--- title: "Physical vs logical clock; sync method" subject: "Distributed System" chapter: 5 key: "semester_7/distributed_system:physical-clock-sync-difficulty" source: https://www.bctengineer.com/q/semester_7/distributed_system/physical-clock-sync-difficulty site: BCT Engineering --- # Physical vs logical clock; sync method Distributed System · Chapter 5: Time and State in Distributed Systems - Asked in: 2072 Chaitra, 2+2+6 marks - Video: [Clock Synchronisation | Distributed Systems | Lec-49 | Bhanu Priya](https://www.youtube.com/watch?v=aiJTAqu4gxs) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/distributed_system/physical-clock-sync-difficulty - Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-5 ## Question Differentiate between physical clock and logical clock. Why is it difficult to synchronize physical clock? Describe a method for physical clock synchronization. ## Short answer Physical clocks differ from logical clocks because they track real time and drift. Synchronization is difficult due to clock drift, variable and asymmetric network delays, single points of failure, and the need not to run clocks backward. Cristian's algorithm sets the clock to server time plus half the round-trip delay, assuming symmetric delay. ## Answer #### Physical vs logical clock | Point | Physical clock | Logical clock | |---|---|---| | What it keeps | Actual time of day | Counter giving event order | | Based on | Hardware oscillator | Happened-before relation | | Unit | Seconds | Dimensionless numbers | | Needs sync | Yes, to UTC or among themselves | No external sync | | Drift | Yes | None | | Examples | Cristian, Berkeley, NTP | Lamport, vector clock | | Use | Real-time stamping, billing | Causal ordering, consistency | #### Why synchronising physical clocks is difficult 1. **No global clock**: every node has its own crystal; they run at different rates (clock drift, ~1 s in 10⁵–10⁶ s) and differ in skew. 2. **Variable network delay**: the time to carry a message is unpredictable (congestion, queueing, routing), so the time received is already out of date. 3. **Asymmetric delays** for request and reply make delay estimates inexact. 4. **Failures**: time server or links may fail, clocks may be faulty. 5. Clocks must not run backward (breaks ordering such as file timestamps), so correction must be gradual. 6. Cost and scale: many nodes and wide area networks make frequent synchronisation expensive. 7. Even after synchronisation, drift makes clocks diverge again, so it must be repeated. #### Method: Cristian's algorithm A client sends a request to a time server (which has a UTC source) at T0 and receives the server's time T_S at T1. It sets its clock to T_new = T_S + (T1 − T0) / 2 assuming equal delay in both directions. Accuracy is within ± ((T1 − T0)/2 − Tmin). To improve: repeat several times and use the reading with the minimum round trip time; if T_new is behind the current time, slow the clock down gradually instead of jumping back. Example: T0 = 5.000 s, T1 = 5.040 s (local), T_S = 9.000 s. T_new = 9.000 + 0.020 = 9.020 s. Limit: the single server is a point of failure; Berkeley and NTP overcome this. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does variable network delay make physical synchronisation hard? A. Messages cannot be sent at all B. It causes clocks to run backwards C. It changes the crystal frequency D. The received time value is already out of date and delays may differ by direction Answer: D. Unpredictable and asymmetric delays make delay estimates inexact. (Key point: Delay problem) 2. Why must a clock that is ahead be slowed rather than set back? A. Moving backwards breaks ordering of events such as file timestamps B. Hardware cannot be reset C. It is cheaper D. UTC forbids it Answer: A. Time going backwards would give later events earlier timestamps. (Key point: Backward clock) 3. T0 = 5.000 s, T1 = 5.040 s, T_S = 9.000 s. What is T_new? A. 9.040 s B. 9.020 s C. 9.000 s D. 9.080 s Answer: B. Half the RTT 0.040 is 0.020, added to 9.000. (Key point: Cristian calc) 4. What limitation of Cristian's algorithm do Berkeley and NTP overcome? A. Need for vector clocks B. Use of logical timestamps C. Single time server as a point of failure D. Lack of a round trip Answer: C. Berkeley elects another master; NTP uses many servers. (Key point: Cristian limit) ## Close variants set in other exams - [Synchronising physical clock using logical clock](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Async-physical-with-logical) — Distributed System, chapter 5, asked 2× ## Related in Distributed System - [Physical clock synchronization algorithm](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aphysical-clock-sync-algorithm) — Distributed System, chapter 5 - [Need of logical clock synchronization](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Alogical-clock-sync-need) — Distributed System, chapter 5 - [Physical/logical clocks; NTP and Berkeley](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aphysical-logical-ntp-berkeley) — Distributed System, chapter 5 - [Physical vs logical clocks; Lamport](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Alamport-physical-compare) — Distributed System, chapter 5 - [Lamport's logical clock algorithm](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Alamport-clock) — Distributed System, chapter 5, asked 5× - [Clock sync problems; Chandy-Lamport](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aphysical-clock-problems-chandy-lamport) — Distributed System, chapter 5 ## Related in other subjects - [Clock synchronization in DFS](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Aclock-synchronization-dfs) — Big Data Technologies, chapter 1 - [Logical and physical address bits](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Aresidence-monitor-address-bits) — Operating System, chapter 4