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Synchronous Ethernet (SyncE) is an ITU-T standard for computer networking that facilitates the transference of clock signals over the Ethernet physical layer. This signal can then be made traceable to an external clock.
The aim of Synchronous Ethernet is to provide a synchronization signal to those network resources that may eventually require such a type of signal. The Synchronous Ethernet signal transmitted over the Ethernet physical layer should be traceable to an external clock, ideally a master and unique clock for the whole network. Applications include cellular networks, access technologies such as Ethernet passive optical network, and applications such as IPTV or VoIP, as well as CERN's White Rabbit Project for sub-nanosecond time synchronization of data acquisition equipment for their high-energy experiments.
Unlike time-division multiplexing networks, the Ethernet family of computer networks do not carry clock synchronization information. Several means are defined to address this issue. IETF's Network Time Protocol, IEEE's 1588-2008 Precision Time Protocol are some of them.
SyncE was standardized by the ITU-T, in cooperation with IEEE, as three recommendations:
SyncE architecture minimally requires replacement of the internal clock of the Ethernet card by a phase locked loop in order to feed the Ethernet PHY.
Extension of the synchronization network to consider Ethernet as a building block (ITU-T G.8261). This enables Synchronous Ethernet network equipment to be connected to the same synchronization network as Synchronous Digital Hierarchy (SDH). Synchronization for SDH can be transported over Ethernet and vice versa.
ITU-T G.8262 defines Synchronous Ethernet clocks compatible with SDH clocks. Synchronous Ethernet clocks, based on ITU-T G.813 clocks, are defined in terms of accuracy, noise transfer, holdover performance, noise tolerance and noise generation. These clocks are referred to as Ethernet Equipment Slave clocks. While the IEEE 802.3 standard specifies Ethernet clocks to be within ±100 ppm, EECs accuracy must be within ±4.6 ppm. In addition, by timing the Ethernet clock, it is possible to achieve Primary Reference Clock (PRC) traceability at the interfaces.
G.8262/Y.1362 is an ITU-T recommendation for Synchronous Ethernet that defines "timing characteristics of synchronous Ethernet equipment slave clock (EEC). " [1] It was first published in August 2007, amended in 2008 and 2010 and a new version published in 2010. [1]
In SDH, the Synchronization Status Message (SSM) provides traceability of synchronization signals and it is therefore required to extend the SSM functionality to Synchronous Ethernet to achieve full interoperability with SDH equipment.
In SDH, the SSM message is carried in fixed locations within the SDH frame. However, in Ethernet there is no equivalent of a fixed frame. The mechanisms needed to transport the SSM over Synchronous Ethernet are defined by the ITU-T in G.8264 in cooperation with IEEE. More specifically, the ESMC, defined by the ITU-T is based on the Organization Specific Slow Protocol (OSSP), currently specified in IEEE 802.3ay. The ITU-T G.8264 defines a background or heart-beat message to provide a continuous indication of the clock quality level. However, event type messages with a new SSM quality level are generated immediately.
The ESMC protocol is composed of the standard Ethernet header for a slow protocol, an ITU-T specific header, a flag field and a type length value (TLV) structure. The SSM encoded within the TLV is a four-bit field whose meaning is described in ITU-T G.781.
A general requirement for SyncE was that any network element (NE) should have at least two reference clocks, and in addition, Ethernet interfaces must be able to generate their own synchronization signal in case they lose their external reference. If such is the case, it is said that the Ethernet node (EN) is in holdover. The synchronous signal must be filtered and regenerated by phase locked loop (PLL) at the Ethernet nodes since it degrades when passing through the network.
The synchronization and transport networks are partially mixed, since some NEs both transmit data and distribute clock signals to other NEs. The most common topologies are:
SyncE networks do not usually have only one topology, but rather a combination of all of them. Duplication and security involving more than one master clock, and the existence of some kind of synchronization management protocol, are important features of modern networks. The aim is to minimize the problems associated with signal transport, and to avoid depending on only one clock in case of failure. As a result, we get an extremely precise, redundant, and solid synchronization network.
There are two basic ways to distribute synchronization:
Several type of networks can be used to transport the synchronous signal and could be combined indeed. Some of these networks are T1/E1, SONET/SDH and any rate, and SyncE. However legacy Ethernet is not suitable for transmitting synchronization signals. This is important because if the signal crosses a legacy Ethernet island then the synchronization is lost.
There are many signals suitable for transporting synchronization:
In SyncE, there are several ways to synchronize nodes:
A timing loop is in bad synchronization when the clock signal has closed itself, but there is no clock, either master or slave, that would autonomously generate a non-deficient clock signal. This situation can be caused by a fault affecting an NE in such a way that it has been left without a reference clock, and therefore it has chosen an alternative synchronization: a signal that has turned out to be the same signal, returning by another route. A synchronization loop is a completely unstable situation that may provoke an immediate collapse of part of the network within the loop.
The plesiochronous digital hierarchy (PDH) 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. The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronized.
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.
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to transmit on, the line code to use and similar low-level parameters, are specified by the physical layer.
