Understanding the Basics of SDH Mapping and Multiplexing
The mapping and multiplexing process in SDH can be likened to loading goods onto a train. In this analogy, the "goods" represent the information to be transmitted, while the "train" corresponds to the SDH transmission module. This process is fundamental to modern fiber optic communication systems, including att fiber optic internet networks that deliver high-speed connectivity worldwide.
Figure 1-1: The SDH multiplexing process analogy
Adapting a tributary signal into a Container (C) is comparable to placing goods into small cartons. The fixed stuffing bits (R bits) act like packing material inside these cartons. Adding Path Overhead (POH) to a Container (C) to form a Virtual Container (VC) is similar to labeling these small cartons – this constitutes the mapping process.
A lower-order VC (equivalent to a labeled small carton) combined with a Tributary Unit Pointer (TU-PTR) – which functions like numbering multiple cartons to determine their positions – forms a Tributary Unit (TU), representing the alignment process. Multiplexing multiple tributary units into a Tributary Unit Group (TUG) is analogous to placing several labeled and numbered cartons together into a large container, which is part of the multiplexing process.
Adding POH to a TUG (like labeling the large container) forms a higher-order VC, another mapping step in the process. A higher-order VC combined with an Administrative Unit Pointer (AU-PTR) forms an Administrative Unit (AU-4), which is like numbering the large containers – this is another alignment process. An AU-4 is placed into an AUG-4, and finally, adding Section Overhead (equivalent to the train's service and management systems) forms the STM-1, which represents the complete train in our analogy.
In modern communication networks, including advanced att fiber optic internet services, this structured approach ensures efficient and reliable data transmission across vast distances. The standardization of these processes allows for interoperability between different network equipment and service providers.
Mapping and Multiplexing 139,264 kbit/s into STM-1
The process of mapping and multiplexing a 139,264 kbit/s PDH signal into an STM-1 frame involves four distinct steps, each critical for maintaining signal integrity in high-speed networks like att fiber optic internet backbones.
1 Adaptation into Container C-4
The 139,264 kbit/s PDH signal is rate-adjusted and adapted into the C-4 SDH container. The C-4 container has a period of 125 μs, with a structure of 9 rows and 260 columns, totaling 18,720 bits (9 × 260 × 8).
The corresponding data rate is 149,760 kbit/s (18,720 bits / 125 × 10⁻⁶ seconds). The 139,264 kbit/s signal is inserted into the C-4 using positive rate adjustment.
2 Adaptation into VC-4
The C-4 container is adapted into a VC-4 by adding 9 overhead bytes: J1, B3, C2, G1, F2, H4, F3, K3, and N1. This forms a VC-4 structure with 9 rows and 261 columns.
The corresponding data rate of the VC-4 is 150,336 kbit/s (261 × 9 × 8 × 8), a crucial specification for att fiber optic internet infrastructure that relies on precise timing and bandwidth allocation.
3 Alignment into AU-4
The VC-4 is aligned into an AU-4 by adding the AU-4 pointer. This pointer mechanism allows the VC-4 to be located within the AU-4, even if there are frequency variations between the VC-4 and the higher-level frame structure.
This alignment capability is essential for maintaining signal integrity in dynamic networks, including att fiber optic internet systems that must adapt to varying traffic conditions.
4 Formation of STM-1
The AU-4 is placed into an AUG-4 (Administrative Unit Group). For a 155.520 Mbit/s signal, the AUG-4 rate equals the AU-4 rate since N=1 in this configuration.
Finally, Section Overhead is added to form the complete STM-1 frame, which serves as the fundamental transmission unit in many fiber optic networks, including att fiber optic internet backbones.
Detailed Bit Rate Analysis
From the PDH 139,264 kbit/s stream, we extract 125 μs intervals, containing approximately 17,408 bits (calculated as 139,264 × 125 × 10⁻⁶ = 17,408 bits). However, PDH allows for a ±15 ppm tolerance, resulting in bit count variations within a 0.261 bit range around 17,408 bits within each 125 μs interval.
These bits are arranged in a consistent structure across the 9 rows of the C-4 container. This standardized structure ensures compatibility across different vendors' equipment, a key requirement for large-scale networks like att fiber optic internet that rely on multi-vendor environments.
The addition of path overhead bytes in the VC-4 provides essential management and monitoring capabilities, allowing network operators to track signal quality and performance throughout the transmission path – a critical feature for maintaining the high service levels expected from modern att fiber optic internet connections.
Technical Note
The precise alignment and pointer mechanisms in SDH allow for synchronous operation across large networks, minimizing latency and ensuring reliable transmission – factors that contribute to the exceptional performance of att fiber optic internet services even during peak usage periods.
Mapping and Multiplexing 2048 kbit/s into STM-1
The process of mapping and multiplexing a 2048 kbit/s PDH signal into an STM-1 frame involves six distinct steps, reflecting the more complex adaptation required for this lower-bit-rate signal. This process is vital for integrating legacy systems into modern fiber optic networks, including att fiber optic internet infrastructure that must support various service types.
| Step | Process | Key Parameters |
|---|---|---|
| 1 | Adapt 2048 kbit/s into C-12 | 4 rows × 34 columns, 500 μs frame period |
| 2 | Map C-12 to VC-12 | Add 4 low-order POH bytes |
| 3 | Align VC-12 into TU-12 | Add TU-12 pointer |
| 4 | Multiplex TU-12s into TUG-2 | 7 TU-12s per TUG-2 |
| 5 | Multiplex TUG-2s into TUG-3 | 3 TUG-2s per TUG-3 |
| 6 | Multiplex TUG-3s into VC-4 and beyond | 7 TUG-3s form C-4, then VC-4, AU-4, STM-1 |
Detailed Process Explanation
Step 1: Adapting 2048 kbit/s PDH Signal into C-12
The C-12 frame consists of 4 basic frames forming a multiframe, with a total structure of 4 rows and 34 columns. Each basic frame has a period of 125 μs, making the C-12 frame period 500 μs (4 × 125 μs). This structure spans 4 consecutive STM-1 frames, resulting in a frame frequency of 2 kHz, which is 1/4 of the STM-1 frame frequency.
