UK The short answer
PTP vs NTP is a choice between two ways of keeping computer clocks in step over a network. The Network Time Protocol (NTP), maintained by the IETF and defined for version 4 in RFC 5905, has synchronised internet and enterprise clocks since the 1980s. The Precision Time Protocol (PTP), defined by IEEE 1588, was designed in the early 2000s for measurement and control systems that need far closer agreement than NTP can give.
The practical difference is accuracy, and what it costs to get it. NTP runs in software on almost any device and over any IP network, including the internet, and typically keeps machines on a local network within about a millisecond of each other. PTP takes its timestamps in hardware at the network interface and, with PTP-aware switches between the devices, typically reaches sub-microsecond accuracy. Specialist profiles go further still.
Neither replaces the other. A plant usually runs NTP for servers, historians and log files, and PTP only where cameras, protection relays or motion controllers must act within the same microsecond.
How each protocol measures time
At heart, both protocols solve the problem the same way: exchange timestamped packets, estimate how long a packet takes to cross the network, and correct the local clock by the difference. They differ in where the timestamps are taken and who starts the conversation.
In NTP the client asks. It sends a request to a server, the server replies, and the client uses four times (its send time, the server’s receive and send times, and its own receive time) to work out its offset and the round-trip delay. Clients typically poll every 64 to 1,024 seconds and filter the results over time. Servers form a hierarchy of strata: stratum 1 servers take time from a reference such as GNSS, stratum 2 servers follow them, and so on down.
In PTP the master speaks first. It sends Sync messages, typically once a second or faster, and each slave measures the path delay with its own Delay_Req exchange or, in peer-to-peer mode, link by link. The biggest gain comes from timestamping. PTP-capable network cards stamp each packet as it crosses the physical layer, so operating system and application delays drop out of the measurement. NTP timestamps are usually taken in software, where interrupt handling and scheduling add jitter of tens to hundreds of microseconds.
PTP vs NTP compared
| Feature | PTP | NTP |
|---|---|---|
| Defined by | IEEE 1588 (current edition 2019) | IETF RFC 5905 (NTPv4, 2010) |
| Typical accuracy | Sub-microsecond with hardware timestamping; nanoseconds with specialist profiles | Around 1 ms on a LAN; typically tens of milliseconds over the internet |
| Timestamping | Hardware, at the network interface | Usually software |
| Who starts the exchange | Master sends to slaves | Client polls a server |
| Transport | UDP ports 319 and 320, or directly over Ethernet (EtherType 0x88F7) | UDP port 123 |
| Reference hierarchy | One grandmaster per domain, chosen by the best master clock algorithm | Strata, from stratum 1 servers downwards |
| Network needs | Best with boundary or transparent clock switches on every hop | Any IP network |
| Security | Optional security mechanisms in IEEE 1588-2019; most sites rely on network segregation | Network Time Security (RFC 8915) |
| Typical uses | Machine vision, substations, motion control, broadcast, mobile networks | Servers, PCs, logging, SCADA historians |
The accuracy figures are typical, not guaranteed. A busy network of ordinary switches can push PTP into the tens of microseconds, and a well-tuned NTP client on a quiet LAN can do better than a millisecond.
Which one does your system need?
Start from the accuracy the application needs, then check what the network can support.
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Logs, alarms and historians. NTP. Lining up events across PCs, PLCs and SCADA servers to within a few milliseconds is well within its reach, and most industrial devices already include an NTP or SNTP client.
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Multi-camera machine vision. PTP. Triggering or time-stamping frames from several cameras within microseconds needs hardware timestamps, and GigE Vision cameras commonly support IEEE 1588 for this.
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Substations and protection. PTP with the power profile of IEC 61850-9-3 and IEEE C37.238, used where sampled values and fault records must line up to around a microsecond. Our explainer on IEC 61850 covers the wider standard.
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Time-sensitive networking. gPTP (IEEE 802.1AS), the PTP profile that TSN scheduling depends on.
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Financial trading records. Usually PTP. The EU’s MiFID II rules (RTS 25) require high-frequency trading clocks to stay within 100 microseconds of UTC, which is hard to guarantee with NTP.
Running PTP and NTP together
Most networks that use PTP also run NTP. Many grandmaster appliances serve both from the same GNSS reference, so PTP devices and ordinary IT equipment trace to the same time. On a Linux computer, the open-source linuxptp tools lock the network card’s hardware clock to PTP and then steer the system clock from it, and an NTP server such as chrony can pass that time on to other machines.
The hardware has to be chosen for it. A PTP client is only as good as its network card, so check for hardware timestamping, not just “PTP support”. The multi-port LAN cards among our Ethernet and PoE frame grabbers list IEEE 1588 PTP, which lets an industrial PC time-stamp camera traffic accurately. In the network, several of our managed industrial switches list SNTP or NTP time synchronisation, and selected models add IEEE 1588 v2 PTP. Our white paper on synchronising machine vision with IEEE 1588 works through a full PTP design, from grandmaster to camera, and where satellites are the reference, our explainer on GNSS vs GPS covers the satellite side.
Quick answers on PTP vs NTP
PTP vs NTP: common questions
Is PTP more accurate than NTP?
Yes, typically by around a thousand times. With hardware timestamping and PTP-aware switches, PTP reaches sub-microsecond accuracy, where NTP manages around a millisecond on a local network.
Does NTP use TCP or UDP?
UDP, on port 123. Time packets never use TCP. Only the key exchange for Network Time Security (RFC 8915) runs over TLS on TCP port 4460.
Can PTP work over the internet?
Not in practice. PTP relies on short, symmetric paths, ideally with PTP-aware switches on every hop. Internet paths vary too much, so NTP is used there.
What is SNTP?
A simplified NTP client. Simple NTP uses the same packets without NTP’s filtering and server selection, which suits switches, PLCs and other devices that only follow one server.
Can NTP ever match PTP?
Not as standard. NTP with hardware timestamps on a dedicated network can approach microseconds, but few devices support it. PTP is the standard route to that accuracy.
We supply PTP- and NTP-capable network cards and switches inside complete industrial systems, and as standard we set the time sources, profiles and clock roles before the system ships.