Cisco ONS 15454 Engineering Planning Guide, Release 5.0
Appendix B, SONET Primer
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SONET Primer

Table Of Contents

SONET Primer

Background of SONET

Synchronous, Asynchronous, and Plesiochronous Signals

SONET Signal Hierarchy

SONET Frame Structure

SONET Layers

Section Overhead

Line Overhead

STS-1 Path Overhead

SPE Values

VT Path Overhead

SONET Multiplexing

Asynchronous Multiplexing

M13 Format

Synchronous Multiplexing

SONET Network Configurations





Benefits of SONET


Reduced Back-to-Back Multiplexing

Optical Interconnect

OAM&P Functionality

Multipoint Configurations

Convergence of Voice, Data, and Video


Reduced Cabling and Use of DSX Panels

Enhanced Performance Monitoring

Convergence of SONET and SDH Hierarchies


Asynchronous and Synchronous Tributaries

SONET Primer

This appendix provides an introduction to the Synchronous Optical NETwork (SONET) standard. For additional information on SONET, refer to:

ANSI T1.105 - 1995 American National Standard for Telecommunications, Synchronous Optical Network (SONET)

ANSI T1.106-1988 American National Standard for Telecommunications - Digital Hierarchy Optical Interface Specifications, Single Mode

ITU Recommendations G.707, G.708, G.709

Telcordia GR-253-CORE - Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria

The following topics are covered in this chapter:

Background of SONET

SONET Signal Hierarchy

SONET Frame Structure

SONET Layers

SONET Multiplexing

SONET Network Configurations

Benefits of SONET

Convergence of SONET and SDH Hierarchies

Background of SONET

Before SONET, the first generations of fiber optic systems in the public telephone network used proprietary architectures, equipment, line codes, multiplexing formats, and maintenance procedures. The users of this equipment included the Regional Bell Operating Companies (RBOCs) and Interexchange Carriers (IXCs) in the U.S., Canada, Korea, Taiwan, and Hong Kong. These operating companies wanted an industry standard so they could mix and match equipment from different suppliers. The task of creating such a standard was taken up in 1984 by the Exchange Carriers Standards Association (ECSA) to establish a standard for connecting one fiber system to another. This standard became known as SONET, which stands for Synchronous Optical NETwork.

SONET is an ANSI standard for synchronous data transmission on optical media. SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic based transmission hierarchy. It was formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), which sets industry standards in the U.S. for telecommunications and other industries. Internationally, Synchronous Digital Hierarch (SDH) is the equivalent of SONET. Together, these standards ensure that world-wide digital telecommunications networks can interconnect.

SONET defines the following standards:

Optical carrier (OC) parameters

Multiplexing schemes to map existing tributary signals (i.e., DS-1 and DS-3) into SONET payload signals

Overhead channels to support standard operation, administration, maintenance, and provisioning (OAM&P) functions

Criteria for optical line automatic protection switching (APS)

Synchronous, Asynchronous, and Plesiochronous Signals

To understand correctly the concepts and details of SONET, it's important to be clear about the meaning of Synchronous, Asynchronous, and Plesiochronous.

In a set of Synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may, however, be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation time delays or jitter introduced into the transmission network. In a synchronous network, all the clocks are traceable to one Primary Reference Clock (PRC). The accuracy of the PRC is better than ±1 in 1011 and is derived from a cesium atomic standard.

If two digital signals are Plesiochronous, their transitions occur at "almost" the same rate, with any variation being constrained within tight limits. For example, if two networks need to interwork, their clocks may be derived from two different PRCs. Although these clocks are extremely accurate, there is a difference between one clock and the other. This is known as a plesiochronous difference.

In the case of Asynchronous signals, the transitions of the signals do not necessarily occur at the same nominal rate. Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference. For example, if two clocks are derived from free-running quartz oscillators, they could be described as asynchronous.

