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Clock Synchronization in Distributed Systems Using NTP and PTP
Published in Richard Zurawski, Industrial Communication Technology Handbook, 2017
Reinhard Exel, Thilo Sauter, Paolo Ferrari, Stefano Rinaldi
In software timestamping systems, a timestamp at the transmitter is taken when handing a packet over to the network stack of the OS, and at the receiver when retrieving the packet from the OS. This is shown by the egress timestamps t1^ and t3^, and the ingress timestamps t2^ and t4^ in Figure 21.10. Thus, p1 and p3 are dependent on the implementation of the network stack in the master or slave. As the ingress timestamps t2^ and t4^ are usually generated after the entire frame is received, p2 and p4 contain the transmission delay as well. When the link speed is asymmetric, the transmission delays and therefore the processing delays are different (p2 ≠ p4), leading to a clock offset. Thus, in any case, it is important to correct all timestamps to the reference plane; that is, for software-based timestamping, the frame length, link speed, and other parameters need to be taken into account. In fact, the issue of correcting timestamps to the reference plane also applies to hardware timestamping. For instance, the DP83848 Ethernet PHY from Texas Instruments [16] has a receive latency of 255 ns and a transmit latency of 50 ns. Thus, a synchronization bias is induced when using PHYs with different latencies (p2 − p3 ≠ p4 − p1), for example, different PHY devices.
‘Un’-blocking the industry 4.0 value chain with cyber-physical social thinking
Published in Enterprise Information Systems, 2023
Subodh Mendhurwar, Rajhans Mishra
Psaila (2017) explored blockchain-impact as a culture shift on traditional audit and assurance process. Sutton and Samavi (2017) proposed a blockchain-based ‘linked data’ methodology to devise tamper-proof audit logs (recording participants’ actions in line with privacy policies of data sharing environments) to offer proof of log manipulation and non-repudiation during audit compliance checking. Sahlin and Levenby (2018) feel that blockchains (featuring real-time verification of general ledger-driven chronological journal entries to audit trail maintained by organisational management) improve audit efficiency, permitting auditors to appreciate the data in lieu of sampling to verify transactions. This could be specifically relevant in the context of various audits and certifications pertaining to Industry 4.0 processes, smart factories, and goods. Savelyev (2018) believes that blockchain can enable trusted timestamping about copyright information replacing existing inelegant methods (e.g. registration in patent offices); and provide an enhanced level of trust and scalability as compared with digital fingerprint mechanisms (e.g. ContentID in YouTube, Russian social network VKontakte, etc.); coupled with business organisational law to fill gaps in blockchain smart contracts (Rodrigues 2018).