Table of Contents - G.781 (01/2026) - Synchronization layer functions for frequency synchronization based on the physical layer

1	Scope
2	References
3	Definitions
	3.1	Terms defined elsewhere
	3.2	Terms defined in this Recommendation
4	Abbreviations and acronyms
5	Conventions
6	Synchronization principles
	6.1	Network synchronization
	6.2	Synchronization distribution trails
	6.3	Synchronization interfaces
	6.4	Clock-source quality-level
	6.5	Synchronization status messages (SSMs), extended QL TLV and timing marker (TM) channels
	6.6	Selection process
	6.7	Signal fail
	6.8	Hold-off time
	6.9	Wait-to-restore time
	6.10	Synchronization source priorities
	6.11	External commands
	6.12	Automatic reference selection process
	6.13	Timing loop prevention
	6.14	Delay times for NEs with SEC/ST3/SIC
	6.15	Delay times for NEs with SSU/ST2 or for SASE/BITS
	6.16	Synchronization layer functions
	6.17	Overview of the processes performed within the atomic functions
	6.18	Interworking between option I, II and III synchronization networks
7	Synchronization distribution layer atomic functions
	7.1	SD connection function (SD_C)
	7.2	SD trail termination functions
	7.3	SD adaptation functions
8	Network synchronization layer atomic functions
	8.1	NS_connection functions (NS_C)
9	Transport layer to SD layer atomic functions
	9.1	STMn multiplex section adaptation functions
	9.2	Pqs adaptation functions
	9.3	P12s layer adaptation functions
	Page
	9.4	T12 layer adaptation functions
	9.5	P11s layer adaptation functions
	9.6	T01 layer adaptation functions
	9.7	T02 layer adaptation functions
	9.8	T21 layer adaptation functions
	9.9	ETH layer adaptation functions
	9.10	OTUk layer adaptation functions
	9.11	OSC layer adaptation functions
	9.12	FlexO layer adaptation functions
	9.13	FlexE layer adaptation functions
	9.14	MTNS layer adaptation functions
10	Equipment clock to transport layers clock adaptation functions
	10.1	STM-N layer
	10.2	VC layers
	10.3	Pxx layers
	10.4	T12 layer
	10.5	T01 layer
	10.6	T02 layer
	10.7	T21 layer
	10.8	ETYn layer
	10.9	ODU layer
	10.10	OSC layer
	10.11	FlexO layer
	10.12	FlexE layer
	10.13	MTNS layer
11	T12 section layer functions
	11.1	T12 connection function T12_C
	11.2	T12 trail termination functions
12	T01 section layer functions
	12.1	T01 connection function T01_C
	12.2	T01 trail termination functions
13	T02 section layer functions
	13.1	T02 connection function T02_C
	13.2	T02 trail termination functions
14	T21 section layer functions
	14.1	T21 connection function T21_C
	14.2	T21 trail termination functions
Annex A – Synchronization selection process
Annex B – SyncPhy data sets
	Page
	B.1	Introduction of SyncPhy dataset
	B.2	Derived data type specifications
	B.3	defaultDS data set member specifications
	B.4	currentDS data set member specifications
	B.5	parentDS data set member specifications
	B.6	portDS data set member specifications
Annex C – Hybrid equipment selection function
Appendix I – Transport layer models for synchronization information
Appendix II – Examples of synchronization functionality in the NE
	II.1	Examples of synchronization functionality in the NE for option I networks
	II.2	Examples of synchronization functionality in the NE for option II networks
	II.3	Examples of synchronization functionality in the NE for option III networks
Appendix III – Delay time allocation for option I and III networks
	III.1	Delay and processing times for the synchronization selection process
	III.2	Nonswitching message delay TNSM
	III.3	Switching message delay TSM
	III.4	Holdover message delay THM
	III.5	Wait-to-restore time TWTR
Appendix IV – Interworking of option II equipment supporting second-generation SSM and first-generation SSM using a translation function
Appendix V – SDL descriptions
Appendix VI – Clarification of the use of the acronym 'SEC'
Appendix VII – Use cases of mixing SSM and eSSM
Appendix VIII – ePRC QL information processes using an overwrite function