Synchronous optical networking
Encyclopedia
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing
protocols that transfer multiple digital
bit streams over optical fiber
using laser
s or highly coherent light from light-emitting diode
s (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 synchronization problems. SONET generic criteria are detailed in Telcordia Technologies
Generic Requirements document GR-253-CORE. Generic criteria applicable to SONET and other transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found in Telcordia GR-499-CORE.
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, but they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The primary difficulty in doing this prior to SONET/SDH was that the synchronization sources of these various circuits were different. This meant that each circuit was actually operating at a slightly different rate and with different phase. SONET/SDH allowed for the simultaneous transport of many different circuits of differing origin within a single framing protocol. SONET/SDH is not itself a communications protocol per se, but a transport protocol.
Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the obvious choice for transporting Asynchronous Transfer Mode
(ATM) frames. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as Ethernet
), the internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as OC-3c) into which ATM cells, IP packets, or Ethernet frames are placed.
Both SDH and SONET are widely used today: SONET in the United States
and Canada
, and SDH in the rest of the world. Although the SONET standards were developed before SDH, it is considered a variation of SDH because of SDH's greater worldwide market penetration.
The SDH standard was originally defined by the European Telecommunications Standards Institute
(ETSI), and is formalized as International Telecommunications Union (ITU) standards G.707, G.783, G.784, and G.803. The SONET standard was defined by Telcordia and American National Standards Institute
(ANSI) standard T1.105.
(PDH) in that the exact rates that are used to transport the data on SONET/SDH are tightly synchronized
across the entire network, using atomic clock
s. 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 they can be used to directly 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; they are generic, all-purpose transport containers for moving both voice and data. The basic format of a SONET/SDH signal allows it to carry many different services in its virtual container (VC), because it is bandwidth-flexible.
The protocol is a heavily-multiplexed structure, with the header interleaved between the data in a complex way. This permits the encapsulated data to have its own frame rate and 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 microsecond
s (µs), compared to a frame rate of 125 µs
; many competing protocols buffer the data during such transits 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, as the data is clocked at a different rate than the frame rate. The protocol is made more complex by the decision to permit this padding at most levels of the multiplexing structure, but it improves all-around performance.
(Synchronous Transport Module, level 1), which operates at 155.520 megabits per second (Mbit/s). SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal 3, concatenated) or OC-3c, depending on whether the signal is carried electrically (STS) or optically (OC), 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) or OC-1, operating at 51.84 Mbit/s—exactly one third of an STM-1/STS-3c/OC-3c carrier. This speed is dictated by the bandwidth requirements for PCM-encoded telephonic voice signals: at this rate, an STS-1/OC-1 circuit can carry the bandwidth equivalent of a standard DS-3 channel, which can carry 672 64-kbit/s voice channels. In SONET, the STS-3c/OC-3c signal is composed of three multiplexed STS-1 signals; the STS-3C/OC-3c may be carried on an OC-3 signal. Some manufacturers also support the SDH equivalent of the STS-1/OC-1, known as STM-0.
, a packet frame usually consists of a header and a payload. The header is 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 instead of being transmitted before the payload, is interleaved with it during transmission. Part of the overhead is 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 2,430 octets in size. For STS-1, the frame is transmitted as three octets of overhead, followed by 87 octets of payload. This is repeated nine times, until 810 octets have been transmitted, taking 125 µs
. In the case of an STS-3c/STM-1, which operates three times faster than an STS-1, nine octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times until 2,430 octets have been transmitted, also taking 125 µs
. For both SONET and SDH, this is often represented by displaying the frame graphically: as a block of 90 columns and nine rows for STS-1, and 270 columns and nine 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. Their standards are extremely similar in implementation, making it easy to interoperate between SDH and SONET at any given bandwidth.
In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC designation 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.
The STM-1 (Synchronous Transport Module, level 1) frame is the basic transmission format for SDH—the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted in exactly 125 µs
, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic circuit.2,430 octets × 8 bits per octet × 8,000 frames per second = 155.52 Mbit/s 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.2,349 octets of payload × 8 bits per octet × 8,000 frames per second = 150.336 Mbit/s
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.
Section overhead
Line overhead
AU Pointer
Payload overhead (POH)
Payload
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.
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 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 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. Where fiber exhaustion is a concern, multiple SONET signals can be transported over multiple wavelengths on a single fiber pair by means of wavelength-division multiplexing
, including dense wavelength-division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). DWDM circuits are the basis for all modern submarine communications cable
systems and other long-haul circuits.
