Synchronous optical networking

Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates, data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization.

SONET and SDH, which are essentially the same, were originally designed to transport circuit mode communications, e.g. DS1, DS3, from a variety of different sources. However, they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The STM-1 frame consists of overhead and pointers plus information payload. The first nine columns of each frame make up the section overhead and administrative unit pointers, and the last 261 columns make up the information payload. The pointers (H1, H2, H3 bytes) identify administrative units (AU) within the information payload. Thus, an OC-3 circuit can carry 150.336 Mbit/s of payload, after accounting for the overhead.

Carried within the information payload, which has its own frame structure of nine rows and 261 columns, are administrative units identified by pointers. Also within the administrative unit are one or more virtual containers (VCs). VCs contain path overhead and VC payload. The first column is for path overhead; it is followed by the payload container, which can itself carry other containers. Administrative units can have any phase alignment within the STM frame, and this alignment is indicated by the pointer in row four.

The section overhead (SOH) of a STM-1 signal is divided into two parts: the regenerator section overhead (RSOH) and the multiplex section overhead (MSOH). The overheads contain information from the transmission 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.

Transport overhead

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

:;Section overhead: Called regenerator section overhead (RSOH) in SDH terminology: 27 octets containing information about the frame structure required by the terminal equipment.

:;Line overhead: Called multiplex section overhead (MSOH) in SDH: 45 octets containing information about error correction and Automatic Protection Switching messages (e.g., alarms and maintenance messages) as may be required within the network. The error correction is included for STM-16 and above.

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 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 seven virtual tributary groups (VTGs). Each VTG can then be subdivided into four VT1.5 signals, each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into three VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is a TUG-2; VT1.5 is equivalent to VC-11, and VT2 is equivalent to VC-12.

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 signal is multiplexed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2,430 bytes and transmitted in 125 μs.

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 OC-12 or STM-4.

The highest rate commonly deployed is the OC-768 or STM-256 circuit, which operates at rate of just under 38.5 Gbit/s.

SONET/SDH and relationship to 10 Gigabit Ethernet

Another type of high-speed data networking circuit is 10 Gigabit Ethernet (10GbE). The Gigabit Ethernet Alliance created two 10 Gigabit Ethernet variants: a local area variant (LAN PHY) with a line rate of 10.3125 Gbit/s, and a wide area variant (WAN PHY) with the same line rate as OC-192/STM-64 (9,953,280 kbit/s).

The WAN PHY variant encapsulates Ethernet data using a lightweight SDH/SONET frame, so as to be compatible at a low level with equipment designed to carry SDH/SONET signals, whereas the LAN PHY variant encapsulates Ethernet data using 64B/66B line coding.

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 wavelength-division multiplexing systems (CWDM and DWDM) that currently support OC-192 SONET signals, which can normally support thin-SONET–framed 10 Gigabit Ethernet.

SONET/SDH data rates

<!---

-- ATTENTION:

-- The numbers in this table are in kbit/s.

-- Please do not change the units to Mbit/s.

-- Per http://en.wikipedia.org/wiki/MOS:NUM#Large_numbers

-- numbers in Wikipedia use commas to break the sequence every three places left of the decimal point

--->

{| class="wikitable" style="font-size:95%;"

|+ SONET/SDH Designations and bandwidths

! SONET Optical Carrier level

! SONET frame format

! SDH level and frame format

! Payload bandwidth (kbit/s)

! Line rate (kbit/s)

| OC-1

| STS-1

| STM-0

| align=right |50,112

| align=right |51,840

| OC-3

| STS-3

| STM-1

| align=right |150,336

| align=right |155,520

| OC-12

| STS-12

| STM-4

| align=right |601,344

| align=right |622,080

| OC-24

| STS-24

| –

| align=right |1,202,688

| align=right |1,244,160

| OC-48

| STS-48

| STM-16

| align=right |2,405,376

| align=right |2,488,320

| OC-192

| STS-192

| STM-64

| align=right |9,621,504

| align=right |9,953,280

| OC-768

| STS-768

| STM-256

| align=right |38,486,016

| align=right |39,813,120

User throughput must not deduct path overhead from the payload bandwidth, 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 155 Mbit/s and increases by multiples of four. The only exception is OC-24, which is standardized in ANSI T1.105, but not a SDH standard rate in ITU-T G.707. The ATM and SDH layers are the regenerator section level, digital line level, transmission path level, virtual path level, and virtual channel level. The physical layer is modeled on three major entities: transmission path, digital line and the regenerator section. The regenerator section refers to the section and photonic layers. The photonic layer is the lowest SONET layer and it is responsible for transmitting the bits to the physical medium. The section layer is responsible for generating the proper STS-N frames which are to be transmitted across the physical medium. It deals with issues such as proper framing, error monitoring, section maintenance, and orderwire.

