SONET / SDH Technical Summary

What is SONET & SDH?
SONET and SDH are a set of related standards for synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical NETwork and SDH is an acronym for Synchronous Digital Hierarchy. SONET is the United States version of the standard published by the American National Standards Institutue (ANSI). SDH is the international version of the standard published by the International Telecommunications Union (ITU).
The SONET/SDH Digital Hierarchy
The following table lists the hierarchy of the most common SONET/SDH data rates:

Optical Level
Electrical Level
Line Rate (Mbps)
Payload Rate (Mbps)
Overhead Rate (Mbps)
SDH Equivalent
OC-1
STS-1
51.840
50.112
1.728
-
OC-3
STS-3
155.520
150.336
5.184
STM-1
OC-12
STS-12
622.080
601.344
20.736
STM-4
OC-48
STS-48
2488.320
2405.376
82.944
STM-16
OC-192
STS-192
9953.280
9621.504
331.776
STM-64
OC-768
STS-768
39813.120
38486.016
1327.104
STM-256

Other rates (OC-9, OC-18, OC-24, OC-36, OC-96) are referenced in some of the standards documents but were never widely implemented. It is possible other higher rates (e.g. OC-3072) may be defined in in the future.
The "line rate" refers to the raw bit rate carried over the optical fiber. A portion of the bits transferred over the line are designated as "overhead". The overhead carries information that provides OAM&P (Operations, Administration, Maintenance, and Provisioning) capabilities such as framing, multiplexing, status, trace, and performance monitoring. The "line rate" minus the "overhead rate" yields the "payload rate" which is the bandwidth available for transferring user data such as packets or ATM cells.
The SONET/SDH level designations sometimes include a "c" suffix (such as "OC-48c"). The "c" suffix indicates a "concatenated" or "clear" channel. This implies that the entire payload rate is available as a single channel of communications (i.e. the entire payload rate may be used by a single flow of cells or packets). The opposite of concatenated or clear channel is "channelized". In a channelized link the payload rate is subdivided into multiple fixed rate channels. For example, the payload of an OC-48 link may be subdivided into four OC-12 channels. In this case the data rate of a single cell or packet flow is limited by the bandwidth of an individual channel.
ANSI SONET Standards
The American National Standards Institute (ANSI) coordinates and approves SONET standards. The standards are actually developed by Committee T1 which is sponsored by the Alliance for Telecommunications Industry Solutions (ATIS) and accredited by ANSI to create network interconnection and interoperability standards for the United States. T1X1 and T1M1 are the primary T1 Technical Subcommittees responsible for SONET. T1X1 deals with "digital hierarchy and synchronization". T1M1 deals with "internetworking operations, administration, maintenance, and provisioning (OAM&P). Listed below are some of the most commonly cited SONET standards available from ANSI. ANSI T1.105: SONET - Basic Description including Multiplex Structure, Rates and Formats
ANSI T1.105.01: SONET - Automatic Protection Switching
ANSI T1.105.02: SONET - Payload Mappings
ANSI T1.105.03: SONET - Jitter at Network Interfaces
ANSI T1.105.03a: SONET - Jitter at Network Interfaces - DS1 Supplement
ANSI T1.105.03b: SONET - Jitter at Network Interfaces - DS3 Wander Supplement
ANSI T1.105.04: SONET - Data Communication Channel Protocol and Architectures
ANSI T1.105.05: SONET - Tandem Connection Maintenance
ANSI T1.105.06: SONET - Physical Layer Specifications
ANSI T1.105.07: SONET - Sub-STS-1 Interface Rates and Formats Specification
ANSI T1.105.09: SONET - Network Element Timing and Synchronization
ANSI T1.119: SONET - Operations, Administration, Maintenance, and Provisioning (OAM&P) - Communications
ANSI T1.119.01: SONET: OAM&P Communications Protection Switching Fragment
ITU-T SDH Standards
The International Telecommunications Union (ITU) coordinates the development of SDH standards. ITU was formerly known as the CCITT. It is sponsored by the United Nations and coordinates the development of telecommunications standards for the entire world. Listed below are some of the most commonly cited SDH standards available from ITU. ITU-T G.707: Network Node Interface for the Synchronous Digital Hierarchy (SDH)
ITU-T G.781: Structure of Recommendations on Equipment for the Synchronous Digital Hierarchy (SDH)
ITU-T G.782: Types and Characteristics of Synchronous Digital Hierarchy (SDH) Equipment
ITU-T G.783: Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks
ITU-T G.803: Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH)
Telcordia Documents
Telcordia Technologies (formerly Bell Communications Research, or "Bellcore") has issued over 50 documents that relate to SONET. The document most commonly cited is listed below. Telcordia documents are very expensive with the following document listing for US $2250 (at last check).
GR 253 CORE: SONET Transport Systems: Common Generic Criteria
SONET Interoperability Forum (SIF)
The SONET Interoperability Form (SIF) was formed in 1994 to identify SONET interoperability issues. As solutions are defined, reviewed, and approved they are published as SIF Approved Documents.
ATM over SONET
The following standard from ITU defines the mapping of an ATM cell stream into an SDH frame structure.
ITU-T I.432: B-ISDN User-Network Interface - Physical Layer Specification
Similar specifications are available from the ATM Forum with the following being one example:
622.08 Mb/s Physical Layer Specification
Packet Over SONET (POS)
The Internet Engineering Task Force (IETF) has released RFCs that describe the use of Point-to-Point Protocol for transferring IP traffic natively over SONET and SDH circuits. These RFCs were developed by the PPP Extensions Working Group of IETF.
IETF RFC2615: PPP over SONET/SDH
IETF RFC1661: The Point-to-Point Protocol (PPP)
IETF RFC1662: PPP in HDLC-like Framing