(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.
User throughput must also 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. Other rates, such as OC-9, OC-18, OC-36, OC-96, and OC-1536, are defined but not commonly deployed; most are considered orphaned rates.
The next rate level of 160 Gbit/s OC-3072/STM-1024 has not yet been standardized, due to the cost of high-rate transceivers and the ability to more cheaply multiplex wavelengths at 10 and 40 Gbit/s.
actually comprises a large number of sublayers, only one of which is the optical transmission layer. The SONET and SDH standards come with a host of features for isolating and identifying signal defects and their origins.
, CORBA
, or XML
.
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 NE
s have a limited number of management interfaces defined:
Electrical interface
Craft interface
Data communication channels (DCCs)
To handle all of the possible management channels and signals, most modern network elements contain a router for the network commands and underlying (data) protocols.
The main functions of network management include:
Network and network-element provisioning
Software upgrade
Performance management
) 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.
Since the late 1990s, regenerators have been largely replaced by optical amplifier
s. Also, some of the functionality of regenerators has been absorbed by the transponders of wavelength-division multiplexing systems.
s (ADMs) are the most common type of network elements. 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 network element and sent out from the high-speed side, or vice-versa.
s (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.
(SNCP); SNCP does not impose a ring topology, but may also be used in mesh topologies.
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. In the best case, 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).
Synchronization sources available to a network element are:
Local external timing
Line-derived timing
Holdover
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).
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
protocols that transfer multiple digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
bit streams over optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
using laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s or highly coherent light from light-emitting diode
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
s (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital Hierarchy
Plesiochronous Digital Hierarchy
The Plesiochronous Digital Hierarchy is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems...
(PDH) system for transporting large amounts of telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
calls and data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
traffic over the same fiber without synchronization problems. SONET generic criteria are detailed in Telcordia Technologies
Telcordia Technologies
Telcordia Technologies, formerly Bell Communications Research, Inc. or Bellcore, is a telecommunications research and development company based in the United States created as part of the 1982 Modification of Final Judgment that broke up American Telephone & Telegraph...
Generic Requirements document GR-253-CORE. Generic criteria applicable to SONET and other transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found in Telcordia GR-499-CORE.
SONET and SDH, which are essentially the same, were originally designed to transport circuit mode communications (e.g., DS1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
, DS3
Digital Signal 3
A Digital Signal 3 is a digital signal level 3 T-carrier. It may also be referred to as a T3 line.*The data rate for this type of signal is 44.736 Mbit/s.*This level of carrier can transport 28 DS1 level signals within its payload....
) from a variety of different sources, but they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The primary difficulty in doing this prior to SONET/SDH was that the synchronization sources of these various circuits were different. This meant that each circuit was actually operating at a slightly different rate and with different phase. SONET/SDH allowed for the simultaneous transport of many different circuits of differing origin within a single framing protocol. SONET/SDH is not itself a communications protocol per se, but a transport protocol.
Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the obvious choice for transporting Asynchronous Transfer Mode
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM) frames. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
), the internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as OC-3c) into which ATM cells, IP packets, or Ethernet frames are placed.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, and SDH in the rest of the world. Although the SONET standards were developed before SDH, it is considered a variation of SDH because of SDH's greater worldwide market penetration.
The SDH standard was originally defined by the European Telecommunications Standards Institute
European Telecommunications Standards Institute
The European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
(ETSI), and is formalized as International Telecommunications Union (ITU) standards G.707, G.783, G.784, and G.803. The SONET standard was defined by Telcordia and American National Standards Institute
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
(ANSI) standard T1.105.
Difference from PDH
SDH differs from Plesiochronous Digital HierarchyPlesiochronous Digital Hierarchy
The Plesiochronous Digital Hierarchy is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems...
(PDH) in that the exact rates that are used to transport the data on SONET/SDH are tightly synchronized
Synchronization (computer science)
In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, so as to reach an agreement or...
across the entire network, using atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
s. This synchronization system
Synchronization in telecommunications
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if switches do not operate with the same clock rates, then slips will occur and degrade performance...
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 they can be used to directly support either Asynchronous Transfer Mode (ATM) or so-called packet over SONET/SDH
Packet over SONET/SDH
Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes over optical fibre at high...
(POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves; they are generic, all-purpose transport containers for moving both voice and data. The basic format of a SONET/SDH signal allows it to carry many different services in its virtual container (VC), because it is bandwidth-flexible.
