NTP vs. PTP: Which protocol to choose according to your needs?

It is paramount to make sure that all machines on a computer network are synchronised for software or industrial operations of many economic sectors. There are several ways to time synchronise machines via the network. To do so, time synchronisation protocols are used to synchronise different machines by broadcasting timestamps. These protocols usually broadcast the time of a reference clock such as an atomic clock or a GPS. However, the more extended a network, the more difficult it can be to synchronise machines.

Nowadays, the most widely used protocols are NTP (Network Time Protocol) and PTP (Precision Time Protocol). Each one of these protocols meets specific purposes which are more or less suitable depending on required applications.

All you need to know about PTP

Precision Time Protocol (PTP) is a time synchronisation protocol which ensures that all clocks within the same network are synchronised with the minimum possible offset. To do so, the PTP protocol will broadcast the time of a reference clock, which is most of the time an atomic clock or a GPS.

The PTP protocol is standardised under the name IEEE-1588. The first version dates back to 2002. It has been developed to offer greater accuracy than existing protocols (such as NTP Network Time Protocol). PTP offers a theoretical accuracy in the nanosecond range on LAN networks.

Computer redundancy and time servers

The breakdown of a service within a computer network is an issue that must be avoided where possible, and resolved as quickly as possible when it could not be avoided.

When the service in question turns out to be an essential component of several other services such as time synchronisation via a time server implementing the PTP or NTP protocol, then the breakdown can have disastrous consequences. Fortunately, there are several ways to avoid such breakdowns to happen.

PTP for broadcast networks

Whether they broadcast video (television or streaming) or audio (radio) content, time synchronisation is paramount for broadcast networks.

Indeed, if equipment is not perfectly synchronised, various errors can occur such as audio/video desynchronisation, or editing errors caused by inverted frames.

These issues can be amplified due to poor network quality or very important traffic. It is therefore important for time synchronisation to be optimum in order to offer a satisfying user experience.

Why is it essential to display an accurate time in an airport?

Beyond a simple time display, an airport must deliver a reliable, accurate and highly secure time to all equipment present in the different airport terminals.

An exact time enables to coordinate operations across the airport network, to organise departures and arrivals, to anticipate delays, etc. A single time reference must be deployed across all devices on the IT network to achieve synchronisation (clocks, computers, check-in counters, video surveillance systems, and so on). To meet the need of high time reliability and ongoing availability of time information, airports equip their control tower and operations centre with high-precision time servers.

Accuracy and resolution of IRIG signals

IRIG standards (Inter-Range Instrumentation Group) define a set of timestamp broadcast parameters for synchronising devices over a limited network. Each standard defines a frame format and a transmission method (carrier frequency, modulation, etc). Not all standards have the same resolution and are able to obtain the same accuracy.

Interoperability between different IRIG, NTP and PTP protocols

IRIG (Inter-Range Instrumentation Group) is a set of standards for sending timestamp frames via an electrical signal. Each IRIG standard defines a frame type and the way it is transported (carrier, modulation, and so on). IRIG standards are widely used since they are robust and often implemented in industrial and technical environments. IRIG require specific hardware and has been designed to operate over short distances.

PTP (Precision Time Protocol) is a protocol that enables clocks to be synchronised with a theoretical accuracy of the order of a nanosecond. PTP is the protocol to be used for all sensitive applications (energy, industry, audiovisual, finance, transport, and so on). This protocol works on local networks, but it is also able to operate over very long distances. This makes it particularly useful for extensive networks with several remote sites.

When a network becomes very extensive, it is possible that not all clocks are used for the same purposes and that they do not require the same quality of synchronisation. With no way of dividing a network, it is mandatory to maintain the highest accuracy for all clocks, which involves unnecessary costs. Indeed, the PTP protocol only provides one master clock (GMC, for Grandmaster Clock) by network.

What are the GNSS vulnerabilities and alternative solutions for maintaining accurate time synchronisation?

The Global Navigation Satellite System (GNSS) consists of a constellation of satellites placed in space. They aim at providing accurate information on position and time to receivers located on Earth. This allows providing positioning, navigation and time synchronisation data (PNT for Positioning, Navigation, Timing).