Every NTP server reports a stratum: a number that says how far it is, in network hops, from a reference clock. It is one of the first things you see when checking a time server, and one of the most frequently misunderstood. This guide explains what each level means and how to use it when designing a network.
The levels
Stratum 0 — reference clocks. The devices that actually measure time: GPS and other GNSS receivers, atomic clocks and radio time receivers. They are not on the network themselves; they connect directly to a server.
Stratum 1 — primary time servers. Servers connected directly to a Stratum 0 reference. A GPS NTP time server is a Stratum 1 server: its built-in GPS receiver is the reference clock. Our ORNATE 7000 Series time servers work this way.
Stratum 2 and below. A server that synchronizes over the network from a Stratum 1 server becomes Stratum 2; one that synchronizes from Stratum 2 becomes Stratum 3, and so on. NTP (RFC 5905) allows stratum values up to 15. A value of 16 means the server is unsynchronized, and clients should not use it.
Does a lower stratum mean more accurate time?
Not necessarily. Stratum counts hops, not error. Most of the error in NTP comes from network delay and its variation, so a Stratum 2 server on the same local network can deliver better time than a distant Stratum 1 server reached across the internet. NTP also reports root delay and root dispersion — the accumulated delay and uncertainty back to the reference clock — and these are better indicators of accuracy than stratum alone.
How clients choose a server
An NTP client configured with several servers does not simply pick the lowest stratum. It measures each server, discards any whose time disagrees with the majority, and combines the remaining sources, favouring those with lower stratum and lower root distance. This is why configuring several sources matters; see NTP for data centres for a practical design.
Why run your own Stratum 1 server
Public internet time servers are typically Stratum 2 or lower and are reached over network paths you don't control. A GPS-referenced Stratum 1 server on your own network gives every device a short, predictable path to the reference, keeps working without an internet connection, and gives you one source to monitor and audit. If GPS reception is interrupted, the server falls back to its internal oscillator; our GNSS resilience guide explains how holdover works.
Stratum in practice
For a typical site: a GPS time server at Stratum 1, core infrastructure at Stratum 2, and everything else at Stratum 2 or 3 — all within the same building or campus network. Network slave clocks that display the time synchronize the same way, so the clocks on the wall agree with the timestamps in your logs.
Related products
ORNATE 7000 TSDRNT GPS time server → ORNATE 7000 TSDRNPT GPS time server → All NTP time servers →