Synchronous optical networking

Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH), are two closely related multiplexing protocols for transferring multiple digital bit streams using lasers or light-emitting diodes (LEDs) over the same optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fiber wire without synchronization problems.

SONET and SDH are based on circuit mode communication, meaning that each connection achieves a constant bit rate and delay. For example, SDH or SONET may be utilized to allow several Internet Service Providers to share the same optical fiber, without being affected by each other's traffic load, and without being able to temporarily borrow free capacity from each other. Only certain integer multiples of 64 kbit/s are possible bit rates.

Since SONET and SDH are characterized as pure time division multiplexing (TDM) protocols (not to be confused with Time Division Multiple Access, TDMA), offering permanent connections, and do not involve packet mode communication, they are considered as physical layer protocols.

Both SDH and SONET are widely used today: SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET now is considered the variation.

The two protocols are standardized according to the following:

* SDH or Synchronous Digital Hierarchy standard developed by the International Telecommunication Union (ITU), documented in standard G.707 and its extension G.708
* SONET or Synchronous Optical Networking standard as defined by GR-253-CORE from Telcordia and T1.105 from American National Standards Institute

Difference from PDH

Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either Asynchronous Transfer Mode (ATM) or so-called Packet over SONET/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves, but rather as generic and all-purpose transport containers for moving both voice and data. The basic format of an SDH signal allows it to carry many different services in its Virtual Container (VC) because it is bandwidth-flexible.

Structure of SONET/SDH signals

SONET and SDH often use different terms to describe identical features or functions, sometimes leading to confusion that exaggerates their differences. With a few exceptions, SDH can be thought of as a superset of SONET. The two main differences between the two:
* SONET can use either of two basic units for framing while SDH has one
* SDH has additional mapping options which are not available in SONET.

Protocol overview

The protocol is an extremely heavily multiplexed structure, with the header interleaved between the data in a complex way. This is intended to permit the encapsulated data to have its own frame rate and to be able to float around relative to the SDH/SONET frame structure and rate. This interleaving permits a very low latency for the encapsulated data- data passing through equipment can be delayed by at most 32 microseconds, compared to a frame rate of 125 microseconds; many competing protocols buffer the data for at least one frame or packet before sending it on. Extra padding is allowed for the multiplexed data to move within the overall framing due to it being on a different clock to the frame rate, and the decision to allow this at most of the levels of the multiplexing structure makes the protocol complex, but gives high all-round performance.

The basic unit of transmission

The basic unit of framing in SDH is a STM-1 (Synchronous Transport Module level - 1), which operates at 155.52 Mbit/s. SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal - 3, concatenated), but its high-level functionality, frame size, and bit-rate are the same as STM-1.

SONET offers an additional basic unit of transmission, the STS-1 (Synchronous Transport Signal - 1), operating at 51.84 Mbit/s - exactly one third of an STM-1/STS-3c. Some manufacturers also support the SDH equivalent STM-0, but this is not part of the standard.


In packet oriented data transmission such as Ethernet, a packet frame usually consists of a header and a payload, with the header of the frame being transmitted first, followed by the payload (and possibly a trailer, such as a CRC). In synchronous optical networking, this is modified slightly. The header is termed the overhead and the payload still exists, but instead of the overhead being transmitted before the payload, it is interleaved, with part of the overhead being transmitted, then part of the payload, then the next part of the overhead, then the next part of the payload, until the entire frame has been transmitted. In the case of an STS-1, the frame is 810 octets in size while the STM-1/STS-3c frame is 2430 octets in size. For STS-1, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload. This is repeated nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of an STS-3c/STM-1 which operates three times faster than STS-1, 9 octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times over until 2,430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH, this is normally represented by the frame being displayed graphically as a block: of 90 columns and 9 rows for STS-1; and 270 columns and 9 rows for STM1/STS-3c. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.

The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures. However, the standards are extremely similar in implementation, such that it is easy to interoperate between SDH and SONET at particular bandwidths.

