Essay On Clock Drift

2.4 Clock drift and clock skew

An inevitable problem when working with synchronization is the one of clock drift, which just as the name implies is the drift of a clock often compared to a nominal perfect reference clock. No matter the accuracy of the clock, all clocks will drift sooner or later. Therefore it needs to be considered especially when working with synchronization. Clock skew is another important phenomenon that needs to be considered.

When talking about clock skew in general it often refers to the fact that not all components in an electrical circuit receive the clock signal at exactly the same time. It may also refer to the difference in readings of clocks at different nodes on a network, such as the Internet. Simply speaking this means that there may

Relevant Work

23

2.4.1 Clock drift

The clock skew can be said to be almost constant if it would not be for the clock drift. Furthermore solutions for clock skew have been discussed, such as synchronizing the clocks using NTP or PTP as explained in Section 2.3. The upcoming sections will discuss how and to what extent this drift occurs.

2.4.2 Reasons for clock drift

As mentioned previously, an oscillating crystal is typically used as a clock in com- puters. Readings of the number of oscillations will then determine how much the clock has ticked. As can be expected, all crystals may not oscillate at the same pace, which introduces the problem of drifting clocks.

2.4.3 The magnitude of clock drift

The next question is how much it will occur, which also typically differs depending on the crystal used. The accuracy itself is measured in parts per million (ppm), or some times parts per billion (ppb), which states how many seconds per second the clock may drift in general. For example an accuracy of 1 ppm, which can be said to be typical for a normal computer grade crystal [48], would result in a drift of

1

×