Comprehensive Guide of Transport Network

Transport networks, which encompass transmission network telecom infrastructure, form the backbone of modern communication by enabling seamless data exchange across vast distances using optical fiber. Their crucial role lies in ensuring reliable signal transmission, made possible by cutting-edge technologies such as coherent light. These networks, which support a variety of interfaces and robust protection mechanisms, redefine modern communication by optimizing bandwidth usage and offering flexible management options. They serve as the essential link connecting different devices and systems, without which the intricate web of modern connectivity would grind to a halt. In this comprehensive guide, we explore the fundamental concepts, defining characteristics, and pivotal role of network transport technologies in the realm of telecommunications.

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.

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).

Figure 1 : Transmission Network Infrastructure Diagram

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.

• Interoperability: Transport networks need to support interoperability between different types of equipment and network technologies to enable seamless communication and service delivery. Standards-based protocols and interfaces facilitate interoperability and compatibility between diverse network elements.

Technology Development of the Transport Network

The progression of transport network technology encompasses a range of transmission methodologies, from the conventional PDH to the state-of-the-art OTN. Each of these technologies represents a continuous advancement in network efficiency, reliability, and adaptability. As we move forward, we'll thoroughly explore these technologies in detail.

Figure 2 : Evolution of Transport Networks Diagram

PDH

PDH, or Plesiochronous Digital Hierarchy, is a telecommunications network transmission technology designed to efficiently handle the transfer of large data volumes across extensive digital networks. Unlike synchronous systems, where clocks are perfectly synchronized, PDH operates with clocks that run very close but not exactly in time with one another. This slight variation allows for the multiplexing of signals, although arrival times may differ due to the direct link between transmission rates and clock rates.

The following are technical and historical characteristics of PDH:

• One notable feature of PDH is its ability to compensate for timing differences by employing bit stuffing for each stream of a multiplexed signal, ensuring that the original data stream can be precisely reconstructed as it was sent.

• However, despite its historical significance, PDH has become obsolete in recent years and has been replaced by more advanced technologies such as synchronous optical networking and synchronous digital hierarchy schemes, which offer significantly higher transmission rates.

SDH

With the emergence of intelligent network elements supported by microprocessors, the integration of high-speed, high-capacity optical fiber transmission technology with intelligent networking technology has led to the birth of SDH synchronous optical transmission networks. SDH, which stands for Synchronous Digital Hierarchy, standardizes the frame structure, multiplexing methods, transmission rate levels, and interface coding of digital signals. It not only improves upon the shortcomings of PDH that hinder large-capacity transmission but is also widely adopted in optical fiber networks, providing a standardized protocol for the transmission of voice, data, and video traffic. As a high-speed synchronous networking technology, SDH ensures smooth data transmission, preventing timing errors or data loss. Its "synchronous" feature refers to the synchronization of network clocks, while "digital hierarchy" denotes a structured approach to effectively multiplex signals of various data rates onto optical fibers.

The following are the key features of SDH:

• The operational principle of SDH involves multiplexing multiple digital signals into a single optical signal, transmitted through fiber optic cables synchronously, aligning all signals with a common clock.

• Seamless data handling from diverse sources and bit rates, facilitated by the SDH frame structure accommodating different data rates simultaneously.

Figure 3：PDH VS SDH Diagram

MSTP

With the rise in SDH transmission's popularity and the increasing reliance on data services in telecommunications networks, the demand for carrying various access services on SDH has grown, leading to the gradual evolution of MSTP technology. MSTP, or Multi-Service Transmission Platform, addresses the limitations of traditional SDH devices, which typically offer only E1, E3, and E4 ports. To accommodate Ethernet services, a protocol converter is required, with five converters needed on each side of the SDH device to handle a 10M Ethernet service. MSTP simplifies this process by integrating protocol conversion, signal adaptation, and encapsulation into the service board of SDH devices, streamlining service access. Furthermore, MSTP leverages SDH's protection and recovery functions, as well as OAM capabilities, supporting multi-service transmission and access including 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.

Figure 4：MSTP Architecture Diagram

PTN

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.

The following are the primary performance characteristics of PTN:

• Supports High-Volume Service Forwarding and Large Interfaces: PTN systems facilitate efficient handling of extensive service traffic and accommodate high-capacity interfaces, including 40 GE, 50 GE, 100 GE, and 400 GE.

• Robust Routing and VPN Capabilities: They offer robust routing and VPN functionalities, enabling seamless management of diverse interfaces with flexibility.

• Redundancy Design for Enhanced Reliability: PTN architectures feature redundancy designs for critical components like control boards, switching boards, power modules, and fans. This ensures both device-level and network-level protection, enhancing system reliability.

• Compatibility with Legacy Services: PTN solutions seamlessly integrate with traditional services such as PSTN lines, private leased lines, and clock signal paths, ensuring compatibility with existing infrastructure.

• Adaptability to Emerging Technologies: They are designed to evolve alongside emerging technologies, such as SDN and low-cost L3 switch clustering, ensuring future-proof network deployments.

WDM

Wavelength Division Multiplexing (WDM) is an innovative technology designed to enhance bandwidth capacity by enabling the simultaneous transmission of multiple data streams at varying frequencies across a single optical fiber network. By leveraging this approach, WDM optimizes fiber utilization, thereby maximizing the efficiency of network investments. If you want to delve deeper into WDM technology, you can check out this article: CWDM vs DWDM: What’s the Difference?

The typical structure of a WDM system is as follows:

• 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.

Figure 5：WDM System Structure Diagram

OTN

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.

The advantages of OTN over SDH and WDM are as follows:

• 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.

Figure 6：Hierarchy and Interface of OTN Diagram 

FS 1.6T High Capacity OTN Solution

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Conclusion

In conclusion, the transport network plays a crucial role in transmitting various service data and enabling seamless communication and service delivery. It achieves this through its high capacity, long-distance transmission, and multi-service access capabilities. By integrating advanced technologies like PDH, SDH, MSTP, PTN, WDM, and OTN, the transport network consistently enhances network capabilities, management flexibility, and business support. These advancements not only improve communication services but also offer growth opportunities for network operators, including excess telecom network providers. Looking ahead, the transport network will remain central in driving the telecommunications industry towards greater intelligence, efficiency, and reliability.