SDH Mapping and Multiplexing Units

The fundamental multiplexing units of SDH form a hierarchical structure that enables the efficient transmission of various data rates over optical fiber networks. These units work together to ensure that different signal types can be seamlessly integrated into the SDH framework, providing a robust and flexible solution for modern telecommunications over optical fiber infrastructure.

The primary SDH multiplexing units include:

• Standard Containers (C)
• Virtual Containers (VC)
• Tributary Units (TU)
• Tributary Unit Groups (TUG)
• Administrative Units (AU)
• Administrative Unit Groups (AUG)

This hierarchical structure allows for flexible multiplexing of lower-rate signals into higher-rate optical fiber transmission systems, enabling efficient use of optical fiber bandwidth while maintaining compatibility across different network elements. The standardization of these units ensures interoperability between equipment from different manufacturers, a crucial factor in the global optical fiber telecommunications infrastructure.

Containers are information structures used to load various rates of service signals in SDH systems. Their primary function is to perform adaptation, which involves converting the code type and speed between input and output signals. This adaptation is crucial for ensuring that different signal formats can be efficiently transmitted over optical fiber networks.

Five standard containers are specified in SDH standards, each designed to accommodate specific signal rates commonly used in telecommunications over optical fiber:

In China, the commonly used containers are C-12, C-3, and C-4, which align with the country's telecommunications standards and optical fiber infrastructure requirements. These containers provide the necessary flexibility to handle various services over optical fiber networks, from basic telephony to high-speed data transmission.

A loaded container C serves as the information payload for a Virtual Container (VC). This transition from container to virtual container is a critical step in the SDH multiplexing process, as it prepares the signal for further processing and transmission over optical fiber links.

The standardization of container types ensures that different equipment manufacturers can produce interoperable systems, which is essential for the global optical fiber telecommunications network. By defining these standard containers, SDH enables service providers to efficiently utilize optical fiber bandwidth while supporting a wide range of services with different bandwidth requirements.

A Virtual Container (VC) is an information structure used to support SDH path layer connections. It consists of the information payload from a container output plus Path Overhead (POH). This structure is crucial for managing and maintaining signal integrity across optical fiber networks.

The Path Overhead (POH) contains information necessary for managing and maintaining the signal along its path through the optical fiber network. This includes performance monitoring, error detection, and management information that enables network operators to ensure reliable transmission over optical fiber links.

Classification of Virtual Containers

Virtual containers can be divided into low-order and high-order virtual containers, each serving different roles in the SDH hierarchy over optical fiber networks:

The VC is the smallest information unit in SDH that can be transmitted, switched, and processed. The path that a VC takes through an SDH transmission network is called a channel. In China, due to the elimination of the AU-3 channel, VC-12 and VC-3 are both considered low-order channels in the country's optical fiber network infrastructure.

One of the key advantages of virtual containers is their ability to maintain their structure and payload content during transmission through the optical fiber network, even when they are being switched or multiplexed. This feature ensures that the integrity of the signal is preserved throughout its journey across the optical fiber network, from source to destination.

The standardization of virtual containers across the global telecommunications industry has been instrumental in enabling interoperability between different network elements and service providers. This standardization is particularly important for optical fiber networks, which form the backbone of international communications, requiring seamless integration between different operators and technologies.

Tributary Units (TUs) are information structures that provide adaptation between the low-order path layer and the high-order path layer in SDH systems. Their primary function is to assemble low-order containers into high-order virtual containers for efficient transmission over optical fiber networks.

A Tributary Unit consists of a low-order VC-n and a corresponding Tributary Unit Pointer (TU-n-PTR):

The Tributary Unit Pointer (TU-n-PTR) is a critical component that indicates the position of the start of the VC-n payload within the TU. This pointer mechanism allows for flexibility in aligning different signals, which is essential when multiplexing multiple signals onto a single optical fiber transmission path.

Tributary Unit Groups (TUG)

A Tributary Unit Group (TUG) consists of one or more tributary units that occupy fixed, determined positions within the payload of a higher-order VC. TUGs facilitate the efficient multiplexing of multiple lower-rate signals into higher-rate structures for transmission over optical fiber networks.

The TUG structure provides a flexible and efficient way to aggregate lower-rate signals into higher-rate signals suitable for transmission over optical fiber. This hierarchical approach allows network operators to optimize bandwidth usage on optical fiber links while maintaining the ability to easily access and manage individual lower-rate signals when needed.

In modern optical fiber networks, the efficient multiplexing provided by TUs and TUGs is critical for supporting the ever-increasing demand for bandwidth. By allowing different types of services with varying bandwidth requirements to coexist on the same optical fiber infrastructure, these units enable service providers to offer a diverse range of services while maximizing the utilization of their optical fiber assets.

Administrative Units (AUs) are information structures that provide adaptation between the high-order path layer and the multiplex section layer in SDH systems. Their primary responsibility is to assemble high-order virtual containers into STM-N frames for transmission over optical fiber networks.