The frame length is 1088 bits (4 × 34 × 8). The 2048 kbit/s signal is inserted into the C-12 using positive/zero/negative rate adjustment. This flexibility in rate adjustment allows for precise synchronization with the higher-level SDH structure, a capability that enhances the robustness of att fiber optic internet networks.
Figure 2-1: C-12 Frame Structure
The left side of the C-12 frame contains 4 bytes (the first byte of each row), including 1 fixed stuffing byte. The remaining 3 bytes contain C1 and C2 bits (6 bits total) for adjustment control, S1 bits for negative rate adjustment, and S2 bits for positive adjustment opportunities.
The right side of the C-12 frame contains 4 bytes, all of which are fixed stuffing bytes. It's important to note that the 4 basic frames within a multiframe are placed in parallel and are not multiplexed into the same STM-1 frame but rather into 4 consecutive STM-1 frames. This time-division approach optimizes bandwidth utilization in att fiber optic internet systems.
Step 2: Mapping from C-12 to VC-12
To enable real-time monitoring of each 2 Mbit/s channel signal performance in the SDH network, the C-12 is augmented with 4 low-order path overhead bytes (LP-POH) to form the VC-12 information structure. This monitoring capability is essential for maintaining the high service quality standards of att fiber optic internet connections.
A single set of path overhead bytes monitors the transmission status of the entire multiframe across the network. Since one C-12 multiframe carries 4 frames of 2 Mbit/s signals, one set of low-order path overhead (LP-POH) monitors the transmission status of 4 frames of 2 Mbit/s signals.
Steps 3-6: Alignment and Multiplexing
The VC-12 is aligned into a TU-12 (Tributary Unit) by adding a TU-12 pointer, which enables precise location of the VC-12 within the TU-12 structure. This pointer mechanism compensates for frequency variations, ensuring signal integrity even in large networks like att fiber optic internet backbones that span vast geographical distances.
Seven TU-12s are multiplexed to form a TUG-2 (Tributary Unit Group 2). This grouping allows for efficient management of multiple 2 Mbit/s channels. Three TUG-2s are then multiplexed to form a TUG-3, further aggregating the signals.
Finally, seven TUG-3s are combined to form a C-4 container. From this point, the process continues as described in the 139,264 kbit/s mapping procedure: the C-4 is adapted into a VC-4 by adding path overhead, aligned into an AU-4 with the addition of an AU-4 pointer, and ultimately formed into an STM-1 frame with the addition of section overhead.
Key Advantage in att fiber optic internet Networks
This hierarchical multiplexing structure allows for efficient aggregation of multiple lower-bit-rate signals into the higher-capacity STM-1 frame, optimizing bandwidth utilization in att fiber optic internet networks while maintaining individual channel integrity and manageability.
Hierarchical Structure
The layered approach allows for flexible mapping of different signal rates, making SDH and att fiber optic internet networks adaptable to various service requirements.
Efficient Bandwidth
Precise rate adjustment and multiplexing ensure optimal use of available bandwidth, a critical factor in maximizing the performance of att fiber optic internet services.
Robust Monitoring
Comprehensive overhead bytes enable end-to-end performance monitoring, ensuring high reliability in att fiber optic internet networks.
Practical Applications in Modern Networks
The mapping and multiplexing processes described play a crucial role in modern telecommunications infrastructure, including att fiber optic internet networks that deliver high-speed connectivity to homes and businesses. By standardizing how different PDH signals are integrated into SDH frames, these processes enable interoperability between equipment from different vendors and facilitate the efficient use of fiber optic bandwidth.
One of the key advantages of the SDH architecture is its ability to support multiple signal types within a single transmission framework. This flexibility allows network operators to efficiently manage a mix of legacy services and new high-bandwidth applications on the same physical infrastructure, maximizing the return on investment in fiber optic networks like att fiber optic internet.
Why 34 Mbit/s is Less Commonly Used
As noted earlier, a single STM-1 frame can only accommodate 3 x 34 Mbit/s tributary signals, resulting in relatively low channel utilization. This inefficiency has led to the 34 Mbit/s interface being less commonly deployed in modern networks compared to the 139,264 kbit/s and 2048 kbit/s interfaces.
In att fiber optic internet networks, where bandwidth efficiency is paramount, the focus has shifted toward higher-capacity interfaces and more efficient multiplexing techniques. However, the fundamental principles of mapping and multiplexing remain consistent across different signal rates, forming the basis for reliable high-speed communication.
Future Developments
While SDH technology has been foundational in telecommunications, modern networks are evolving toward more packet-oriented architectures. However, the principles of efficient signal mapping and multiplexing remain relevant, with new techniques emerging to address the challenges of packet-based transmission over fiber optic infrastructure.
Att fiber optic internet continues to evolve, incorporating advanced multiplexing technologies like dense wavelength division multiplexing (DWDM) alongside traditional SDH principles to deliver ever-increasing bandwidth capabilities. Understanding the fundamentals of PDH to STM-1 mapping provides a valuable foundation for comprehending these advanced technologies and their applications in modern communication networks.