It is worth noting that the choice of a 125 microsecond interval is not an arbitrary one. What it means is that the same octet position in each frame comes past every 125 microseconds. If one octet is extracted from the bitstream every 125 microseconds, this gives a data rate of 8 bits per 125 microseconds - or 64 kbit/s, the basic DS0 telecommunications rate. This relation allows an extremely useful behaviour of synchronous optical networking, which is that low data rate channels or streams of data can be extracted from high data rate streams by simply extracting octets at regular time intervals - there is no need to understand or decode the entire frame. This is not possible in PDH networking. Furthermore, it shows that a relatively simple device is all that is needed to extract a datastream from an SDH framed connection and insert it into a SONET framed connection and vice versa.

In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

SDH Frame

The STM-1 (Synchronous Transport Module level - 1) frame is the basic transmission format for SDH or the fundamental frame or the first level of the synchronous digital hierarchy. The STS-1 frame is transmitted in exactly 125 microseconds, therefore there are 8000 frames per second on a fiber-optic circuit designated OC-1 (optical carrier one). The STM-1 frame consists of overhead plus a virtual container capacity. The first 9 columns of each frame make up the Section Overhead, and the last 261 columns make up the Virtual Container (VC) capacity. The VC plus the pointers (H1, H2, H3 bytes) is called the AU (Administrative Unit).

Carried within the VC capacity, which has its own frame structure of 9 rows and 261 columns, is the Path Overhead and the Container. The first column is for Path Overhead; it’s followed by the payload container, which can itself carry other containers. Virtual Containers can have any phase alignment within the Administrative Unit, and this alignment is indicated by the Pointer in row four,

The Section overhead of an STM-1 signal (SOH) is divided into two parts: the Regenerator Section Overhead (RSOH) and the Multiplex Section Overhead (MSOH). The overheads contain information from the system itself, which is used for a wide range of management functions, such as monitoring transmission quality, detecting failures, managing alarms, data communication channels, service channels, etc.

The STM frame is continuous and is transmitted in a serial fashion, byte-by-byte, row-by-row.

STM–1 frame contains

* Total content : 9 x 270 bytes = 2430 bytes:*overhead : 9 rows x 9 bytes :*payload : 9 rows x 261 bytes

* Period : 125 μsec
* Bitrate : 155.520 Mbit/s (2430 x 8 bits x 8000 frame/s ):*payload capacity : 150.336 Mbit/s (2349 x 8 bits x 8000 frame/s)

The transmission of the frame is done row by row, from the top left corner.

Framing Structure

The frame consists of two parts, the transport overhead and the path virtual envelope.

Transport overhead

The transport overhead is used for signaling and measuring transmission error rates, and is composed as follows:

*Section overhead - called RSOH (Regenerator Section Overhead) in SDH terminology: 27 octets containing information about the frame structure required by the terminal equipment.
*Line overhead - called MSOH (Multiplex Section Overhead) in SDH: 45 octets containing information about alarms, maintenance and error correction as may be required within the network.
*Pointer – It points to the location of the J1 byte in the payload.

Path virtual envelope

Data transmitted from end to end is referred to as path data. It is composed of two components:
*Payload overhead (POH): 9 bytes used for end to end signaling and error measurement.
*Payload: user data (774 bytes for STS-1, or 2349 bytes for STM-1/STS-3c)

For STS-1, the payload is referred to as the synchronous payload envelope (SPE), which in turn has 18 stuffing bytes, leading to the STS-1 payload capacity of 756 bytes. [ [ International Engineering Consortium SONET Tutorial] , undated, URL retrieved on 21 April 2007]

The STS-1 payload is designed to carry a full PDH DS3 frame. When the DS3 enters a SONET network, path overhead is added, and that SONET network element (NE) is said to be a path generator and terminator. The SONET NE is said to be line terminating if it processes the line overhead. Note that wherever the line or path is terminated, the section is terminated also. SONET Regenerators terminate the section but not the paths or line.