The line layer ensures reliable transport of the payload and overhead generated by the path layer. It provides synchronization and multiplexing for multiple paths. It modifies overhead bits relating to quality control. The path layer is SONET's highest level layer. It takes data to be transmitted and transforms them into signals required by the line layer, and adds or modifies the path overhead bits for performance monitoring and protection switching.

SONET/SDH network management protocols

Overall functionality

Network management systems are used to configure and monitor SDH and SONET equipment either locally or remotely.

The systems consist of three essential parts, covered later in more detail:

Software running on a network management system terminal, e.g. workstation, dumb terminal or laptop housed in an exchange/central office.
Transport of network management data between the network management system terminal and the SONET/SDH equipment, e.g. using TL1/Q3 protocols.
Transport of network management data between SDH/SONET equipment using dedicated embedded data communication channels (DCCs) within the section and line overhead.

The main functions of network management thereby include:

;Network and network-element provisioning

:In order to allocate bandwidth throughout a network, each network element 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

:Network-element software upgrades are done mostly through the SONET/SDH management network in modern equipment.

;Performance management

:Network elements have a very large set of standards for performance management. The performance-management criteria allow not only monitoring the health of individual network elements, but isolating and identifying most network defects or outages. Higher-layer network monitoring and management software allows the proper filtering and troubleshooting of network-wide performance management, so that defects and outages can be quickly identified and resolved.

Consider the three parts defined above:

Network management system terminal

;Local Craft interface

:Local "craftspersons" (telephone network engineers) can access a SDH/SONET network element on a "craft port" and issue commands through a dumb terminal or terminal emulation program running on a laptop. This interface can also be attached to a console server, allowing for remote out-of-band management and logging.

; Network management system (sitting at a higher layer)

This will often consist of software running on a Workstation covering a number of SDH/SONET network elements

TL1/Q3 Protocols

;TL1

SONET equipment is often managed with the TL1 protocol. TL1 is a telecom language for managing and reconfiguring SONET network elements. The command language used by a SONET network element, such as TL1, must be carried by other management protocols, such as SNMP, CORBA, or XML.

;Q3

SDH has been mainly managed using the Q3 interface protocol suite defined in ITU recommendations Q.811 and Q.812. With the convergence of SONET and SDH on switching matrix and network elements architecture, newer implementations have also offered TL1.

Most SONET NEs have a limited number of management interfaces defined:

;TL1 Electrical interface

:The electrical interface, often a 50-ohm coaxial cable, sends SONET TL1 commands from a local management network physically housed in the central office where the SONET network element is located. This is for local management of that network element and, possibly, remote management of other SONET network elements.

Dedicated embedded data communication channels (DCCs)

: SONET and SDH have dedicated data communication channels (DCCs) 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: 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.

Synchronization sources available to a network element are:

;Local external timing

:This is generated by an atomic cesium clock or a satellite-derived clock by a device in the same central office as the network element. The interface is often a DS1, with sync-status messages supplied by the clock and placed into the DS1 overhead.

;Line-derived timing

:A network element 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, a network element can go into a holdover mode until higher-quality external timing becomes available again. In this mode, the network element uses its own timing circuits as a reference.

Timing loops

A timing loop occurs when network elements in a network are each deriving their timing from other network elements, 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, concatenation was developed, whereby smaller multiplexing containers (e.g., STS-1) are inversely multiplexed to build up a larger container (e.g., 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. More important is the need for all intermediate network elements to support newly introduced concatenation sizes. This problem was overcome with the introduction of Virtual Concatenation.

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 network elements to support this particular form of concatenation. Virtual concatenation leverages the X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container.

The 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 that enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).

End of life and retirement

SONET/SDH was used by internet access providers for large customers, and is no longer competitive in the supply of private circuits. Development has stagnated for the last decade (2020) and both suppliers of equipment and operators of SONET/SDH networks are migrating to other technologies such as OTN and wide area Ethernet.

British Telecom has recently (March 2020) closed down their KiloStream and Mega Stream products which were the last large scale uses of the BT SDH. BT has also ceased new connections to their SDH network which indicates withdrawal of services soon.

Notes

References

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: the future looks bright
The Future of SONET/SDH (pdf)
Telcordia GR-253-CORE, SONET Transport Systems: Common Generic Criteria
Telcordia GR-499-CORE, Transport Systems Generic Requirements (TSGR): Common Requirements
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)