on corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. That clears things up, doesn't it? This definition refers to the reference used with atomic clocks. Atomic clocks are the most accurate clocks available to the public. They are considered to be the reference by which all other clocks should be set. If we consider atomic clocks to be the gold standard by which other clocks should be measured, we can also categorize other clocks by how accurate they are compared to the atomic clock standard. In NTP, each clock exists inside a stratum. An actual atomic clock is a stratum-zero time source. A device that serves NTP requests that is directly connected to the atomic clock is considered to be at stratum one. If you set up an NTP server that learns time from a stratum-one NTP server, your server will become a stratum-two server, and SO on down the line. Strata in NTP reference how many levels of clocks you are removed from the actual time source. A clock operating in stratum 16 is considered to be unsynchronized. Strata are defined differently in NTP than they are in telecommunica- tions systems. In telecom, stratum levels reference the holdover performance of an oscillator should synchronization be lost. If an oscillator is accurate to 1.0 * 1011 it is considered to be stratum-one. Stratum-two oscillators are accurate to 1.6 108. In other words, the level of accuracy is not tied to the distance from the source clock, as it is with NTP. What Is Accurate Time? NTP Design NTP is often not designed, but rather implemented in the simplest possible way. Many people mistakenly believe all they need to do is configure a single NTP source and their time problems will be solved. This idea is perpetuated because it usually works. But what would happen to your network if the original time source stopped responding or became inaccurate? I learned about NTP the hard way. I configured a single time source for the core switches on a large network. I thought I was being clever 