Synchronization in Packet Networks: Timing Metrics and Monitoring

Abstract : Packet Delay Variation (PDV) is a major impediment to transferring timing from a source (Master/Server) to destination (Slave/Client) over a packet-switched network. PDV directly affects frequency alignment (synchronization) and asymmetry of PDV impacts time alignment (phase). There is considerable interest in the industry to establish a relationship between PDV and the ability to recover time/phase/frequency via a packet-based method (e.g., PTP, NTP). Recent developments in PDV analysis have indicated that it is feasible to build accurate models of network behavior under varying conditions of load, number of switches, forwarding algorithms, QoS implementation, and so on. It has also been shown that no single metric (e.g., TDEV, minTDEV) is sufficient to characterize PDV and that a suite of metrics is necessary. Study of PDV also develops intuition and permits heuristic approaches to be devised that use nonlinear processing to filtering of the PDV, greatly enhancing the performance of clock recovery compared to linear PLL methods. In this presentation, Brilliant will provide experimental results and demonstrate that: (1) Practice does indeed have relationship to theory. Proper measurements are indeed consistent, repeatable, and significantly predictable; (2) Proper heuristics and multiple metrics provide high-quality clock recovery and knowledge of the transport layer (e.g., GigE, xDSL) can be applied to improve performance.