Clock Strata Explained

NTP uses a hierarchical, semi-layered system of time sources. Each level of this hierarchy is termed a "stratum" and is assigned a number starting with zero at the top. A server synchronized to a stratum n server will be running at stratum n + 1. The number represents the distance from the reference clock and is used to prevent cyclical dependencies in the hierarchy. Stratum is not always an indication of quality or reliability; it is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources. Telecommunication systems use a different definition for clock strata. A brief description of strata 0, 1, 2 and 3 is provided below.

Stratum 0
These are high-precision timekeeping devices such as atomic (cesium, rubidium) clocks, GPS clocks or other radio clocks. They generate a very accurate pulse per second (PPS) signal that triggers an interrupt and timestamp on a connected computer. Stratum 0 devices are also known as reference clocks.

Stratum 1
These are computers whose system clocks are synchronized to within a few microseconds of their attached stratum 0 devices. Stratum 1 servers may peer with other stratum 1 servers for sanity checking and backup. They are also referred to as primary time servers.

Stratum 2
These are computers that are synchronized over a network to stratum 1 servers. Often a stratum 2 computer will query several stratum 1 servers. Stratum 2 computers may also peer with other stratum 2 computers to provide more stable and robust time for all devices in the peer group.

Stratum 3
These are computers that are synchronized to stratum 2 servers. They employ exactly the same algorithms for peering and data sampling as stratum 2, and can themselves act as servers for stratum 4 computers, and so on.