scope:

This Recommendation outlines minimum requirements for timing devices used in synchronizing network equipment that support synchronous clocks. It supports clock distribution based on the network-synchronous line-code methods (e.g., synchronous Ethernet, synchronous OTN to deliver frequency synchronization).

This Recommendation focuses on the requirements for the synchronous Ethernet equipment clock (EEC) and the synchronous OTN equipment clock (OEC).

The Recommendation allows for proper network operation when a synchronous equipment clock (Option 1 or 2) is timed from another network equipment clock or a higher-quality clock.

Included in this Recommendation are requirements for clock accuracy, noise transfer, holdover performance, noise tolerance and noise generation. These requirements apply under the normal environmental conditions specified for the equipment.

This Recommendation contains two options for synchronous equipment clocks. The first option, referred to as "Option 1", applies to synchronous equipment designed to interwork with networks optimized for the 2048 kbit/s hierarchy. These networks allow the worst-case synchronization reference chain as specified in Figure 8-5 of [ITU-T G.803]. The second option, referred to as "Option 2", applies to synchronous equipment designed to interwork with networks optimized for the 1544 kbit/s hierarchy. The synchronization reference chain for these networks is defined in clause II.3 of [ITU-T G.813].

A synchronous equipment slave clock should comply with all the requirements specific to one option and should not mix requirements between Options 1 and 2. In the clauses where one requirement is specified, the requirements are common to both options.

The intent of the synchronous Ethernet and synchronous OTN is to interoperate with each other and with existing synchronization networks based on [ITU-T G.813].

Careful consideration should be taken when interworking between networks with synchronous equipment clocks based on Option 1 and networks with synchronous equipment clocks based on Option 2.

Some synchronous network elements (NEs) may have a higher-quality clock. This Recommendation allows for proper network operation when synchronous equipment (Option 1 or 2) is timed from either another synchronous equipment (like option) clock, or a higher-quality clock. Hierarchical timing distribution is recommended for synchronous networks. Timing should not be passed from a synchronous clock in free-run/holdover mode to a higher-quality clock since the higher-quality clock should not follow the free-run/holdover signal during fault conditions.

OTN 3R regenerators, as specified in [ITU-T G.8251], provide through-timing capability and can transmit timing via synchronous OTN.

More information on synchronous Ethernet can be found in [ITU-T G.781], [ITU-T G.8261] and [ITU-T G.8264], and for synchronous OTN can be found in [ITU-T G.709] and [ITU-T G.7041].