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Network-based Synchronization

    Last Modified: May 11, 2018

    IEEE 1588, also known as the precision time protocol (PTP), is an Ethernet-based synchronization method designed for cabled, local networks. The PTP protocol provides a fault tolerant method of synchronizing all participating clocks to the highest quality clock in the network. This method of synchronization between networked devices uses packet-based communication and is possible over the long distances allowed for each Ethernet link, without signal propagation impact. IEEE 1588 has many different profiles, such as IEEE 802.1AS-2011, each of which use different features. Because the profiles are not interoperable with each other, make sure it is known which profile is implemented on the device. For devices on the network to synchronize with each other using IEEE 1588, all devices must be compatible with the desired IEEE 1588 profile and must all be connected within the selected IEEE 1588 profile-compliant network infrastructure.

    The cRIO-904x controllers are compatible with both the IEEE 802.1AS-2011 profile and the IEEE 1588-2008 (1588v2) Delay Request-Response profile. However, each network port must be configured individually to the specific profile required for the network.

    Differences Between IEEE 802.1AS-2011 and IEEE 1588-2008

    IEEE 802.1AS-2011, also known as the generalized precision time protocol (gPTP), is a profile of IEEE 1588. A cRIO-904x controller can be configured to use either the IEEE 802.1AS-2011 profile or the IEEE 1588-2008 profile by configuring the port’s time reference. If a user does not explicitly specify which time reference to use a cRIO-904x controller will default to use the IEEE 802.1AS-2011 profile. There are some differences between the IEEE 802.1AS-2011 profile and the IEEE 1588-2008 profile which are called out below:
    • IEEE 802.1AS-2011 assumes all communication between devices is done on the OSI layer 2, while IEEE 1588-2008 can support various layer 2 and layer 3-4 communication methods. The IEEE 1588-2008 profile National Instruments implements on the cRIO-904x only supports layer 3-4 communication methods. Operating on the layer 2 yields better performance for the IEEE 802.1AS-2011.

    • IEEE 802.1AS-2011 only communicates gPTP information directly with other IEEE 802.1AS devices within a system. Therefore, there must be IEEE 802.1AS-2011 support along the entire path from one IEEE 802.1AS-2011 device to another. With IEEE 1588-2008, it is possible to use non-IEEE 1588-2008 switches between two IEEE 1588-2008 devices. The benefit of having IEEE 802.1AS-2011 support along the entire path is a faster performance and lower jitter compared to IEEE 1588-2008.

    • With IEEE 802.1AS-2011 there are only two types of time-aware systems: time-aware end stations and time-aware bridges. Whereas with IEEE 1588-2008, there are the following: ordinary clock, boundary clock, end-to-end transparent clock and a time-aware bridges. Based on these factors, IEEE 802.1AS-2011 can reduce complexity and configuration challenges compared to IEEE 1588-2008. A cRIO-904x controller acts as a time-aware end station for both protocols.

    IEEE 1588 External Switch Requirements

    To take advantage of the network synchronization features of the cRIO-904x controllers, ensure that your network infrastructure meets certain requirements depending on which IEEE 1588 profile is implemented for your application:
    • IEEE 802.1AS-2011 support—Automatically enables timebase synchronization and enables the use of time-based triggers and timestamping between devices across the network. Synchronization performance will meet NI product specifications.

    • IEEE 1588-2008 support—Enables timebase synchronization and enables the use of time-based triggers and timestamping between devices across the network. Synchronization performance will vary and may not meet NI product specifications. As a default configuration for IEEE 1588-2008, NI supports the IEEE 1588 Delay Request-Response profile, using the UDP over IP transport (layer 3-4).

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