In telecommunications, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly synchronised. According to ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. A sender and receiver operate plesiosynchronously if they operate at the same nominal clock frequency but may have a slight clock frequency mismatch, which leads to a drifting phase. The mismatch between the two systems' clocks is known as the plesiochronous difference.
Resilient Packet Ring (RPR), as defined by IEEE standard 802.17, is a protocol designed for the transport of data traffic over optical fiber ring networks. The standard began development in November 2000 and has undergone several amendments since its initial standard was completed in June 2004. The amended standards are 802.17a through 802.17d, the last of which was adopted in May 2011. It is designed to provide the resilience found in SONET and Synchronous Digital Hierarchy networks but, instead of setting up circuit oriented connections, provides a packet based transmission, in order to increase the efficiency of Ethernet and IP services.
Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of clock rate differences and several solutions, some being more acceptable than others in certain contexts.
A metropolitan-area Ethernet, Ethernet MAN, carrier Ethernet or metro Ethernet network is a metropolitan area network (MAN) that is based on Ethernet standards. It is commonly used to connect subscribers to a larger service network or for internet access. Businesses can also use metropolitan-area Ethernet to connect their own offices to each other.
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems. PTP is employed to synchronize financial transactions, mobile phone tower transmissions, sub-sea acoustic arrays, and networks that require precise timing but lack access to satellite navigation signals.
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, then bit slips will occur and degrade performance. Telecommunication networks rely on the use of highly accurate primary reference clocks which are distributed network-wide using synchronization links and synchronization supply units.
Virtual concatenation (VCAT) 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. Virtual concatenation has been defined for SONET/SDH, OTN and PDH path signals.
EtherCAT is an Ethernet-based fieldbus system developed by Beckhoff Automation. The protocol is standardized in IEC 61158 and is suitable for both hard and soft real-time computing requirements in automation technology.
Ethernet over PDH over SONET/SDH (EoPoS) is one of many techniques that provided Ethernet connectivity over non-Ethernet networks. EoPoS is a standardized method for transporting native Ethernet frames over the existing telecommunications optical infrastructure use both the established Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SONET/SDH) transport technologies.
Ethernet Ring Protection Switching (ERPS) is an effort at ITU-T under G.8032 Recommendation to provide sub-50ms protection and recovery switching for Ethernet traffic in a ring topology and at the same time ensuring that there are no loops formed at the Ethernet layer. This ITU-T specification is directly based on and derived from the Ethernet Automatic Protection Switching technology developed and patented by Extreme Networks. G.8032v1 supported a single ring topology and G.8032v2 supports multiple rings/ladder topology.
Parallel Redundancy Protocol (PRP) is a network protocol standard for Ethernet that provides seamless failover against failure of any network component. This redundancy is invisible to the application.
White Rabbit is the name of a collaborative project including CERN, GSI Helmholtz Centre for Heavy Ion Research and other partners from universities and industry to develop a fully deterministic Ethernet-based network for general purpose data transfer and sub-nanosecond accuracy time transfer. Its initial use was as a timing distribution network for control and data acquisition timing of the accelerator sites at CERN as well as in GSI's Facility for Antiproton and Ion Research (FAIR) project. The hardware designs as well as the source code are publicly available. The name of the project is a reference to the White Rabbit appearing in Lewis Carroll's novel Alice's Adventures in Wonderland.
Time-Sensitive Networking (TSN) is a set of standards under development by the Time-Sensitive Networking task group of the IEEE 802.1 working group. The TSN task group was formed in November 2012 by renaming the existing Audio Video Bridging Task Group and continuing its work. The name changed as a result of the extension of the working area of the standardization group. The standards define mechanisms for the time-sensitive transmission of data over deterministic Ethernet networks.
IEC/IEEE 61850-9-3 or PUP is an international standard for precise time distribution and clock synchronization in electrical grids with an accuracy of 1 μs.
It supports precise time stamping of voltage and current measurement for differential protection, wide area monitoring and protection, busbar protection and event recording.
It can be used to ensure deterministic operation of critical functions in the automation system.
It belongs to the IEC 61850 standard suite for communication networks and systems for power utility automation.
Industrial automation systems consisting of several distributed controllers need a precise synchronization for commands, events and process data. For instance, motors for newspaper printing are synchronized within some 5 microseconds to ensure that the color pixels in the different cylinders come within 0.1 mm at a paper speed of some 20 m/s. Similar requirements exist in high-power semiconductors and in drive-by-wire vehicles. This synchronisation is provided by the communication network, in most cases Industrial Ethernet. Many ad-hoc synchronization schemes exist, so IEEE published a standard Precision Time Protocol IEEE 1588 or "PTP", which allows sub-microsecond synchronization of clocks. PTP is formulated generally, so concrete applications need a stricter profile. In particular, PTP does not specify how the clocks should operate when the network is duplicated for better resilience to failures.
Audio Video Bridging (AVB) is a common name for a set of technical standards that provide improved synchronization, low latency, and reliability for switched Ethernet networks. AVB embodies the following technologies and standards:
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