An STS-1 payload can also be subdivided into 7 VTGs, or Virtual Tributary Groups. Each VTG can then be subdivided into 4 VT1.5 signals, each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into 3 VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is a TUG2; VT1.5 is equivalent to VC11, and VT2 is equivalent to VC12.

Three STS-1 signals may be multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2,430 bytes and transmitted in 125 microseconds.

Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four STS-3 or AU4 signals can be aggregated to form a 622.08 Mbit/s signal designated as OC-12 or STM-4.

The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are technically viable and are under evaluation. [Few vendors are offering STM-256 rates now, with speeds of nearly 40Gbit/s] . Where fiber exhaust is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of Wavelength division multiplexing, including Dense Wave Division Multiplexing (DWDM) and Coarse Wave Division Multiplexing (CWDM). DWDM circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.

SONET/SDH and relationship to 10 Gigabit Ethernet

Another circuit type amongst data networking equipment is 10 Gigabit Ethernet (10GbE). This is similar to the line rate of OC-192/STM-64 (9.953 Gbit/s). The Gigabit Ethernet Alliance created two 10 Gigabit Ethernet variants: a local area variant (LAN PHY), with a line rate of exactly 10,000,000 kbit/s and a wide area variant (WAN PHY), with the same line rate as OC-192/STM-64 (9,953,280 kbit/s). The Ethernet wide area variant encapsulates its data using a light-weight SDH/SONET frame so as to be compatible at low level with equipment designed to carry those signals.

However, 10 Gigabit Ethernet does not explicitly provide any interoperability at the bitstream level with other SDH/SONET systems. This differs from WDM System Transponders, including both Coarse- and Dense-WDM systems (CWDM, DWDM) that currently support OC-192 SONET Signals, which can normally support thin-SONET framed 10 Gigabit Ethernet.

SONET/SDH data rates

In the above table, Payload bandwidth is the line rate less the bandwidth of the line and section overheads. User throughput must also deduct path overhead from this, but path overhead bandwidth is variable based on the types of cross-connects built across the optical system.

Note that the data rate progression starts at 155Mb/s and increases by multiples of 4. The only exception is OC-24 which is standardised in ANSI T1.105, but not a SDH standard rate in ITU-T G.707. STM-0 is not a standard SDH rate in G.707 but is commonly used for compatibility with STS/OC-1. Other rates such as OC-9, OC-18, OC-36, and OC-96, and OC-1536 are sometimes described, but it is not clear if they were ever deployed, and are certainly not common, and are not standards compliant.

The next logical rate of 160 Gb/s OC-3072/STM-1024 has not yet been standardised, due to the cost of high-rate transceivers and the ability to more cheaply multiplex wavelengths at 10 and 40 Gb/s.

Physical layer

The physical layer actually comprises a large number of layers within it, only one of which is the optical/transmission layer (which includes bitrates, jitter specifications, optical signal specifications and so on). The SONET and SDH standards come with a host of features for isolating and identifying signal defects and their origins.

SONET/SDH Network Management Protocols

SONET equipment is often managed with the TL1 protocol. TL1 is a traditional telecom language for managing and reconfiguring SONET network elements. TL1 (or whatever command language a SONET Network Element utilizes) must be carried by other management protocols, including SNMP, CORBA and XML.

There are some features that are fairly universal in SONET Network Management. First of all, most SONET NEs have a limited number of management interfaces defined. These are:

* Electrical Interface. The electrical interface (often 50 Ω) sends SONET TL1 commands from a local management network physically housed in the Central Office where the SONET NE is located. This is for "local management" of that NE and, possibly, remote management of other SONET NEs.

* Craft Interface. Local "craftspersons" can access a SONET NE on a "craft port" and issue commands through a dumb terminal or terminal emulation program running on a laptop. This interface can also be hooked-up to a console server, allowing for remote out-of-band management and logging.

* SONET and SDH have dedicated Data Communication Channels (DCC)s within the section and line overhead for management traffic. Generally, section overhead (regenerator section in SDH) is used. According to ITU-T G.7712, there are three modes used for management:

:*IP-only stack, using PPP as data-link:*OSI-only stack, using LAP-D as data-link:*Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate between stacks.

