Linuxptp Servo Optimization for Network Adapters with High-Precision Oscillators

Network Adapters with high-precision oscillators (typically Oven Controlled Oscillator, OCXO) and Global Navigational Satellite System (GNSS) receivers appeared on the market for time and frequency synchronization-related use cases in telecommunications and data center applications. They are designed for the grandmaster clock role primarily; however, we introduced alternative use cases for these cards in this paper, which also use time Receiver role (IEEE 1588g-2022). The user-space software framework for these cards is provided by the Linux PTP (linuxptp) project typically, in which ptp4l is the IEEE 1588 compliant clock implementation. A servo (controller) sets the PTP Hardware Clock (PHC) according to the network-acquired offset samples in the timeReceiver role of ptp4l, which can be selected from a set of implemented servos (pi, linreg, ntpshm, nullf). This paper investigates how the relevant servos (pi, linreg) behave in normal timeReceiver/synchronized operation and how the servo selection and configuration influence the holdover mode. The results are relevant to the proposed use cases. The investigation was done by comparing the performance of the Intel E810-XXVDA4 Network Adapter and an E810-XXVDA4T with a high-precision OCXO and Digital Phase-Locked Loop (DPLL). The presence of two different oscillators gives a very special insight into how the oscillator and servo influence synchronization and holdover performance.