Interoperability between different IRIG, NTP and PTP protocols

Why interoperating IRIG with other synchronisation mechanisms?

An IRIG network can evolve by integrating other equipment that do not have the same time synchronisation needs. In that case, the question arises of integrating a new time synchronisation protocol into the network in order for it to interoperate with the existing IRIG standard.

The different synchronisation protocols have not been developed to be interoperable: they do not offer the same guarantees and do not all require the same hardware to operate.

As a result, there is no solution available for interoperating different protocols. Even different standards such as IRIG A and IRIG B cannot be simply interoperated. Indeed, they do not use the same frame type, carrier frequency and modulation. As a result, it is not always possible to use the same hardware. It is also impossible to encapsulate IRIG frames in NTP or PTP protocols (and vice versa).

IRIG interoperability in practice

The practical solution for interoperating a synchronisation protocol with IRIG is to convert the protocols at the edge of the application. This conversion can be carried out via software or hardware.

Converting via software can introduce a delay and therefore degrade the accuracy of time synchronisation. It is therefore recommended to use specific hardware in order to interoperate IRIG with other time synchronisation protocols such as NTP or PTP.

To do so, time servers will be used with IRIG inputs and NTP or PTP outputs (and vice versa). This type of server, for example Netsilon 9 and 11 time servers, is responsible for converting one protocol to another. However, attention should be paid to minimise the number of round trips, since each change of protocol is a source of additional delay which will degrade the accuracy of the system clocks.

Another possibility is to connect a multi-protocol time server directly to the reference clock. With its multiple outputs, the time server delivers the reference time on both the IRIG network and an NTP or PTP network. This ensures that each part of the network has the same time reference with a synchronisation protocol suitable for each equipment.

Some key points

IRIG offers high accuracy. As such, it is important not to use reference clocks from a less accurate protocol as part of interoperability.

For example, NTP (Network Time Protocol) offers a precision of milliseconds. As a result, it is not recommended to use it as reference clock and convert messages to synchronise an IRIG network. Nevertheless, it is possible to use a PTP (Precision Time Protocol) time server to synchronise two remote IRIG networks, as PTP accuracy is sufficient for this purpose.

Since IRIG is limited to local networks, it is advisable to switch to a network synchronisation protocol such as NTP or PTP when the reference clock must synchronise remote machines. PTP seems to be a great option due to its accuracy. However, its implementation is complex, as it requires defining a network architecture beforehand as well as specific hardware. If an accuracy of milliseconds is enough, then NTP will generally be the best compromise. Whatever solution is chosen, it is important to make sure that equipment are compatible with each other so that the change of protocol is carried out correctly with the minimum additional latency.

To conclude

Interoperating IRIG with other synchronisation protocols such as NTP or PTP presents a number of challenges if there is the need to maintain time accuracy. However, none of these challenges is insurmountable as long as the network is well thought out and the equipment is suitable.