An interesting fact about modern NEs is that, to handle all of the possible management channels and signals, most NEs actually contain a router for routing the network commands and underlying (data) protocols.

The main functions of Network Management include:

* Network and NE Provisioning. In order to allocate bandwidth throughout a network, each NE must be configured. Although this can be done locally, through a craft interface, it is normally done through a Network Management System (sitting at a higher layer) that in turn operates through the SONET/SDH Network Management Network.

* Software Upgrade. NE Software Upgrade is in modern NEs done mostly through the SONET/SDH Management network.

* Performance Management. NEs have a very large set of standards for Performance Management. The PM criteria allow for monitoring not only the health of individual NEs, but for the isolation and identification of most network defects or outages. Higher-layer Network monitoring and management software allows for the proper filtering and troubleshooting of network-wide PM so that defects and outages can be quickly identified and responded to.


With recent advances in SONET and SDH chipsets, the traditional categories of NEs are breaking down. Nevertheless, as Network architectures have remained relatively constant, even newer equipment (including "Multiservice Provisioning Platforms") can be examined in light of the architectures they will support. Thus, there is value in viewing new (as well as traditional) equipment in terms of the older categories.


Traditional regenerators terminate the section overhead, but not the line or path. Regens extend long haul routes in a way similar to most regenerators, by converting an optical signal that has already traveled a long distance into electrical format and then retransmitting a regenerated high-power signal.

Since the late 1990s, regenerators have been largely replaced by Optical Amplifiers. Also, some of the functionality of regens has been absorbed by the Transponders of Wavelength Division Multiplexing systems.

add-drop multiplexer (ADM)

ADMs are the most common type of NEs. Traditional ADMs were designed to support one of the Network Architectures, though new generation systems can often support several architectures, sometimes simultaneously. ADMs traditionally have a "high speed side" (where the full line rate signal is supported), and a "low speed side", which can consist of electrical as well as optical interfaces. The low speed side takes in low speed signals which are multiplexed by the NE and sent out from the high speed side, or vice versa.

Digital Cross Connect system

Recent Digital Cross Connect systems (DCSs or DXCs) support numerous high-speed signals, and allow for cross connection of DS1s, DS3s and even STS-3s/12c and so on, from any input to any output. Advanced DCSs can support numerous subtending rings simultaneously.

Network Architectures

Currently, SONET (and SDH) have a limited number of architectures defined. These architectures allow for efficient bandwidth usage as well as protection (i.e. the ability to transmit traffic even when part of the network has failed), and are key in understanding the almost worldwide usage of SONET and SDH for moving digital traffic. The three main architectures are:

* Linear APS (Automatic Protection Switching), also known as 1+1: This involves 4 fibers: 2 working fibers in each direction, and two protection fibers. Switching is based on the line state, and may be unidirectional, with each direction switching independently, or bidirectional, where the NEs at each end negotiate so that both directions are generally carried on the same pair of fibers.
* UPSR (Unidirectional Path Switched Ring): In a UPSR, two redundant (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines the higher-quality copy and decides to use the best copy, thus coping if deterioration in one copy occurs due to a broken fiber or other failure. UPSRs tend to sit nearer to the edge of a network and, as such, are sometimes called "collector rings". Because the same data is sent around the ring in both directions, the total capacity of a [ UPSR] is equal to the line rate N of the OC-N ring. For example if we had an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, then 100% of the ring bandwidth (N=3) would be consumed by nodes A and D. Any other nodes on the ring, say B and C could only act as pass through nodes. The SDH analog of UPSR is Subnetwork Connection Protection (SNCP); however, SNCP does not impose a ring topology, but may also be used in mesh topologies.

