NTP stratum levels and accuracy budgets

In NTP architectures you will hear terms like "stratum 1" or "stratum 3". That is a hierarchical ranking of clocks based on distance from the original time source, and it strongly determines how accurate each layer is.

What does each stratum level mean?

• Stratum 0: the fundamental time source, an atomic clock in a national laboratory, a GPS satellite, a GNSS receiver. Not directly accessible over the network.
• Stratum 1: an NTP server that synchronises directly with a stratum 0 source via hardware (GNSS antenna, IRIG cable, etc.). Very accurate, typically sub-100 μs.
• Stratum 2: synchronises with one or more stratum 1 servers over the network. Accuracy: 1–10 ms on LAN, tens of ms over WAN.
• Stratum 3: synchronises with stratum 2. Accuracy: typically 10–100 ms.
• Stratum 4–15: derived even further. Rarely used for IT purposes in practice.
• Stratum 16: unsynced (the server does not trust its own time).

What does that mean for your network?

In a typical office or data-centre deployment:

• Place one or two stratum 1 servers (an NTP100-GNSS plus GMR redundant for example) as the source
• Let key application servers (database, monitoring, SIEM) synchronise against that stratum 1, they become stratum 2
• Workstations, IoT devices, clocks synchronise against the stratum 2 servers, they become stratum 3
• Redundancy: every client can synchronise against 3+ NTP servers at once, so that a single failure is not a disaster

How much accuracy do you lose per stratum layer?

Every hop adds uncertainty. A rough rule of thumb:

Layer	Typical LAN accuracy
Stratum 1 (your own GNSS server)	10–100 μs
Stratum 2 (server in this data centre)	0.5–5 ms
Stratum 3 (client PC)	5–50 ms
Stratum 4+	depends

Enough for 99% of business applications, TLS certificate validation, audit-trail sorting, database replication.

When is it not enough?

• High-frequency trading: requires sub-millisecond, so stratum 1 directly plus PTP
• Broadcast frame sync: stratum 1 plus PTP or even SMPTE 2110
• Digital substation IEC 61850: PTP grandmaster instead of an NTP hierarchy
• Scientific timing: direct atomic clock or GPS disciplined Rubidium

In those cases you are talking about a fundamentally different architecture, PTP, not NTP. See our article NTP vs PTP.

In practice via Daylight

For most customers, 1 stratum 1 server plus redundancy is sufficient. Too many customers buy 3 stratum 1 servers because they "want more". Call Daylight and we will sketch the architecture out together, less hardware can work perfectly fine.

Sources

• RFC 5905 — NTPv4 specification section 3 on stratum
• NTP Pool Project, an example of a worldwide stratum 2/3 distribution