scope:

This Recommendation outlines minimum requirements for timing devices used as node clocks in synchronization networks. The function of a node clock is to select one of the external synchronization links coming into a telecommunication station as the active synchronization reference, to attenuate its jitter and wander and subsequently to distribute the reference to the telecommunication equipment in the station. The requirements in this Recommendation apply under the normal environmental conditions specified for telecommunications equipment.

This Recommendation specifies node clocks suitable for use in SDH and PSTN network applications. They may provide acceptable performance for other applications, but that has to be investigated for each case individually.

In normal operation, a node clock is operating as a slave clock, traceable to a primary reference clock. For purposes of redundancy, a node clock will in general have multiple reference inputs. In the event that all links between the master(s) and the node clock fail, the node clock should be capable of maintaining operation within prescribed performance limits (the hold-over mode of operation).

A node clock can be a separate piece of equipment called a Stand Alone Synchronization Equipment (SASE) or it can be a part of another equipment such as a telephony exchange or an SDH cross-connect.

This Recommendation defines six Types of clocks. Applications for each clock Type are described in clause 2.