Protocol overview
SONET and SDH often use different terms to describe identical features or functions. This can cause confusion and exaggerate their differences. With a few exceptions, SDH can be thought of as a superset of SONET.The protocol is a heavily-multiplexed structure, with the header interleaved between the data in a complex way. This permits the encapsulated data to have its own frame rate and be able to "float around" relative to the SDH/SONET frame structure and rate. This interleaving permits a very low latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
for the encapsulated data. Data passing through equipment can be delayed by at most 32 microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s (µs), compared to a frame rate of 125 µs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
; many competing protocols buffer the data during such transits 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, as the data is clocked at a different rate than the frame rate. The protocol is made more complex by the decision to permit this padding at most levels of the multiplexing structure, but it improves all-around performance.
The basic unit of transmission
The basic unit of framing in SDH is a STM-1STM-1
The STM-1 is the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 155.52 Mbit/s. Higher levels go up by a factor of 4 at a time: the other currently supported levels are STM-4, STM-16, STM-64 and STM-256...
(Synchronous Transport Module, level 1), which operates at 155.520 megabits per second (Mbit/s). SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal 3, concatenated) or OC-3c, depending on whether the signal is carried electrically (STS) or optically (OC), 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) or OC-1, operating at 51.84 Mbit/s—exactly one third of an STM-1/STS-3c/OC-3c carrier. This speed is dictated by the bandwidth requirements for PCM-encoded telephonic voice signals: at this rate, an STS-1/OC-1 circuit can carry the bandwidth equivalent of a standard DS-3 channel, which can carry 672 64-kbit/s voice channels. In SONET, the STS-3c/OC-3c signal is composed of three multiplexed STS-1 signals; the STS-3C/OC-3c may be carried on an OC-3 signal. Some manufacturers also support the SDH equivalent of the STS-1/OC-1, known as STM-0.
Framing
In packet-oriented data transmission, such as EthernetEthernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, a packet frame usually consists of a header and a payload. The header is transmitted first, followed by the payload (and possibly a trailer, such as a CRC
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
). In synchronous optical networking, this is modified slightly. The header is termed the overhead, and instead of being transmitted before the payload, is interleaved with it during transmission. Part of the overhead is 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
Octet (computing)
An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:...
in size, while the STM-1/STS-3c frame is 2,430 octets in size. For STS-1, the frame is transmitted as three octets of overhead, followed by 87 octets of payload. This is repeated nine times, until 810 octets have been transmitted, taking 125 µs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
. In the case of an STS-3c/STM-1, which operates three times faster than an STS-1, nine octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times until 2,430 octets have been transmitted, also taking 125 µs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
. For both SONET and SDH, this is often represented by displaying the frame graphically: as a block of 90 columns and nine rows for STS-1, and 270 columns and nine 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. Their standards are extremely similar in implementation, making it easy to interoperate between SDH and SONET at any given bandwidth.
In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC designation 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
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic circuit.2,430 octets × 8 bits per octet × 8,000 frames per second = 155.52 Mbit/s 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.2,349 octets of payload × 8 bits per octet × 8,000 frames per second = 150.336 Mbit/s
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 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 error correction and Automatic Protection Switching messages (e.g., alarms and maintenance messages) as may be required within the network.
AU Pointer
- Points to the location of the J1 byte in the payload (the first byte in the virtual container).
Path virtual envelope
Data transmitted from end to end is referred to as path data. It is composed of two components:Payload overhead (POH)
- Nine octets used for end-to-end signaling and error measurement.
Payload
- User data (774 bytes for STM-0/STS-1, or 2,340 octets 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.
The STS-1 payload is designed to carry a full PDH DS3
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
frame. When the DS3 enters a SONET network, path overhead is added, and that SONET network element
Network element
A network element is usually defined as a manageable logical entity uniting one or more physical devices. This allows distributed devices to be managed in a unified way using one management system....
(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
VT1.5
VT1.5 is a type of virtual tributary in SONET.SONET bandwidth is defined in multiples of an OC-1/STS-1, each of which can transport up to 51.84 Mbit/s. However, it is frequently desirable to address much smaller portions of bandwidth. To meet this need, sub-STS-1 facilities called Virtual...
signals, each of which can carry a PDH DS1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
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 TUG2; VT1.5 is equivalent to VC11, and VT2 is equivalent to VC12.