SONET Signal Hierarchy

SONET uses a basic transmission rate of Synchronous Transport Signal level-1 (STS-1), which is equivalent to 51.84 Mb/s. There are 8,000 STS-1 frames per second in a STS-1 signal. Higher-level signals are integer multiples of the base rate. For example, STS-3 is three times the rate of STS-1 (3 x 51.84 = 155.52 Mb/s). An STS-12 rate would be 12 x 51.84 = 622.08 Mb/s. Table B-1 shows the hierarchy of STS-1 signals.

Table B-1 STS-1 Signal Hierarchy 

STS-N Signal
OC-N Signal
Bit Rate
Channel Capacity



51.84 Mb/s

28 DS-1s or 1 DS-3



155.520 Mb/s

84 DS-1s or 3 DS-3s



622.080 Mb/s

336 DS-1s or 12 DS-3s



2488.320 Mb/s

1344 DS-1s or 48 DS-3s



9953.280 Mb/s

5376 DS-1s or 192 DS-3s

SONET Frame Structure

The frame format of the STS-1 signal is shown in Figure B-1. The STS-1 frame has a recurring rate of 8000 frames a second and a frame rate of 125 microseconds The STS-1 frame consists of 90 columns and 9 rows. In general, the frame can be divided into two main areas: Transport overhead and the STS-1 synchronous payload envelope (STS-1 SPE).

Figure B-1 STS-1 Frame Format

The first three columns in each of the nine rows carry the section and line overhead bytes. Collectively, these 27 bytes are referred to as Transport Overhead. Transport Overhead is composed of section overhead and line overhead. The STS-1 path overhead is part of the synchronous payload envelope. The STS-1 payload has the capacity to transport up to:

28 DS-1s

1 DS-3

21 2.048 Mb/s signals or combinations of above.

Columns 4 through 90 are reserved for payload signals (i.e., DS1 and DS3) and is referred to as the STS-1 SPE. The optical counterpart of the STS-1 is the optical carrier level 1 signal (OC-1), which is the result of a direct optical conversion after scrambling. The STS-1 SPE can be divided into two parts: STS path overhead and the payload, as shown in Figure B-2. The payload is the revenue-producing traffic being transported and routed over the SONET network. Once the payload is multiplexed into the synchronous payload envelope, it can be transported and switched through SONET without having to be examined and possibly demultiplexed at intermediate nodes. Thus, SONET is said to be service-independent or transparent.

Figure B-2 STS-1 SPE Format

SONET Layers

SONET divides the overhead and transport functions into three layers:




These three layers are associated with both the physical equipment that segments the network and the bytes of information that flows through the network elements. Figure B-3 shows the various layers of a typical SONET network.

Figure B-3 Section, Line and Path Network Layers

The overhead layers are described in Table B-2.

Table B-2 SONET Overhead Layers

Overhead Layer


Section overhead is used for communications between adjacent network elements, including regenerators.


Line overhead is used for the STS-N signal between SONET equipment except regenerators.


Path-level overhead is carried within the SPE from end-to-end. It is added to DS-1 signals when they are mapped into virtual tributaries (VTs) and for STS-1 payloads that travel across the path end-to-end.

Section Overhead

The Section Overhead is found in the first three rows of Columns 1 through 3 (see Figure B-4). Table B-3 lists and defines the various Section Overhead bytes.

Figure B-4 Section Overhead Bytes within Rows 1 to 3 of Transport Overhead

Table B-3 Description of Section Overhead Bytes 


A1 & A2

Framing bytes - These two bytes indicate the beginning of an STS-1 frame.


Section Trace (J0)/Section Growth (Z0) - The byte in each of the N STS-1s in an STS-N that was formally defined as the STS-1 ID (C1) byte has been refined either as the Section Trace byte (in the first STS-1 of the STS-N), or as a Section Growth byte (in the second through Nth STS-1s).


Section bit interleaved parity code (BIP-8) byte - This is a parity code (even parity) used to check for transmission errors over a regenerator section. Its value is calculated over all bits of the previous STS-N frame after scrambling, then placed in the B1 byte of STS-1 before scrambling. Therefore, this byte is defined only for STS-1 number 1 of an STS-N signal.


