Synchronization Architectures Network topologies 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. If such is the case, it is said that the Ethernet node (EN) is in holdover. The SSM encoded within the TLV is a four-bit field whose meaning is described in ITU-T G.781.Ī 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. 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. However, event type messages with a new SSM quality level are generated immediately. The ITU-T G.8264 defines a background or heart-beat message to provide a continuous indication of the clock quality level. 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 mechanisms needed to transport the SSM over Synchronous Ethernet are defined by the ITU-T in G.8264 in cooperation with IEEE. However, in Ethernet there is no equivalent of a fixed frame. In SDH, the SSM message is carried in fixed locations within the SDH frame.
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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. Messaging channel Įthernet Synchronization Message Channel (ESMC) protocol data unit rec. " It was first published in August 2007, amended in 20 and a new version published in 2010. G.8262/Y.1362 is an ITU-T recommendation for Synchronous Ethernet that defines "timing characteristics of synchronous Ethernet equipment slave clock (EEC). In addition, by timing the Ethernet clock, it is possible to achieve Primary Reference Clock (PRC) traceability at the interfaces. While the IEEE 802.3 standard specifies Ethernet clocks to be within ☑00 ppm, EECs accuracy must be within ±4.6 ppm. These clocks are referred to as Ethernet Equipment Slave 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. ITU-T G.8262 defines Synchronous Ethernet clocks compatible with SDH clocks. Synchronization for SDH can be transported over Ethernet and vice versa. This enables Synchronous Ethernet network equipment to be connected to the same synchronization network as Synchronous Digital Hierarchy (SDH). Synchronization network model for Synchronous Ethernet, SONET and SDHĮxtension of the synchronization network to consider Ethernet as a building block (ITU-T G.8261). 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. G.8264 that describes the specification of Ethernet Synchronization Messaging Channel (ESMC) G.8262 that specifies Synchronous Ethernet clocks for SyncE G.8261 that defines aspects about the architecture and the wander performance of SyncE networks SyncE was standardized by the ITU-T, in cooperation with IEEE, as three recommendations: IETF’s Network Time Protocol, IEEE's 1588-2008 Precision Time Protocol are some of them. Several means are defined to address this issue. Unlike time-division multiplexing networks, the Ethernet family of computer networks do not carry clock synchronization information. 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. 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. The aim of Synchronous Ethernet is to provide a synchronization signal to those network resources that may eventually require such a type of signal. Mobile Networks require a kind of synchronization
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