Three STS-1 signals may be multiplexed
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
by time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
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
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
.
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
STM-4
The STM-4 is a SDH ITU-T fiber optic network transmission standard. It has a bit rate of 622.080 Mbit/s....
.
The highest rate commonly deployed is the OC-768 or STM-256 circuit, which operates at rate of just under 38.5 Gbit/s. Where fiber exhaustion is a concern, multiple SONET signals can be transported over multiple wavelengths on a single fiber pair by means of wavelength-division multiplexing
Wavelength-division multiplexing
In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...
, including dense wavelength-division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). DWDM circuits are the basis for all modern submarine communications cable
Submarine communications cable
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean....
systems and other long-haul circuits.
SONET/SDH and relationship to 10 Gigabit Ethernet
Another type of high-speed data networking circuit is 10 Gigabit Ethernet10 Gigabit Ethernet
The 10 gigabit Ethernet computer networking standard was first published in 2002. It defines a version of Ethernet with a nominal data rate of 10 Gbit/s , ten times faster than gigabit Ethernet.10 gigabit Ethernet defines only full duplex point to point links which are generally connected by...
(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
64b/66b encoding
In data networking and transmission, 64b/66b is a line code that transforms 64-bit data to 66-bit line code to provide enough state changes to allow reasonable clock recovery and facilitate alignment of the data stream at the receiver....
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
| SONET Optical Carrier Level | |SDH level and Frame Format | |Line Rate (kbit/s) | ||
|---|---|---|---|---|
| OC-1 | STS-1 | STM-0 | 50,112 | 51,840 |
| OC-3 | STS-3 | STM-1 | 150,336 | 155,520 |
| OC-12 | STS-12 | STM-4 | 601,344 | 622,080 |
| OC-24 | STS-24 | – | 1,202,688 | 1,244,160 |
| OC-48 | STS-48 | STM-16 | 2,405,376 | 2,488,320 |
| OC-192 | STS-192 | STM-64 | 9,621,504 | 9,953,280 |
| OC-768 | STS-768 | STM-256 | 38,486,016 | 39,813,120 |
User throughput must also 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. Other rates, such as OC-9, OC-18, OC-36, OC-96, and OC-1536, are defined but not commonly deployed; most are considered orphaned rates.
The next rate level of 160 Gbit/s OC-3072/STM-1024 has not yet been standardized, due to the cost of high-rate transceivers and the ability to more cheaply multiplex wavelengths at 10 and 40 Gbit/s.
Physical layer
The physical layerPhysical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
actually comprises a large number of sublayers, only one of which is the optical transmission layer. 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 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 SNMPSimple Network Management Protocol
Simple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks. Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more." It is used mostly in network management systems to monitor...
, CORBA
Çorba
Chorba , ciorbă , shurpa , shorpo , or sorpa is one of various kinds of soup or stew found in national cuisines across Middle East...
, or XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
.
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 NE
Network element
A network element is usually defined as a manageable logical entity uniting one or more physical devices. This allows distributed devices to be managed in a unified way using one management system....
s have a limited number of management interfaces defined:
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.
Craft interface
- Local "craftspersons" (telephone network engineers) can access a 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 serverConsole serverA console server is a device or service that provides access to the system console of a computing device via networking technologies....
, allowing for remote out-of-band managementOut-of-band managementIn computing, out-of-band management involves the use of a dedicated management channel for device maintenance...
and logging.
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-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.7712, there are three modes used for management:- IPInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
-only stack, using PPPPoint-to-Point ProtocolIn networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
as data-link - OSIOpen Systems InterconnectionOpen Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
-only stack, using LAP-DLink Access Procedures, D channelLink Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...
as data-link - Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate between stacks.
- IP
To handle all of the possible management channels and signals, most modern network elements contain a router for the network commands and underlying (data) protocols.
The main functions of network management 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.
Equipment
With advances in SONET and SDH chipsets, the traditional categories of network elements are no longer distinct. Nevertheless, as network architectures have remained relatively constant, even newer equipment (including multi-service provisioning platformsAdd-drop multiplexer
An add-drop multiplexer is an important element of an optical fiber network. A multiplexer combines, or multiplexes, several lower-bandwidth streams of data into a single beam of light...
) 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.
Regenerator
Traditional regenerators terminate the section overhead, but not the line or path. Regenerators 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 amplifier
Optical amplifier
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed...
s. Also, some of the functionality of regenerators has been absorbed by the transponders of wavelength-division multiplexing systems.