Section orderwire byte - This byte is allocated to be used as a local orderwire channel for voice communication between regenerators, hubs, and remote terminal locations.


Section user channel byte - This byte is set aside for users' purposes. It terminates at all section terminating equipment within a line; that is, it can be read and/or written to at each section terminating equipment in that line.

D1 to D3

Section data communications channel (DCC) bytes - Together, these three bytes form a 192 kbps message channel providing a message-based channel for Operations, Administration, Maintenance, and Provisioning (OAM&P) between pieces of section-terminating equipment. The channel is used from a central location for alarms, control, monitoring, administration, and other communication needs. It's available for internally generated, externally generated, or manufacturer-specific messages.

Line Overhead

The Line Overhead is found in Rows 4 to 9 of Columns 1 through 3 (see Figure B-5). Table B-4 lists and defines the various Line Overhead bytes.

Figure B-5 Line Overhead Bytes in Rows 4 to 9 of Transport Overhead

Table B-4 Description of Line Overhead Bytes 


H1, H2

STS Payload Pointer (H1 and H2) - Two bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The pointer bytes are used in all STS-1s within an STS-N to align the STS-1 Transport Overhead in the STS-N, and to perform frequency justification. These bytes are also used to indicate concatenation, and to detect STS Path Alarm Indication Signals (AIS-P).


Pointer Action Byte (H3) - The pointer action byte is allocated for SPE frequency justification purposes. The H3 byte is used in all STS-1s within an STS-N to carry the extra SPE byte in the event of a negative pointer adjustment. The value contained in this byte when it's not used to carry the SPE byte is undefined.


Line bit interleaved parity code (BIP-8) byte - This parity code byte is used to determine if a transmission error has occurred over a line. It's even parity, and is calculated over all bits of the line Overhead and STS-1 SPE of the previous STS-1 frame before scrambling. The value is placed in the B2 byte of the line Overhead before scrambling. This byte is provided in all STS-1 signals in an STS-N signal.

K1 & K2

Automatic Protection Switching (APS channel) bytes - These two bytes are used for Protection Signaling between Line Terminating entities for bi-directional automatic protection switching and for detecting alarm indication signal (AIS-L) and Remote Defect Indication (RDI) signals.

D4 to D12

Line Data Communications Channel (DCC) bytes - These nine bytes form a 576kb/s message channel from a central location for OAM&P information (alarms, control, maintenance, remote provisioning, monitoring, administration, and other communication needs) between line entities. Available for internally generated, externally generated, and manufacturer specific messages. A protocol analyzer is required to access the Line-DCC information.


Synchronization Status (S1) - The S1 byte is located in the first STS-1 of an STS-N, and bits 5 through 8 of that byte are allocated to convey the synchronization status of the network element.


Growth (Z1) - The Z1 byte is located in the second through Nth STS-1s of an STS-N (3<N<48), and is allocated for future growth. Note that an OC-1 or STS-1 electrical signal does not contain a Z1 byte.


STS-1 REI-L (M0) - The M0 byte is only defined for STS-1 in an OC-1 or STS-1 electrical signal. Bits 5 through 8 are allocated for a Line Remote Error Indication function (REI-L - formerly referred to as Line FEBE), which conveys the error count detected by an LTE (using the Line BIP-8 code) back to its peer LTE.


STS-N REI-L (M1) - The M1 byte is located in the third STS-1 (in order of appearance in the byte-interleaved STS-N electrical or OC-N signal) in an STS-N (N>3), and is used for a REI-L function.


Growth (Z2) - The Z2 byte is located in the first and second STS-1s of an STS-3, and the first, second, and fourth through Nth STS-1s of an STS-N (12<N<48). These bytes are allocated for future growth. Note that an OC-1 or STS-1 electrical signal does not contain a Z2 byte.


Orderwire byte - This orderwire byte provides a 64 kb/s channel between line entities for an express orderwire. It's a voice channel for use by technicians and will be ignored as it passes through the regenerators.

STS-1 Path Overhead

The STS-1 Path Overhead is found in Rows 1 to 9 of the first column of the STS-1 SPE (see Figure B-6). Table B-5 lists and defines the Path Overhead bytes.

