60-397: 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
120-501: A circuit switched connection, or a packet-mode virtual circuit connection. In the latter case, it may use either a transport layer virtual circuit protocol such as the Transmission Control Protocol (TCP) protocol, allowing data to be delivered in order. Although the lower-layer switching is connectionless, or it may be a data link layer or network layer switching mode, where all data packets belonging to
180-588: 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. 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. SONET and SDH have
240-483: A 'transport' in the OSI Model sense). Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the choice for transporting the fixed length Asynchronous Transfer Mode (ATM) frames also known as cells. 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 ),
300-508: 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
360-409: A SONET/SDH signal allows it to carry many different services in its virtual container (VC), because it is bandwidth-flexible. 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. SONET is a set of transport containers that allow for delivery of
420-588: A circuit establishment phase. Packet switched communication may also be connection-oriented, which is called virtual circuit mode communication. Due to the packet switching, the communication may suffer from variable bit rate and delay, due to varying traffic load and packet queue lengths. Connection-oriented communication does not necessarily imply reliability . Connection-oriented transport-layer protocols provide connection-oriented communications over connectionless communication systems. A connection-oriented transport layer protocol, such as TCP , may be based on
480-522: A connection-oriented protocol identifies traffic flows only by a channel or data stream number, often denoted virtual circuit identifier (VCI). Routing information may be provided to the network nodes during the connection establishment phase, where the VCI is defined in tables within each node. Thus, the actual packet switching and data transfer can be taken care of by fast hardware, as opposed to slower software-based routing. Typically, this connection identifier
540-556: A connectionless network-layer protocol such as IP, but still achieves in-order delivery of a byte-stream by means of segment sequence numbering on the sender side, packet buffering, and data packet reordering on the receiver side. In a connection-oriented packet-switched data-link or network-layer protocol , all data is sent over the same path during a communication session. Rather than using complete routing information for each packet (source and destination addresses) as in connectionless datagram switching such as conventional IP routers,
600-426: A finite number of values from some alphabet , such as letters or digits. An example is a text document , which consists of a string of alphanumeric characters . The most common form of digital data in modern information systems is binary data , which is represented by a string of binary digits (bits) each of which can have one of two values, either 0 or 1. Digital data can be contrasted with analog data , which
660-585: 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 (APS), also known as 1+1 , involves four fibers: two working fibers (one in each direction), and two protection fibers. Switching
SECTION 10
#1732780941396720-418: A single word. This is useful when combinations of key presses are meaningful, and is sometimes used for passing the status of modifier keys on a keyboard (such as shift and control). But it does not scale to support more keys than the number of bits in a single byte or word. Devices with many switches (such as a computer keyboard ) usually arrange these switches in a scan matrix, with the individual switches on
780-549: A switch is pressed, released, and pressed again. This polling can be done by a specialized processor in the device to prevent burdening the main CPU . When a new symbol has been entered, the device typically sends an interrupt , in a specialized format, so that the CPU can read it. For devices with only a few switches (such as the buttons on a joystick ), the status of each can be encoded as bits (usually 0 for released and 1 for pressed) in
840-485: A variety of protocols, including traditional telephony, ATM, Ethernet, and TCP/IP traffic. SONET therefore is not in itself a native communications protocol and should not be confused as being necessarily connection-oriented in the way that term is usually used. 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
900-465: 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
960-638: Is a communication protocol where a communication session or a semi-permanent connection is established before any useful data can be transferred. The established connection ensures that data is delivered in the correct order to the upper communication layer. The alternative is called connectionless communication , such as the datagram mode communication used by Internet Protocol (IP) and User Datagram Protocol (UDP), where data may be delivered out of order, since different network packets are routed independently and may be delivered over different paths. Connection-oriented communication may be implemented with
1020-485: Is a small integer (for example, 10 bits for Frame Relay and 24 bits for ATM). This makes network switches substantially faster. ATM and Frame Relay, for example, are both examples of connection-oriented, unreliable data link layer protocols. Reliable connectionless protocols exist as well, for example AX.25 network layer protocol when it passes data in I-frames, but this combination is rare, and reliable-connectionless
1080-491: 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
1140-679: Is an example of a connection-oriented protocol in which, if a message is not delivered, an error report is sent to the sender, making it a reliable protocol. Because they can keep track of a conversation, connection-oriented protocols are sometimes described as stateful. Circuit switched communication, for example the public switched telephone network , ISDN , SONET/SDH and optical mesh networks , are intrinsically connection-oriented communication systems. Circuit-mode communication provides guarantees that constant bandwidth will be available, and bit stream or byte stream data will arrive in order with constant delay. The switches are reconfigured during
1200-435: 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). Digital data Digital data , in information theory and information systems , is information represented as a string of discrete symbols, each of which can take on one of only
1260-515: Is composed as follows: Data transmitted from end to end is referred to as path data . It is composed of two components: 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
SECTION 20
#17327809413961320-540: Is considered a variation of SDH because of SDH's greater worldwide market penetration. SONET is subdivided into four sublayers with some factor such as the path, line, section and physical layer. The SDH standard was originally defined by the European Telecommunications Standards Institute (ETSI), and is formalised as International Telecommunication Union (ITU) standards G.707, G.783 , G.784, and G.803. The SONET standard
1380-507: 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 signal is composed of three multiplexed STS-1 signals; the STS-3c may be carried on an OC-3 signal. Some manufacturers also support the SDH equivalent of
1440-484: 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
1500-503: 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
1560-524: 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