Add-drop multiplexer
Add-drop multiplexerAdd-drop multiplexer
An add-drop multiplexer is an important element of an optical fiber network. A multiplexer combines, or multiplexes, several lower-bandwidth streams of data into a single beam of light...
s (ADMs) are the most common type of network elements. 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 network element and sent out from the high-speed side, or vice-versa.
Digital cross connect system
Recent digital cross connect systemDigital cross connect system
A digital cross-connect system is a piece of circuit-switched network equipment, used in telecommunications networks, that allows lower-level TDM bit streams, such as DS0 bit streams, to be rearranged and interconnected among higher-level TDM signals, such as DS1 bit streams...
s (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
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 fundamental to the worldwide deployment of SONET and SDH for moving digital traffic. Every SDH/SONET connection on the optical Physical layer uses two optical fibers, regardless of the transmission speed.Linear Automatic Protection Switching
Linear Automatic Protection Switching (APS), also known as 1+1, involves four fibers: two working fibers (one 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 network elements at each end negotiate so that both directions are generally carried on the same pair of fibers).Unidirectional path-switched ring
In unidirectional path-switched rings (UPSRs), two redundant (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines which copy has the highest quality, and uses that copy, thus coping if one copy deteriorates 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, in an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, 100 percent of the ring bandwidth (N=3) would be consumed by nodes A and D. Any other nodes on the ring could only act as pass-through nodes. The SDH equivalent of UPSR is subnetwork connection protectionSubnetwork Connection Protection
In telecommunications, subnetwork connection protection, or SNCP, is a type of protection mechanism associated with synchronous optical networks such as synchronous digital hierarchy....
(SNCP); SNCP does not impose a ring topology, but may also be used in mesh topologies.
Bidirectional line-switched ring
Bidirectional line-switched ring (BLSR) comes in two varieties: two-fiber BLSR and four-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 on the protection fibers. BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability to support "extra traffic" that can be pre-empted when a protection switching event occurs. In four-fiber ring, either single node failures, or multiple line failures can be supported, since a failure or maintenance action on one line causes the protection fiber connecting two nodes to be used rather than looping it around the ring.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. In the best case, 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).
Synchronization
Clock sources used for synchronization in telecommunications networks are rated by quality, commonly called a stratum. 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 Caesium 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 holdoverHoldover in synchronization applications“Synchronization is as important as power at the cell site.” The quote above suggests that we can think of holdover in synchronization applications as analogous to running on backup power....
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
Fast Ethernet
In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the fast Ethernet standards 100BASE-TX is by far the most common and is supported by the...
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
Virtual concatenation
Virtual concatenation is an inverse multiplexing technique creating a large capacity payload container distributed over multiple smaller capacity TDM signals. These signals may be transported or routed independently...
(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
Generic Framing Procedure
Generic Framing Procedure is a multiplexing technique defined by ITU-T G.7041. This allows mapping of variable length, higher-layer client signals over a circuit switched transport network like OTN, SDH/SONET or PDH...
(GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually-concatenated container.
The Link Capacity Adjustment Scheme (LCAS
LCAS
Link Capacity Adjustment Scheme or LCAS is a method to dynamically increase or decrease the bandwidth of virtual concatenated containers. The LCAS protocol is specified in ITU-T G.7042....
) 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
Ethernet over SDH
Ethernet Over SDH or Ethernet over SONET refers to a set of protocols which allow Ethernet traffic to be carried over synchronous digital hierarchy networks in an efficient and flexible way...
(EoS).
See also
- List of device bandwidths
- Routing and wavelength assignment (RWA)
- Multiwavelength optical networkingMultiwavelength optical networkingMultiwavelength optical networking , is a method for communicating digital information using lasers over optical fiber. The method provides the next level of communication networks after SONET optical networks. MONET optical networks provide an even greater bandwidth capacity...
- optical mesh networkOptical mesh networkOptical mesh networks are a type of telecommunications network.Transport networks, the underlying optical fiber-based layer of telecommunications networks, have evolved from DCS -based mesh architectures in the 1980s, to SONET/SDH ring architectures in the 1990s...
- Optical Transport NetworkOptical Transport NetworkITU-T defines an Optical Transport Network as a set of Optical Network Elements connected by optical fibre links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals...
- G.709G.709ITU-T Recommendation G.709 "Interfaces for the Optical Transport Network " describes a means of communicating data over an optical network...
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)
Standards
- 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)