The Optical Transport Network (OTN) is a set of optical network elements connected by optical fiber links, able to provide functionality of transport, multiplexing, routing, management, supervision and survivability of optical channels carrying client signals.
The OTN frame structure is based on a byte-interleaved format. Each frame consists of rows and columns organized in specific areas that carry payload data, overhead information, and Forward Error Correction (FEC) bytes.
| Overhead Type | Size | Primary Functions |
|---|---|---|
| OTU OH | 6 bytes × 4 rows | Section monitoring, APS/PCC, fault type & location indication |
| ODU OH | 8 bytes × 4 rows | Path monitoring, TCM, tandem connection monitoring, fault type indication |
| OPU OH | 2 bytes × 4 rows | Payload type, mapping, justification control |
| FAS (Frame Alignment Signal) | 6 bytes × 4 rows | Frame alignment, multi-frame alignment |
| FEC | 32 bytes per row | Forward Error Correction using Reed-Solomon code |
OTN uses a specific bit and byte structure for its various frame elements. Understanding these structures is essential for proper frame processing and error detection.
Fixed pattern: F6F6F6282828 (hex)
Used for synchronization and frame detection
8-bit counter (0-255)
Used for multi-frame alignment
64-byte message format
Used for path verification
8-bit error detection code
Used for error monitoring
OTN provides several key functions that are essential for robust optical transport networks.
OTN supports multiplexing of lower-rate ODUs into higher-rate ODUs:
OTN employs strong FEC:
Comprehensive monitoring:
Multiple protection schemes:
End-to-end path monitoring:
Supports various client signals:
OTN defines a hierarchical signal structure that enables efficient transport and management of client signals.
| Signal | Line Rate | Payload Rate | Typical Client Signals |
|---|---|---|---|
| OTU1 | 2.666 Gbps | 2.488 Gbps | STM-16/OC-48, FC-1200 |
| OTU2 | 10.709 Gbps | 9.995 Gbps | STM-64/OC-192, 10GbE WAN PHY |
| OTU3 | 43.018 Gbps | 40.150 Gbps | STM-256/OC-768, 40GbE |
| OTU4 | 111.81 Gbps | 104.79 Gbps | 100GbE |
| OTUCn | n × 100 Gbps | n × 100 Gbps with OH | 400GbE, FlexE |
Note: OTN multiplexing is flexible, allowing different client signals to be transported efficiently over the same physical infrastructure.
The OTN overhead structure provides comprehensive management capabilities for the network.
OTN incorporates powerful Forward Error Correction (FEC) mechanisms to ensure reliable data transmission over long distances.
OTN provides standardized multiplexing schemes that allow efficient use of bandwidth and flexible transport of client signals.
OTN uses a standard fixed frame structure for TDM.
Flexible container for client signals with variable bandwidth.
Maps client signals with clock frequency differences.
Supports asynchronous client signals.
Note: The OTN tributary slot structure provides a standardized way to transport and multiplex client signals of different rates efficiently.
OTN defines several mapping procedures to adapt and transport client signals efficiently.
Constant Bit Rate mapping for SDH/SONET and other synchronous signals.
Generic Framing Procedure for packet-based client signals.
Generic Mapping Procedure for flexible client adaptation.
Bit-synchronous Mapping Procedure for transparent transport.
| Client Signal | OPU Container | Mapping Method | Payload Type (PT) |
|---|---|---|---|
| 1GbE | OPU0 | GFP-F | 0x01 |
| 10GbE LAN | OPU2e | GFP-F | 0x02 |
| 10GbE WAN | OPU2 | CBR mapping | 0x03 |
| FC-1200 | OPU1 | GFP-T | 0x04 |
| STM-64/OC-192 | OPU2 | AMP or BMP | 0x05 |
| 40GbE | OPU3 | GFP-F | 0x06 |
| 100GbE | OPU4 | GMP | 0x07 |
OTN provides several protection and restoration mechanisms to ensure network reliability and resiliency.
OTN provides comprehensive performance monitoring capabilities at different layers of the network hierarchy.
OTN networks are controlled and managed through various interfaces and protocols.
