including PDH, SDH, ETH, PCM, and video services. It also supports a variety of network-level and equipment-level protection. Compared with the protection of the service network itself, the protection switching time of the transport network is short and the damage to the service is small.

Technology Development of the Transport Network

Technology Development of the Transport Network


Plesiochronous Digital Hierarchy(PDH) is a digital transmission technology used to transmit Time Division Multiplexing(TDM) signals in digital communication systems. In some old communication systems, PDH technology is still the main transmission means.

However, with the continuous development of digital communication technology, PDH technology has been gradually replaced by more advanced transmission technology. For example, Synchronous Digital Hierarchy(SDH) and High-Speed Digital Subscriber Line(HDSL). These new technologies have higher transmission rates, longer transmission distances, higher reliability and stability, so they are more favored by users.


Synchronous Digital Hierarchy(SDH) is a hierarchy of standardized digital signals that can be transmitted, multiplexed and cross-connected synchronously.

No international optical interface standard
International optical interface STM-N for different devices
Asynchronous multiplexing
Synchronous interbyte multiplexing
Expense on OAM
Little expense on OAM
Much expense and pointers on transmission monitor
Convenient management and flexible networking



A traditional SDH device provides only E1, E3, and E4 ports. To carry Ethernet services, you need to configure a protocol converter. Assuming a 10M Ethernet service, five protocol converters are needed on each side of the SDH device to complete the access. MSTP, Multi-Service Transmission Platform, based on traditional SDH devices, integrates the process of protocol conversion, signal adaptation and encapsulation into the service board of SDH devices to facilitate service access. At the same time, MSTP uses the protection and recovery function, OAM and other functions of SDH, and supports multi-service transmission and access such as PDH, SDH/ETH/ATM/PCM. Simple understanding: MSTP= traditional SDH+ service board. Both MSTP and traditional SDH devices are based on the TDM plane. When a packet plane is added, it becomes a Hybrid MSTP device.


Packet Transport Network(PTN) refers to an optical transport network architecture and specific technology: a layer is set between the IP service and the underlying optical transmission medium, which is aimed at the burstiness and statistical recovery of packet traffic. Designed with the requirements of delivery, with packet services as the core and supporting multi-service provisioning, with lower Total Cost of Ownership(TCO), while adhering to the traditional advantages of optical transmission, including high availability and reliability, efficient bandwidth management mechanisms and traffic engineering, convenient OAM and network management, scalability, high security, etc.


Wavelength Division Multiplexing(WDM) is a way of multiplexing optical signals of different wavelengths into an optical fiber for transmission. The sending end combines the wave, while the receiving end divides the wave.


The overall structure of the WDM system of the N-channel wavelength is: Optical Transform Unit(OTU), Optical Demultiplex Unit/Optical Multiplex Unit(ODU/OMU), Optical Amplifier(OA), Optical/Electrical Supervisory Channel(OSC/ESC).

WDM Structure

  • Optical Transform Unit: The non-standard wavelength is converted to the standard wavelength regulated by ITU-T to meet the requirements of OMU/ODU multiplexing.

  • Wavelength Division Multiplexing: Multiple input wavelengths are multiplexed to an interface output, or demultiplexes a multiplexed input signal to multiple wavelength signal output.

  • Optical Amplification: The signal is amplified to extend the transmission distance.

  • Optical/Electrical Supervisory Channel: To realize the monitoring and management of the system, ITU-T recommends that the wavelength of 1510nm be preferred and the capacity is 2M.


Optical Transport Network is a network of optical network elements connected together through optical fiber links, which can provide the transmission, multiplexing, routing, management, monitoring and protection of customer signals based on optical channels. An obvious feature of OTN is that the transmission settings for any digital customer signal are irrelevant to the customer-specific characteristics, i.e., customer independence.

Hierarchy and Interface of OTN

Hierarchy and Interface of OTN

Advantages of OTN over SDH and WDM

  • Higher bandwidth utilization: OTN uses a virtual channel to encapsulate multiple physical channels into a virtual channel, thereby improving bandwidth utilization and being more economical than SDH and WDM.

  • More flexible network management: OTN uses a centralized control plane to manage and control the network through a Digital Signal Processor(DSP), which is more flexible than SDH and WDM, and can respond faster to network failures and adjustments.

  • Higher reliability: OTN uses technologies such as optical amplifiers and regenerators, which can effectively improve the reliability of the network and is more reliable than SDH and WDM.

  • More flexible network expanding and upgrading: OTN can adopt a distributed network topology that makes network expanding and upgrading easier and can adapt to network development and changes more quickly.

  • Better business support capability: OTN supports different types of business, including digital signal transmission, digital image transmission, voice transmission, etc., which can better meet the needs of different businesses.

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