This Recommendation outlines requirements for timing devices used in synchronizing network equipment that operate according to the principles governed by the Synchronous Digital Hierarchy (SDH). These requirements apply under the normal environmental conditions specified for the SDH equipment. In normal operation, SDH equipment contains a slave clock traceable to a primary reference clock. In general, the SDH equipment clock (SEC) will have multiple reference inputs. In the event that all links between the master and the slave clock fail, the equipment should be capable of maintaining operation (holdover) within prescribed performance limits.

The SEC is part of the SDH equipment, the functions of which are specified in ITU-T Rec. G.783 as the Synchronous Equipment Timing Source (SETS). Slave clocks used in SDH equipment must meet specific requirements in order to comply with network jitter requirements for plesiochronous tributaries.

This Recommendation contains two options for the SEC. The first option, referred to as "Option 1," applies to SDH networks optimized for the 2048 kbit/s hierarchy. These networks allow the worst-case synchronization reference chain as specified in Figure 8-5/G.803. The second option, referred to as "Option 2," applies to SDH networks optimized for the particular 1544 kbit/s hierarchy that includes the rates 1544 kbit/s, 6312 kbit/s, and 44 736 kbit/s. The synchronization reference chain for these networks is defined in Appendix II.

An SDH equipment slave clock should comply with all of 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. It is the intention that Options 1 and 2 should be harmonized in the future.

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

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