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This Concept Map, created with IHMC CmapTools, has information related to: Chapter 11, Coordinated Universal Time (UTC) International Atomic Time is based on very accurate physical clocks (drift rate 10-13) UTC is an international standard for time keeping It is based on atomic time, but occasionally adjusted to astronomical time It is broadcast from radio stations on land and satellite (e.g. GPS) Computers with receivers can synchronize their clocks with these timing signals Signals from land-based stations are accurate to about 0.1-10 millisecond Signals from GPS are accurate to about 1 microsecond also Synchronization in a synchronous system a synchronous distributed system is one in which the time to execute each step of a process has known lower and upper bounds each message transmitted over a channel is received within a known bounded time each process has a local clock whose drift rate from real time has a known bound, Computer clocks and timing events Each computer in a DS has its own internal clock used by local processes to obtain the value of the current time processes on different computers can timestamp their events but clocks on different computers may give different times computer clocks drift from perfect time and their drift rates differ from one another. clock drift rate: the relative amount that a computer clock differs from a perfect clock Even if clocks on all computers in a DS are set to the same time, their clocks will eventually vary quite significantly unless corrections are applied contains Skew between computer clocks in a distributed system Computer clocks are not generally in perfect agreement Skew: the difference between the times on two clocks (at any instant) Computer clocks are subject to clock drift (they count time at different rates) Clock drift rate: the difference per unit of time from some ideal reference clock Ordinary quartz clocks drift by about 1 sec in 11-12 days. (10-6 secs/sec). High precision quartz clocks drift rate is about 10-7 or 10-8 secs/sec, Skew between computer clocks in a distributed system Computer clocks are not generally in perfect agreement Skew: the difference between the times on two clocks (at any instant) Computer clocks are subject to clock drift (they count time at different rates) Clock drift rate: the difference per unit of time from some ideal reference clock Ordinary quartz clocks drift by about 1 sec in 11-12 days. (10-6 secs/sec). High precision quartz clocks drift rate is about 10-7 or 10-8 secs/sec and Coordinated Universal Time (UTC) International Atomic Time is based on very accurate physical clocks (drift rate 10-13) UTC is an international standard for time keeping It is based on atomic time, but occasionally adjusted to astronomical time It is broadcast from radio stations on land and satellite (e.g. GPS) Computers with receivers can synchronize their clocks with these timing signals Signals from land-based stations are accurate to about 0.1-10 millisecond Signals from GPS are accurate to about 1 microsecond, We need to measure time accurately: to know the time an event occurred at a computer to do this we need to synchronize its clock with an authoritative external clock Algorithms for clock synchronization useful for concurrency control based on timestamp ordering authenticity of requests e.g. in Kerberos There is no global clock in a distributed system this chapter discusses clock accuracy and synchronisation so that Computer clocks and timing events Each computer in a DS has its own internal clock used by local processes to obtain the value of the current time processes on different computers can timestamp their events but clocks on different computers may give different times computer clocks drift from perfect time and their drift rates differ from one another. clock drift rate: the relative amount that a computer clock differs from a perfect clock Even if clocks on all computers in a DS are set to the same time, their clocks will eventually vary quite significantly unless corrections are applied