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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 LevelElectrical LevelLine 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 2488.3202405.37682.944 STM-16
OC-192STS-1929953.2809621.504331.776STM-64
OC-768STS-76839813.12038486.0161327.104STM-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. Refer to the ANSI web site at http://www.ansi.org for a complete list of SONET standards along with information on purchasing the documents.

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. Refer to the ITU web site at http://www.itu.int for a complete list of SONET standards along with information on purchasing the documents.

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. Refer to the Telcordia web site at http://www.telcordia.com for a complete list of documents along with information on purchasing the documents. Telcordia documents are very expensive with the following document listing for US $2250 (at last check).

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.

Similar specifications are available from the ATM Forum with the following being one example:

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.

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