Clocks
Sometimes an exact time is needed, not just an order.
UTC: Universal Coordinated Times
Based on the number of transitions of an atom of cesium 133 in one second (atomic clock).
Currently, the UTC value is the average between 50 atomic clocks from all around the globe.
It introduces a leap second to compensate for the fact that days get increasingly longer.
UTC values are radio-broadcasted in short-wave and satellite.
Satellites have a precision of about 5ms.
Synchronization
Precision
The objective is to keep the drift between two clocks under a limit π.
∀t,∀p,q : |Cp(t)−Cq(t)| ≤ π
C(p) is the calculated time value for the p machine at t UTC time instant.
Exactness
The objective is to keep the drift between the clock and the UTC time.
∀t,∀p : |Cp(t)−t| ≤ α
Synchronization
Internal: precise clocks.
External: exact clocks.
Clock Drift
Specifications of a clock:
Maximum clock drift rate - ρ
F(t) is the oscillation frequency of a hardware clock at a given t moment.
F is the ideal clock (constant) frequency.
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