An Administrative Unit consists of a high-order VC and a corresponding Administrative Unit Pointer (AU-PTR):

The Administrative Unit Pointer (AU-n-PTR) indicates the position of the start of the high-order VC-n payload within the AU frame. This pointer mechanism is crucial for aligning the high-order virtual containers within the STM-N frame structure, ensuring proper synchronization and transmission over optical fiber links.

Administrative Unit Groups (AUG)

An Administrative Unit Group (AUG) is composed of one or more administrative units that occupy fixed, determined positions within the payload of an STM-N frame. The AUG structure enables the aggregation of multiple high-order signals into the final STM-N signal transmitted over optical fiber.

The AU and AUG structures are critical for the final assembly of signals before they are transmitted over optical fiber. They provide the necessary adaptation between the high-order path layer and the multiplex section layer, ensuring that signals from different sources can be combined into a single high-speed signal suitable for optical fiber transmission.

In modern optical fiber networks, the ability to efficiently multiplex multiple high-rate signals into even higher-rate STM-N signals is essential for meeting the growing demand for bandwidth. The AU and AUG structures provide a standardized way to achieve this multiplexing, enabling interoperability between different network elements and ensuring reliable transmission over long-distance optical fiber links.

According to the analysis of the aforementioned multiplexing units, any signal entering the SDH network to form an STM-N signal must go through three key processes: mapping, positioning, and multiplexing. These processes are essential for converting various signal types into the standardized format required for transmission over optical fiber networks, ensuring compatibility and efficient bandwidth utilization.

Mapping

Mapping is the process of first adjusting the speed of various G.703 tributary signals through code rate adaptation to load them into corresponding standard containers, and then loading them into virtual containers. This process is essential for converting diverse signal formats into the standardized structure required for efficient transmission over optical fiber networks.

The mapping process ensures that different signal types, regardless of their original format, can be uniformly handled within the SDH network, facilitating their transmission over optical fiber links. This standardization is crucial for interoperability between different network components and between networks operated by different service providers.

The mapping process converts various signal rates into standardized containers for optical fiber transmission

Examples of mapping include:

• Loading 2.048 Mbit/s signals into VC-12 containers for transmission over optical fiber
• Loading 34.368 Mbit/s signals into VC-3 containers
• Loading 139.264 Mbit/s signals into VC-4 containers

During the mapping process, code rate adjustment may be necessary to align the input signal with the container's standard rate. This adjustment ensures that the signal fits perfectly within the container, minimizing inefficiencies in optical fiber bandwidth utilization. The mapping process may also include scrambling to ensure a sufficient number of transitions in the signal, which is important for clock recovery in optical fiber receivers.

Positioning

Positioning is a process that uses pointers attached to VCs to indicate and determine the position of the start of a low-order VC frame within the TU payload, or administrative unit pointers to indicate and determine the position of the start of a high-order VC frame within the AU payload. This mechanism is crucial for maintaining signal integrity and synchronization in optical fiber networks.

The positioning process using pointers allows for flexibility in aligning different signals, accommodating small timing differences between various network elements. This is particularly important in large optical fiber networks where signals may traverse multiple nodes with slightly different clock sources.

Examples of Positioning

• Using the TU-12-PTR attached to VC-12 to indicate and determine the position of the start of VC-12 in the TU-12 payload, ensuring accurate extraction at the receiving end of the optical fiber link
• Using the TU-3-PTR attached to VC-3 to indicate and determine the position of the start of VC-3 in the TU-3 payload
• Using the AU-4-PTR attached to VC-4 to indicate and determine the position of the start of VC-4 in the AU-4 payload for high-speed optical fiber transmission

The pointer mechanism allows for dynamic adjustment of the payload position, which is essential for accommodating frequency variations between different parts of the optical fiber network. This adjustment capability ensures that the receiver can always correctly locate the start of the payload, even if there are small timing variations in the optical fiber transmission path. This feature significantly enhances the robustness and reliability of SDH optical fiber networks.

Multiplexing

Multiplexing is the process of organizing TUs into higher-order VCs or AUs into STM-N frames. This process enables the efficient utilization of optical fiber bandwidth by combining multiple lower-rate signals into a single higher-rate signal for transmission over optical fiber links.

SDH uses synchronous multiplexing, which means that the timing of the multiplexed signals is aligned, simplifying the demultiplexing process at the receiving end. This synchronous approach is particularly beneficial for optical fiber networks, as it reduces the complexity of network equipment and improves signal integrity.

Multiplexing process combines lower-order units into higher-order structures for efficient optical fiber transmission

Examples of multiplexing in SDH optical fiber networks include:

• The process of loading TU-12 into VC-4 through TUG-2 and then TUG-3, a common pathway in Chinese optical fiber networks
• The process of loading TU-3 into VC-4 through TUG-3
• The process of loading AU-4 into the STM-N frame, forming the final signal transmitted over optical fiber

The multiplexing process in SDH is designed to be flexible, allowing network operators to efficiently utilize the available bandwidth on optical fiber links. By enabling different combinations of lower-rate signals, SDH optical fiber networks can be easily adapted to meet changing traffic demands. This flexibility is one of the key reasons why SDH has been widely adopted as a standard for optical fiber telecommunications networks around the world.