Figure B-6 Path Overhead in Rows 1 to 9

Table B-5 Description of Path Overhead Bytes 



STS path trace byte - This user-programmable byte repetitively transmits a 64-byte, or 16-byte E.164 format string. This allows the receiving terminal in a path to verify its continued connection to the intended transmitting terminal.


STS Path Bit Interleaved Parity code (Path BIP-8) byte - This is a parity code (even), used to determine if a transmission error has occurred over a path. Its value is calculated over all the bits of the previous synchronous payload envelope (SPE) before scrambling.


STS Path signal label byte - This byte is used to indicate the content of the STS SPE, including the status of the mapped payloads.


Path status byte - This byte is used to convey the path terminating status and performance back to the originating path terminating equipment. Therefore, the duplex path in its entirety can be monitored from either end, or from any point along the path. Bits 1 through 4 are allocated for an STS Path REI function (REI-P - formerly referred to as STS Path FEBE). Bits 5, 6, and 7 of the G1 byte are allocated for an STS Path RDI (RDI-P) signal. Bit 8 of the G1 byte is currently undefined.


Path user channel byte - This byte is used for user communication between path elements.


Virtual Tributary (VT) multi-frame indicator byte - This byte provides a generalized multi-frame indicator for payload containers. At present, it is used only for tributary unit structured payloads.

Note The Path Overhead Portion of the SPE remains with the payload until it is demultiplexed.

SPE Values

Table B-6 lists and describes the type of STS-1 SPE values.

Table B-6 Description of STS-1SPE Values

Hexidecimal Code




Equipped nonspecific payload


VT-Structured STS-1 SPE


Asynchronous mapping for DS3


DS4NA Asynchronous mapping


Mapping for ATM


Mapping for DQDB


Asynchronous mapping FDDI

VT Path Overhead

VT Path Overhead (VT POH) provides communication between the point of creation of a VT SPE and its point of disassembly. Four bytes (V5, J2, Z6, and Z7) are allocated for VT POH. The first byte of a VT SPE (i.e., the byte in the location pointed to by the VT Payload Pointer) is the V5 byte, while the J2, Z6, and Z7 bytes occupy the corresponding locations in the subsequent 125 microsecond frames of the VT Super-frame.

The V5 byte provides the same functions for VT paths that the B3, C2, and G1 bytes provide for STS paths; namely error checking, signal label, and path status.

SONET Multiplexing

SONET supports two multiplexing schemes:



Asynchronous Multiplexing

Asynchronous multiplexing uses multiple stages. Signals such as asynchronous DS-1s are multiplexed, extra bits are added (bit-stuffing) to account for the variations of each individual stream, and are combined with other bits (framing bits) to form a DS2 stream. Bit-stuffing is used again to multiplex up to DS-3. DS-3s are multiplexed up to higher rates in the same manner. At the higher asynchronous rate, they cannot be accessed without demultiplexing. When these signals are multiplexed to carry DS-3 signals, the signal consists of a combination of the following payloads:

28 DS-1s

14 DS-1s

7 DS-2s

M13 Format

M13 multiplex provides a digital interface between the DS-1 and DS-3 signal levels. M13 takes 28 DS-1 signals and combines them into a single DS-3 using a two-step process. In step one, 4 DS-1 signals are multiplexed using pulse stuffing synchronization to reach a 6.312 Mb/s DS-2 signal. Bit multiplexing is used and the bits are interleaved according to the input numbering order. The second step multiplexes 7 of the DS-2 signals using pulse stuffing synchronization to generate the DS-3 signal. Demultiplexing is also accomplished in a two-step process. In the first step, a DS-3 signal is decomposed into 7 DS-2 signals. In the second step, each of the DS-2 signals is decomposed into 4 DS-1 signals.

Synchronous Multiplexing

Synchronous multiplexing is the SONET process used when multiple lower-order path-layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into the Line Overhead. The multiplexing principles of SONET are:

Mapping - A process used when tributaries are adapted into Virtual Tributaries (VTs) by adding justification bits and Path Overhead (POH) information.

