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# Lec7 - ECE151 Lecture 7 Chapter 5 Synchronization ECE151...

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ECE151 – Lecture 7 1 ECE151 – Lecture 7 Chapter 5 Synchronization

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ECE151 – Lecture 7 2 Clock Synchronization When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time. Physical clocks Logical clocks Vector clocks
ECE151 – Lecture 7 3 Physical Clocks Problem: Sometimes we simply need the exact time, not just an ordering. Solution: Universal Coordinated Time (UTC): Based on the number of transitions per second of the cesium 133 atom (pretty accurate). At present, the real time is taken as the average of some 50 cesium- clocks around the world. Introduces a leap second from time to time to compensate that days are getting longer. UTC is broadcast through short wave radio and satellite. Satellites can give an accuracy of about 0 5 ms. Question: Does this solve all our problems? Don’t we now have some global timing mechanism?

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ECE151 – Lecture 7 4 Physical Clocks (1) Computation of the mean solar day.
ECE151 – Lecture 7 5 Physical Clocks (2) TAI seconds are of constant length, unlike solar seconds. Leap seconds are introduced when necessary to keep in phase with the sun.

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ECE151 – Lecture 7 6 Physical Clocks Problem: Suppose we have a distributed system with a UTC-receiver somewhere in it we still have to distribute its time to each machine. Basic principle: Every machine has a timer that generates an interrupt H times per second. There is a clock in machine p that ticks on each timer interrupt. Denote the value of that clock by Cp(t) , where t is UTC time. Ideally, we have that for each machine p , Cp(t) = t , or, in other words, dC/dt = 1
ECE151 – Lecture 7 7 Clock Synchronization Algorithms The relation between clock time and UTC when clocks tick at different rates. In practice: 1 - ρ < dC/dt < 1 + ρ Goal: Never let two clocks in any system differ by more than δ time units synchronize at least every δ /(2 ρ ) seconds.

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ECE151 – Lecture 7 8 Clock Synchronization Principles Principle I: Every machine asks a time server for the accurate time at least once every δ /(2 ρ ) seconds. Okay, but you need an accurate measure of round trip delay, including interrupt handling and processing incoming messages. Principle II: Let the time server scan all machines periodically, calculate an average, and inform each machine how it should adjust its time relative to its present time. Okay, you’ll probably get every machine in sync. You don’t even need to propagate UTC time (why not?) Fundamental problem: You’ll have to ensure that setting time back is never allowed (smooth adjustments)
ECE151 – Lecture 7 9 Cristian's Algorithm Getting the current time from a time server.

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Lec7 - ECE151 Lecture 7 Chapter 5 Synchronization ECE151...

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