This Recommendation specifies the requirements for primary reference time clocks (PRTCs) suitable for time, phase and frequency synchronization in packet networks. These requirements apply under the normal environmental conditions specified for the equipment.

A typical PRTC provides the reference signal for time, phase and frequency synchronization for other clocks within a network or section of a network. In particular, the PRTC can also provide the reference signal to the telecom grand master (T-GM) within the network nodes where the PRTC is located.

The PRTC provides a reference time signal traceable to a recognized time standard (e.g., coordinated universal time (UTC)). UTC can be obtained from a UTC time laboratory registered at Bureau International des Poids et Mesures (BIPM) (e.g., a national UTC time lab) or, most commonly, from a global navigation satellite system (GNSS).

This Recommendation defines the PRTC output requirements. The accuracy of the PRTC should be maintained as specified in this Recommendation.

This Recommendation also covers the case where a PRTC is integrated with a T-GM clock. In this case it defines the performance at the output of the combined PRTC and T-GM function, i.e., the precision time protocol (PTP) messages.

This Recommendation defines two types of PRTCs, PRTC-A and PRTC-B. The time output of PRTC-B is more accurate than that of PRTC-A. PRTC-B is intended for locations where it is possible to guarantee optimized environmental conditions (e.g., controlled temperature variation in indoor deployments). Examples are central location and large aggregation sites. PRTC-B is related to applications where more accurate absolute time is required. Achieving accurate relative time between timing chains may benefit from deploying PRTC-B clocks in all timing chains.