Aligning - This process takes place when a pointer is included in the STS Path or VT Path Overhead, to allow the first byte of the Virtual Tributary to be located.

Multiplexing - This process is used when multiple lower-order path-layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into the Line Overhead.

Stuffing - SONET has the ability to handle various input tributary rates from asynchronous signals. As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SONET frame to provide enough space for all these various tributary rates

Figure B-7 shows the basic multiplexing structure of SONET. Any type of service, ranging from voice to high-speed data and video, can be accepted by various types of service adapters. A service adapter maps the signal into the payload envelope of the STS-1 or virtual tributary (VT). New services and signals can be transported by adding new service adapters at the edge of the SONET network.

Figure B-7 SONET Multiplexing Hierarchy

Except for concatenated signals, all inputs are eventually converted to a base format of a synchronous STS-1 signal (51.84 Mb/s or higher). Lower speed inputs such as DS1s are first bit- or byte-multiplexed into virtual tributaries. Several synchronous STS-1s are then multiplexed together in either a single- or two-stage process to form an electrical STS-N signal (N = 1 or more).

STS multiplexing is performed at the Byte Interleave Synchronous Multiplexer. Basically, the bytes are interleaved together in a format such that the low-speed signals are visible. No additional signal processing occurs except a direct conversion from electrical to optical to form an OC-N signal.

SONET Network Configurations


The SONET multiplexer acts as a concentrator of DS-1s as well as other tributaries. Its simplest deployment involves two terminal multiplexers linked by fiber with or without a regenerator in the link. This implementation represents the simplest SONET configuration.

In the configuration shown in Figure B-8, the SONET path and the Service path (DS-1 or DS-3 links end-to-end) are identical and this synchronous island can exist within an asynchronous network world. Point-to-point service path connections can span across the whole network and will always originate and terminate in a multiplexer.

Figure B-8 Point-to-Point Network Configuration


The point-to-multipoint (linear add/drop) configuration shown in Figure B-9 includes adding and drop/ping circuits along the way. The SONET add/drop multiplexer (ADM) is a unique network element specifically designed for this task. It avoids the current cumbersome network architecture of demultiplexing, cross-connecting, adding and dropping channels, and than re-multiplexing. The ADM is typically placed along a SONET link to facilitate adding and dropping tributary channels at intermediate points in the network.

Figure B-9 Point-to-Multipoint Configuration


The hub configuration accommodates unexpected growth and change more easily than simple point-to-point networks. The hub configuration shown in Figure B-10 concentrates traffic at a central site and allows easy re-provisioning of the circuits. There are two possible implementations of this type of network:

Using two or more ADMs, and a wideband cross-connect switch which allows cross-connecting the tributary services at the tributary level.

Using a broadband digital cross-connect switch which allows cross-connecting at both the SONET level and the tributary level.

Figure B-10 Hub Architecture


The SONET building block for a ring configuration is the ADM. Multiple ADMs can be put into a ring configuration for either bi-directional or unidirectional traffic as shown in Figure B-11. The main advantage of the ring topology is its survivability; if a fiber cable is cut, the multiplexers have the intelligence to send the services affected via an alternate path through the ring without interruption.

The demand for survivable services, diverse routing of fiber facilities, flexibility to rearrange services to alternate serving nodes, as well as automatic restoration within seconds, have made rings a popular SONET topology.

Figure B-11 Ring Architecture

Benefits of SONET

The transport network using SONET provides much more powerful networking capabilities than existing asynchronous systems. The key benefits provided by SONET include the following:


As a result of SONET transmission, the network's clocks are referenced to a highly stable reference point. Therefore, the need to align the data streams or synchronize clocks is unnecessary. Therefore, a lower rate signal such as DS1 is accessible, and demultiplexing is not needed to access the bitstreams. Also, the signals can be stacked together without bit stuffing.

For those situations in which reference frequencies may vary, SONET uses pointers to allow the streams to "float" within the payload envelope. Synchronous clocking is the key to pointers. It allows a very flexible allocation and alignment of the payload within the transmission envelope.

