Synchronous Ethernet, also referred as SyncE, is an
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
standard for
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
ing that facilitates the
transference of clock signals over the
Ethernet physical layer
The physical-layer specifications of the Ethernet family of computer network standards are published by the Institute of Electrical and Electronics Engineers (IEEE), which defines the electrical or optical properties and the transfer speed of t ...
. This signal can then be made
traceable to an external clock.
Overview
![Synce-network-wless](https://upload.wikimedia.org/wikipedia/commons/3/35/Synce-network-wless.png)
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 network
A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically thre ...
s, access technologies such as
Ethernet passive optical network
Ethernet in the first mile (EFM) refers to using one of the Ethernet family of computer network technologies between a telecommunications company and a customer's premises. From the customer's point of view, it is their first mile, although from th ...
, and applications such as
IPTV
Internet Protocol television (IPTV) is the delivery of television content over Internet Protocol (IP) networks. This is in contrast to delivery through traditional terrestrial, satellite, and cable television formats. Unlike downloaded media, ...
or
VoIP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
, as well as
CERN
The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
's
White Rabbit Project
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 tra ...
for sub-nanosecond time synchronization of data acquisition equipment for their high-energy experiments.
Unlike
time-division multiplexing
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...
networks, the
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
family of
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
s do not carry clock synchronization information. Several means are defined to address this issue. IETF’s
Network Time Protocol
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable- latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in c ...
, IEEE's 1588-2008
Precision Time Protocol
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. ...
are some of them.
SyncE was standardized by the ITU-T, in cooperation with IEEE, as three recommendations:
#
ITU-T Rec. G.8261 that defines aspects about the architecture and the
wander performance of SyncE networks
# ITU-T Rec. G.8262 that specifies Synchronous Ethernet clocks for SyncE
# ITU-T Rec. G.8264 that describes the specification of Ethernet Synchronization Messaging Channel (ESMC)
SyncE architecture minimally requires replacement of the internal clock of the Ethernet card by a
phase locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
in order to feed the
Ethernet PHY
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. This layer may be implemented by a PHY chip.
The ...
.
Architecture
![Synce-network-standard](https://upload.wikimedia.org/wikipedia/commons/7/76/Synce-network-standard.png)
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
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 diode ...
(SDH). Synchronization for SDH can be transported over Ethernet and vice versa.
Clocks
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). "
It was first published in August 2007, amended in 2008 and 2010 and a new version published in 2010.
Messaging channel
![Synce-network-esmc](https://upload.wikimedia.org/wikipedia/commons/f/f3/Synce-network-esmc.png)
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.
Synchronization architectures
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
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
(PLL) at the Ethernet nodes since it degrades when passing through the network.
Network topologies
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:
* Tree: This is a basic topology that relies on a master clock whose reference is distributed to the rest of the slave clocks. It has two weak points: it depends on only one clock, and the signals gradually degrade.
* Ring: Basically, this is a tree topology that uses ring configurations to propagate the synchronization signal. The ring topology offers a way to make a tree secure, but care must be taken to avoid the formation of synchronizing loops.
* Meshed: In this topology, nodes form interconnections between each other, in order to have redundancy in case of failure. However, synchronization loops occur easily and should be avoided.
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.
Interconnection of nodes
![Synce-network-topology](https://upload.wikimedia.org/wikipedia/commons/2/26/Synce-network-topology.png)
There are two basic ways to distribute synchronization:
* ''Intranode'', which is a high-quality slave clock known as either synchronization supply unit (SSU). These are responsible for distributing synchronization to NEs situated inside the node.
* ''Internode'', where the synchronization signal is sent to another node by a link specifically dedicated to this purpose, or by a PHY signal.
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.
Synchronization signals
There are many signals suitable for transporting synchronization:
* Analog, of 1.544 and 2.048 MHz
* Digital, of 1.544 and 2.048 Mbit/s
* SyncE signal at any bit rate
* STM-n/OC-m line codes
Synchronization models
![Synce-network-sample](https://upload.wikimedia.org/wikipedia/commons/3/3d/Synce-network-sample.png)
In SyncE, there are several ways to synchronize nodes:
# ''External timing'': The EEC obtains its signal from an stand-alone synchronization equipment (SASE). This is a typical way to synchronize, and the NE usually also has an extra reference signal for emergency situations.
# ''Line timing'': The NE obtains its clock by deriving it from one of the input signals.
# ''Through timing'': Where the Tx outputs of one interface are synchronized with the Rx inputs of the opposite interface.
# ''Internal timing'': In this mode, the internal clock of the EEC is used to synchronize outputs. It may be a temporary holdover stage after losing the synchronization signal, or it may be a simple line configuration where no other clock is available.
Timing loops
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.
References
External links
SyncE explained under the installation and maintenance point of viewWhite Rabbit Synchronous Network The White Rabbit Project
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 tran ...
{{Ethernet
Ethernet
Network protocols
Synchronization
Timecodes
ITU-T recommendations
ITU-T G Series Recommendations