OTN is standardized primarily by the ITU-T, ensuring multi-vendor interoperability and network consistency.
OTN can be deployed in various network architectures to meet different requirements.
OTN continues to evolve to meet the growing bandwidth demands and new application requirements.
OTN is deployed across various industry segments to provide reliable high-capacity transport services.
Successfully implementing OTN networks requires adherence to best practices in design, deployment, and operations.
This glossary provides definitions for key OTN terms and acronyms.
| Term | Definition |
|---|---|
| OTN | Optical Transport Network, a standardized technology for transporting client signals over optical networks. |
| OPU | Optical Payload Unit, the container that carries client data and adaptation information. |
| ODU | Optical Data Unit, the container that adds path monitoring and tandem connection monitoring to the OPU. |
| OTU | Optical Transport Unit, the container that adds FEC and section monitoring to the ODU. |
| OCh | Optical Channel, the optical path carrying an OTU. |
| OMS | Optical Multiplex Section, carrying multiple wavelengths. |
| OTS | Optical Transmission Section, representing the fiber span. |
| FEC | Forward Error Correction, used to correct transmission errors. |
| TCM | Tandem Connection Monitoring, allows monitoring of specific network segments. |
| GMP | Generic Mapping Procedure, a method for mapping client signals. |
| GCC | General Communication Channel, used for management communication. |
| ODUflex | Flexible-size ODU that can be adjusted to match client signal rate. |
| OTUCn | OTU with n × 100G capacity, for interfaces beyond 100G. |
| TTI | Trail Trace Identifier, used for path verification. |
| BIP-8 | Bit Interleaved Parity-8, used for error detection. |
| BEI | Backward Error Indication, reports errors to the upstream node. |
| BDI | Backward Defect Indication, reports defects to the upstream node. |
| APS | Automatic Protection Switching, protocol for protection coordination. |
| PCC | Protection Communication Channel, used for protection signaling. |
| PSI | Payload Structure Identifier, includes payload type and mapping-specific information. |
| PT | Payload Type, indicates the type of client signal carried. |
| JC | Justification Control, used in asynchronous mapping. |
| PM | Path Monitoring, overhead for end-to-end monitoring. |
| SM | Section Monitoring, overhead for section monitoring. |
| MFAS | Multi-Frame Alignment Signal, for multi-frame synchronization. |
| FAS | Frame Alignment Signal, used for frame synchronization. |
| GFP | Generic Framing Procedure, encapsulation for packet data over OTN. |
| CBR | Constant Bit Rate, mapping for synchronous signals like SONET/SDH. |
| AMP | Asynchronous Mapping Procedure, for CBR client signals. |
| BMP | Bit-synchronous Mapping Procedure, for transparent CBR mapping. |
| TPN | Tributary Port Number, identifies client ports in ODUflex. |
| TS | Tributary Slot, a fixed bandwidth allocation unit in ODU multiplexing. |
| HO | Higher Order, refers to ODUs used for transport (ODU2/3/4). |
| LO | Lower Order, refers to client ODUs (ODU0/1/flex). |
| GMPLS | Generalized Multi-Protocol Label Switching, control plane for OTN. |
| ASON | Automatically Switched Optical Network, architecture for dynamic OTN. |
| FlexO | Flexible OTN, interface standard for beyond 100G OTN. |
| ES | Error Second, performance parameter for monitoring. |
| SES | Severely Errored Second, performance parameter with higher error threshold. |
| BBE | Background Block Error, errors in non-SES seconds. |
| UAS | Unavailable Second, period when service is considered unavailable. |
Additional resources for further study on OTN technology and standards.
The Optical Transport Network (OTN) provides a standardized framework for transporting diverse client signals over optical networks with high reliability, scalability, and manageability.
As telecommunications networks continue to evolve to support emerging applications like 5G, cloud services, edge computing, and the Internet of Things, OTN provides the essential transport foundation to deliver the required capacity, reliability, and performance needed for next-generation services.
OTN's continued evolution ensures it will remain a critical technology in optical networking infrastructure for years to come, adapting to new requirements while maintaining backward compatibility with existing deployments.