Reduced Back-to-Back Multiplexing

Separate M13 multiplexers (DS-1 to DS-3) and fiber optic transmission system terminals are used to multiplex a DS-1 signal to a DS-2, DS-2 to DS-3, and then DS-3 to an optical line rate. The next stage is a mechanically integrated fiber/multiplex terminal.

In the existing asynchronous format, care must be taken when routing circuits in order to avoid multiplexing and demultiplexing too many times since electronics (and their associated capital cost) are required every time a DS-1 signal is processed. With SONET, DS-1s can be multiplexed directly to the OC-N rate. Because of synchronization, an entire optical signal doesn't have to be demultiplexed, only the VT or STS signals that need to be accessed.

Optical Interconnect

Because of different optical formats among vendors' asynchronous products, it's not possible to optically connect one vendor's fiber terminal to another. For example, one manufacturer may use 417 Mb/s line rate, another 565 Mb/s.

A major SONET value is that it allows mid-span meet with multi-vendor compatibility. Today's SONET standards contain definitions for fiber-to-fiber interfaces at the physical level. They determine the optical line rate, wavelength, power levels, pulse shapes, and coding. Current standards also fully define the frame structure, overhead, and payload mappings. Enhancements are being developed to define the messages in the overhead channels to provide increased

OAM&P Functionality

SONET allows optical interconnection between network providers regardless of who makes the equipment. The network provider can purchase one vendor's equipment and conveniently interface with other vendors' SONET equipment at either the different carrier locations or customer premises sites. Users may now obtain the OC-N equipment of their choice and meet with their network provider of choice at that OC-N level.

Multipoint Configurations

The difference between point-to-point and multipoint systems was shown previously in this appendix. Most existing asynchronous systems are only suitable for point-to-point, whereas SONET supports a multipoint or hub configuration.

A hub is an intermediate site from which traffic is distributed to three or more spurs. The hub allows the four nodes or sites to communicate as a single network instead of three separate systems. Hubbing reduces requirements for back-to-back multiplexing and demultiplexing, and helps realize the benefits of traffic grooming.

Network providers no longer need to own and maintain customer-located equipment. A multi-point implementation permits OC-N interconnects or midspan meet, allowing network providers and their customers to optimize their shared use of the SONET infrastructure.

Convergence of Voice, Data, and Video

Convergence is the trend toward delivery of voice, data, and video through diverse transmission and switching systems that supply high-speed transportation over any medium to any location. With its modular, service-independent architecture, SONET provides vast capabilities in terms of service flexibility.


Grooming refers to either consolidating or segregating traffic to make more efficient use of the facilities. Consolidation means combining traffic from different locations onto one facility.

Segregation is the separation of traffic. With existing systems, the cumbersome technique of back hauling might be used to reduce the expense of repeated multiplexing and demultiplexing.

Grooming eliminates inefficient techniques like back hauling. It's possible to groom traffic on asynchronous systems, however to do so requires expensive back-to-back configurations and manual DSX panels or electronic cross-connects. By contrast, a SONET system can segregate traffic at either an STS-1 or VT level to send it to the appropriate nodes.

Grooming can also provide segregation of services. For example, at an interconnect point, an incoming SONET line may contain different types of traffic, such as switched voice, data, or video. A SONET network can conveniently segregate the switched and non-switched traffic.

Reduced Cabling and Use of DSX Panels

Asynchronous systems are dominated by back-to-back terminals because the asynchronous Fiber optic transmission system architecture is inefficient for other than point-to-point networks. Excessive multiplexing and demultiplexing are used to transport a signal from one end to another, and many bays of DSX-1 cross-connect and DSX-3 panels are required to interconnect the systems. Associated expenses are the panel, bays, cabling, the labor installation, and the inconveniences of increased floor space and congested cable racks.

The corresponding SONET system allows a hub configuration, reducing the need for back-to-back terminals. Grooming is performed electronically so DSX panels are not used except when required to interface with existing asynchronous equipment.

