Experimental Evaluation of the Jitter Generated in Timing Transfer (original) (raw)
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2008 IEEE International Workshop on Factory Communication Systems, 2008
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2008 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication, 2008
Clock synchronization protocols for packet-oriented networks, like IEEE 1588, depend on time stamps drawn from a local clock at distinct points in time. Due to the fact that software-generated time stamps suffer from jitter caused by nondeterministic execution times, many implementations for high precision clock synchronization rely on hardware support. This allows time readings for packets with very low jitter close to the physical layer. Nevertheless, approaches using hardware support have to carefully consider influences on synchronization accuracy when it comes to the range of nanoseconds. Among others, limits come from the update interval, oscillator stability, or hardware clock frequency. This paper enlightens the limits for such implementations based on an analysis of the influences of the main factors for jitter. The conclusions give hints for efficiently optimizing current implementations.
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Synchronization over packet-switched networks has been attracting increasing attention lately, e.g. due to migration to IP transport in GSM networks. Timing is distributed by sending packet flows with constant rate. At the receiver, timing is recovered mostly based on PLL schemes that equalize packet random delay. The performance of these schemes depends significantly on the statistics of packet jitter, thus yielding a growing interest for real data measurement. We developed an experimental setup to measure IP packet jitter by active probing, aiming at statistical characterization of packet jitter in real heterogeneous networks. We present some experimental results, measured in networks including bridges and routers. Contrary to previous studies in literature, we emphasize data analysis by means of Modified Allan Variance (MAVAR) and Maximum Time Interval Error (MTIE), two well-known time-domain quantities widely used for synchronization interface specification in international standards. Although our setup is simple, the noise floor is negligible, compared to the network jitter under measurement.
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END-TO-END TIMING TRANSPARENCY IN PACKET NETWORKS: GNSS-BASED SYNCHRONIZATION OF THE END-SYSTEMS
In the past telecommunications were dominated by telephony. But now data communications are getting more and more important. Future networks, often called Next Generation Networks (NGN), will have to cope both with bursty computer data traffic and with realtime data streams such as telephony, video, and multimedia. Packet network technology is currently regarded as the most promising NGN approach. However, the real-time capability of today's packet network technology is still insufficient. This paper proposes a new solution for improving jitter performance of real-time communication services over packet networks in presence of high levels of packet delay variation. The idea is to synchronize both the transmitting and the receiving end systems to a stable common reference clock provided by a Global Navigation Satellite System (GNSS). GNSS-receivers installed in the access networks distribute the common time signal to the end systems. Simulations done for a scenario with G.729 coded voice communication over two medium-sized Ethernet LANs and a long-distance path in the global Internet show clearly, that a substantial reduction of the residual jitter at the receiving end can be achieved. In the case studied, standard deviation of the residual jitter was approximately 200 µs, compared to 3.0 ms with stateof-the-art packet network technology based on the Real-Time Protocol RTP.
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Synchronization Performance of the Precision Time Protocol
2007 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication, 2007
This paper analyzes the factors that affect the synchronization performance in peer-to-peer precision time protocol (PTP). We first study the influence of frequency drift in the absence of jitter and compare the gravity of the master drift with that of the slave drift. Then, we study the influence of jitter under the assumption of constant frequencies and the effect of averaging. The analytic formulas provide a theoretical ground for understanding the simulation results, some of which are presented, as well as the guidelines for choosing both system and control parameters.
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