ITU-T G.8262 PDF

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

This Recommendation allows for proper network operation when a synchronous Ethernet equipment clock (EEC) (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 Ethernet. The first option, referred to as "EEC-Option 1", applies to synchronous Ethernet equipments that are 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 "EEC-Option 2", applies to synchronous Ethernet equipments that are 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 Ethernet equipment slave clock should comply with all of the requirements specific to one option and should not mix requirements between EEC-Options 1 and 2. In the clauses where one requirement is specified, the requirements are common to both options. It is the intention that EEC-Options 1 and 2 should be harmonized in the future. The intent of the synchronous Ethernet is to interoperate with existing synchronization networks based on [ITU-T G.813].

Careful consideration should be taken when interworking between networks with synchronous Ethernet based on EEC-Option 1 and networks with synchronous Ethernet based on EEC-Option 2.

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

Certain Ethernet equipments, such as regenerators/repeaters, must provide through-timing capability to transmit timing via synchronous Ethernet. These equipments are for further study.

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