1620-436: Is rather simpler than conversion of continuous or analog information to digital. Instead of sampling and quantization as in analog-to-digital conversion , such techniques as polling and encoding are used. A symbol input device usually consists of a group of switches that are polled at regular intervals to see which switches are switched. Data will be lost if, within a single polling interval, two switches are pressed, or
1680-408: 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
1740-581: Is represented by a value from a continuous range of real numbers . Analog data is transmitted by an analog signal , which not only takes on continuous values but can vary continuously with time, a continuous real-valued function of time. An example is the air pressure variation in a sound wave . The word digital comes from the same source as the words digit and digitus (the Latin word for finger ), as fingers are often used for counting. Mathematician George Stibitz of Bell Telephone Laboratories used
1800-595: 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. Another type of high-speed data networking circuit
1860-443: 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. 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
Synchronous optical networking - Misplaced Pages Continue
1920-579: 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, 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
1980-559: 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 not deduct path overhead from
2040-504: 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: To handle all of the possible management channels and signals, most modern network elements contain a router for
2100-484: 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 (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
2160-467: The STS-1/OC-1, known as STM-0. In packet-oriented data transmission, such as Ethernet , 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,
2220-458: The STS-3c is carried over OC-3, it is often colloquially referred to as OC-3c , but this is not an official designation within the SONET standard as there is no physical layer (i.e. optical) difference between an STS-3c and 3 STS-1s carried within an OC-3. 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
2280-569: 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. Therefore, 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
2340-536: The entire lifecycle from 'birth' to the destruction of the data. All digital information possesses common properties that distinguish it from analog data with respect to communications: Even though digital signals are generally associated with the binary electronic digital systems used in modern electronics and computing, digital systems are actually ancient, and need not be binary or electronic. Connection-oriented communication In telecommunications and computer networking , connection-oriented communication
2400-448: The first layer in the OSI networking model. 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
2460-575: 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,
Synchronous optical networking - Misplaced Pages Continue
2520-515: The functionality of regenerators has been absorbed by the transponders of wavelength-division multiplexing systems. STS multiplexer and demultiplexer provide the interface between an electrical tributary network and the optical network. Add-drop multiplexers (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
2580-415: 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
2640-473: The internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as STS-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
2700-408: The intersections of x and y lines. When a switch is pressed, it connects the corresponding x and y lines together. Polling (often called scanning in this case) is done by activating each x line in sequence and detecting which y lines then have a signal , thus which keys are pressed. When the keyboard processor detects that a key has changed state, it sends a signal to the CPU indicating the scan code of
2760-406: The key and its new state. The symbol is then encoded or converted into a number based on the status of modifier keys and the desired character encoding . A custom encoding can be used for a specific application with no loss of data. However, using a standard encoding such as ASCII is problematic if a symbol such as 'ß' needs to be converted but is not in the standard. It is estimated that in
2820-404: 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
2880-461: The network commands and underlying (data) protocols. 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 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
2940-439: The older categories. 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 amplifiers . Also, some of
3000-488: 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 framing in SDH is a STM-1 (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). When
3060-636: 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. 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: The main functions of network management thereby include: Consider
SECTION 50
#17327809413963120-617: 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 physical layer refers to
3180-633: The same traffic stream are delivered over the same path, and traffic flows are identified by some connection identifier reducing the overhead of routing decisions on a packet-by-packet basis for the network. Connection-oriented protocol services are often, but not always, reliable network services that provide acknowledgment after successful delivery and automatic repeat request functions in case of missing or corrupted data. Asynchronous Transfer Mode (ATM), Frame Relay and Multiprotocol Label Switching (MPLS) are examples of connection-oriented unreliable protocols. Simple Mail Transfer Protocol (SMTP)
3240-450: 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 Synchronous Transport Module, level 1 (STM-1) frame is the basic transmission format for SDH—the first level of the synchronous digital hierarchy. The STM-1 frame
3300-515: The three parts defined above: This will often consist of software running on a Workstation covering a number of SDH/SONET network elements 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 . SDH has been mainly managed using
3360-406: 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. The transport overhead is used for signaling and measuring transmission error rates , and
3420-435: The word digital in reference to the fast electric pulses emitted by a device designed to aim and fire anti-aircraft guns in 1942. The term is most commonly used in computing and electronics , especially where real-world information is converted to binary numeric form as in digital audio and digital photography . Since symbols (for example, alphanumeric characters ) are not continuous, representing symbols digitally
3480-470: The year 1986, less than 1% of the world's technological capacity to store information was digital and in 2007 it was already 94%. The year 2002 is assumed to be the year when humankind was able to store more information in digital than in analog format (the "beginning of the digital age "). Digital data come in these three states: data at rest , data in transit , and data in use . The confidentiality, integrity, and availability have to be managed during
3540-521: Was defined by Telcordia and American National Standards Institute (ANSI) standard T1.105. which define the set of transmission formats and transmission rates in the range above 51.840 Mbit/s. SDH differs from Plesiochronous Digital Hierarchy (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 clocks . This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing
3600-408: 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 a complete communications protocol in itself, but a transport protocol (not
#395604