Hello,
Today I will share with you an overview on main international standard related to synchronization.
Starting with frequency synchronization based on physical interfaces (through the line code, which is clocked to a reference traceable to the Primary Reference Clock): they need network support hop by hop at every node in the transmission chain
ITU-T G.811/812/813. Frequency Timing over SDH, mandatory for STM-n signals. (Late 90’s)
ITU-T G.8261/8262/8264. Frequency SyncE (2006) for IP/Ethernet networks. It is available but not needed for fixed traffic unless we require circuit emulation or mobile convergence. It uses 100/1000 Base-X and 10GBASE R signals (required in BEP1 and BEP2 networks).
Moving to packet based frequency synchronization that was defined by:
IEEE1588v2 (2008). Frequency (and Time) precision protocol, hardware based, multipurpose and microsecond granularity. IEEE defined a wide set of specifications that is afterwards filtered by ITU-T building recommendation sets for specific telecom uses, adding Interworking profiles and quality objectives for network models and devices
ITU-T G.8265.1 frequency only. (2009) over asynchronous networks. Mandatory on BEP1.0 to transmit frequency over Ethernet or IP managed networks or legacy asynchronous network for managing RAN handover and TDM circuit emulation. It requires IP connectivity and low jitter (QoS)
Finishing with packet based time synchronization that has been defined by ITU-T (2014) with a standard suite represented in Figure. Standards highlighted within green boxes are the reference ones for this document.
Figure : ITU-T Time / Phase G.827x standard suite
In particular, following ITU-T specifications for full on path support (BC hop by hop) holds for Time Sync implementation:
G.8271.1 for Time/Phase distribution reference architecture over packet network and time error allocation
G.8272 for primary reference time clock (Grand Master). Gives the details of GNSS recovered time (+-100ns)
G8273.2 for port based time stamping on Boundary Clock. Quality aspects. Specifically referred to G.8275.1 profile of PTP (1588v2).
G8273.2 for permissible range of Constant Phase/Time Error – cTE (+- 50ns for class A, +-20ns for Class B) on Boundary Clock and Time Slave Clock. Most devices in standard Network should be class B
G.8275.1. for PTP Ethernet multicast per hop (Annex F 1588 format) as reference protocol. Interoperability and protection (BMCA) covered. Joint body with G.8275.1
ITU-T is also working to define specifications for assisted partial on path support self-built strategy (standards within yellow boxes) and finalizing Transparent Clock (TC). Standardization is ready for the format and profile, partial on path has been specified (G.8275.2, similar to G.8265.1, unicast), but the boundary clock and slave devices (with their performance limits to be specified via asymmetry corrections and PDV tolerance under G.8273.4) are expected to be expensive and always with lower performances than fully assisted devices already available.
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