Table of Contents
Advertisement
Network-based Synchronization
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-905x 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-905x 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-905x
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-905x 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-905x controller acts as a
time-aware end station for both protocols.
NI cRIO-905x User Manual | © National Instruments | 17
Hide quick links:
Advertisement
Table of Contents