* BLSR (Bidirectional Line Switched Ring): BLSR comes in two varieties, 2-fiber BLSR and 4-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies from ingress to egress. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring. BLSRs trade cost and complexity for bandwdith efficiency as well as the ability to support "extra traffic", which can be pre-empted when a protection switching event occurs. BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities. Because a [ BLSR] does not send redundant copies from ingress to egress the total bandwidth that a BLSR can support is not limited to the line rate N of the OC-N ring and can actually be larger than N depending upon the traffic pattern on the ring. The best case of this is that all traffic is between adjacent nodes. The worst case is when all traffic on the ring egresses from a single node, i.e. the BLSR is serving as a collector ring. In this case the bandwidth that the ring can support is equal to the line rate N of the OC-N ring. This is why BLSRs are seldom if ever deployed in collector rings but often deployed in inter-office rings. The SDH equivalent of BLSR is called Multiplex Section-Shared Protection Ring (MS-SPRING).


Clock sources used by Synchronization in telecommunications networks are rated by quality, commonly called a 'stratum' level. Typically, a network element uses the highest quality stratum available to it, which can be determined by monitoring the Synchronization Status Messages(SSM) of selected clock sources.

As for Synchronization sources available to an NE, these are:
* Local External Timing. This is generated by an atomic Caesium clock or a satellite-derived clock by a device located in the same central office as the NE. The interface is often a DS1, with Sync Status Messages supplied by the clock and placed into the DS1 overhead.
* Line-derived timing. An NE can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 sync status bytes to ensure quality.
*Holdover. As a last resort, in the absence of higher quality timing, an NE can go into "holdover" until higher quality external timing becomes available again. In this mode, an NE uses its own timing circuits as a reference.

Timing loops

A timing loop occurs when NEs in a network are each deriving their timing from other NEs, without any of them being a "master" timing source. This network loop will eventually see its own timing "float away" from any external networks, causing mysterious bit errors and ultimately, in the worst cases, massive loss of traffic. The source of these kinds of errors can be hard to diagnose. In general, a network that has been properly configured should never find itself in a timing loop, but some classes of silent failures could nevertheless cause this issue.

Next-generation SONET/SDH

SONET/SDH development was originally driven by the need to transport multiple PDH signals like DS1, E1, DS3 and E3 along with other groups of multiplexed 64 kbit/s pulse-code modulated voice traffic. The ability to transport ATM traffic was another early application. In order to support large ATM bandwidths, the technique of concatenation was developed, whereby smaller multiplexing containers (eg, STS-1) are inversely multiplexed to build up a larger container (eg, STS-3c) to support large data-oriented pipes.

One problem with traditional concatenation, however, is inflexibility. Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbit/s Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to considerable waste.

Virtual Concatenation (VCAT) allows for a more arbitrary assembly of lower order multiplexing containers, building larger containers of fairly arbitrary size (e.g. 100 Mbit/s) without the need for intermediate NEs to support this particular form of concatenation. Virtual Concatenation increasingly leverages X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container.

Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamic virtual concatenation, multiplexing containers based on the short-term bandwidth needs in the network.

The set of next generation SONET/SDH protocols to enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).

See also

* Routing Wavelength Assignment (RWA)
* List of device bandwidths


External links

* [ Understanding SONET/SDH]
* [ The Queen's University of Belfast SDH/SONET Primer]
* [ SDH Pocket Handbook from Acterna/JDSU]
* [ SONET Pocket Handbook from Acterna/JDSU]
* [ The Sonet Homepage]
* [ SONET Interoperability Form (SIF)]
* [ Network Connection Speeds Reference]
* [ Next-generation SDH and MSPP]
* [ The Future of SONET/SDH] (pdf)


* [ Telcordia GR 253 CORE: SONET Transport Systems: Common Generic Criteria]
* [ ANSI T1.105: SONET - Basic Description including Multiplex Structure, Rates and Formats]
* [ ANSI T1.119/ATIS PP 0900119.01.2006: SONET - Operations, Administration, Maintenance, and Provisioning (OAM&P) - Communications]
* [ ITU-T recommendation G.707: Network Node Interface for the Synchronous Digital Hierarchy (SDH)]
* [ ITU-T recommendation G.783: Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks]
* [ ITU-T recommendation G.803: Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH)]

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