Enhanced OAM&P

SONET allows integrated network OAM&P (also known as OA&M), in accordance with the philosophy of single-ended maintenance. In other words, one connection can reach all network elements (within a given architecture); separate links are not required for each network element. Remote provisioning provides centralized maintenance and reduced travel for maintenance personnel, which translates to expense savings.

Enhanced Performance Monitoring

Substantial overhead information is provided in SONET to allow quicker troubleshooting and detection of failures before they degrade to serious levels.

Convergence of SONET and SDH Hierarchies

SONET and SDH converge at SONET's 52 Mb/s base level, defined as STM-0 or "Synchronous Transport Module-0". The base level for SDH is STM-1 which is equivalent to SONET's STS-3 (3 x 51.84 Mb/s = 155.5 Mb/s). Higher SDH rates are STM-4 (622 Mb/s) and STM-16 (2.5 Gb/s). STM-64 (10 Gb/s) has also been defined.

Multiplexing is accomplished by combining (or interleaving) multiple lower-order signals (1.5 Mb/s, 2 Mb/s, etc.) into higher-speed circuits (52 Mb/s, 155 Mb/s, etc.). By changing the SONET standard from bit-interleaving to byte-interleaving, it became possible for SDH to accommodate both transmission hierarchies.


Following development of the SONET standard by ANSI, the CCITT undertook to define a synchronization standard that would address inter-working between the CCITT and ANSI transmission hierarchies. That effort culminated in 1989 with CCITT's publication of the Synchronous Digital Hierarchy (SDH) standards. Synchronous Digital Hierarchy is a world standard, and as such, SONET can be considered a subset of SDH.

Transmission standards in the U.S., Canada, Korea, Taiwan, and Hong Kong (ANSI) and the rest of the world (ITU-T, formerly CCITT) evolved from different basic rate signals in the non-synchronous hierarchy. ANSI Time Division Multiplexing (TDM) combines twenty-four 64-kbps channels (DS0s) into one 1.54-Mb/s DS1 signal. ITU-T TDM multiplexes thirty-two 64-kbps channels (E0s) into one 2.048 Mb/s E-1 signal.

The issues between ITU-T and ANSI standards-makers involved how to efficiently accommodate both the 1.5-Mb/s and the 2-Mb/s non-synchronous hierarchies in a single synchronization standard. The agreement reached specifies a basic transmission rate of 52 Mb/s for SONET and a basic rate of 155 Mb/s for SDH.

Asynchronous and Synchronous Tributaries

SDH does away with a number of the lower multiplexing levels, allowing non-synchronous 2-Mb/s tributaries to be multiplexed to the STM-1 level in a single step. SDH recommendations define methods of subdividing the payload area of an STM-1 frame in various ways so that it can carry combinations of synchronous and asynchronous tributaries. Using this method, synchronous transmission systems can accommodate signals generated by equipment operating from various levels of the non-synchronous hierarchy.

Synchronous and non-synchronous line rates and the relationships between each are shown in Table B-7 and Table B-8.

Table B-7 SONET/SDH Hierarchies

SONET Signal
Bit Rate (Mb/s)
SDH Signal
SONET Capacity
SDH Capacity

STS-1, OC-1



28 DS1s or 1 DS3

21 E1s

STS-3, OC-3



84 DS1s or 3 DS3s

63 E1s or 1 E4

STS-12, OC-12



336 DS1s or 12 DS3s

252 E1s or 4 E4s

STS-48, OC-48



1344 DS1s or 48 DS3s

1008 E1s or 16 E4s

STS-192, OC-192



5376 DS1s or 192 DS3s

4032 E1s or 64 E4s

Table B-8 Non-Synchronous Hierarchies 

ITU-T Rate
Bit Rate
Bit Rate


64 kb/s

1 DS0


64 kb/s

1 64-kb/s


1.544 Mb/s

24 DS0s


2.048 Mb/s

1 E1


6.312 Mb/s

96 DS0s


8.45 Mb/s

4 E1s


44.7 Mb/s

28 DS1s


34 Mb/s

16 E1s

Not Defined


144 Mb/s

64 E1s