UAFX defines the use of a time synchronization protocol to ensure a common sense of time among participating IA-stations.
UAFX defines the use of gPTP as defined in IEEE Std 802.1AS2020 for time synchronization of UAFX Stations. Figure 16 illustrates an exemplary time-synchronized network with two gPTP domains (see 18.104.22.168.1). The usage of an application clock as ClockSource or ClockTarget depends on the IEEE Std 802.1AS defined role for synchronization. Each instance of ClockSource or ClockTarget is bound to one PTP Instance. The number and type of PTP Instances residing on a UAFX Station depend on the gPTP domains it participates in and its composition, e.g., the presence of a Bridge Component. A UAFX Station can host more than one PTP Instance, e.g., UAFX Stations 1 and 2. A Grandmaster PTP Instance can reside on any grandmaster-capable UAFX Station, e.g., UAFX Station 3. UAFX Station 2 and IA-station support the forwarding of corrected time information in both gPTP domains via PTP Relay Instances (see 22.214.171.124.2).
The gPTP generates a tree-structure clock relationship between PTP Instances in the network. The clocks in all PTP Instances within a gPTP domain derive their time from a clock known as the Grandmaster Clock.
The following subclauses provide further informative descriptions of IEEE Std 802.1AS2020 terminology and how it applies to UAFX.
IEEE Std 802.1AS2020 defines a gPTP domain in which system timing is consistent. A gPTP domain defines the scope of gPTP message communication, state, operations, parameters, and timescale. The Grandmaster PTP Instance is the root of a gPTP domain.
UAFX supports the following two types of gPTP domains:
A PTP Instance operates in a single Bridge Component or End Station Component within exactly one gPTP domain. A UAFX Station can contain more than one PTP Instance in the same gPTP domain (see 126.96.36.199.4).
A UAFX Station can support one or more gPTP domains. Each gPTP domain is represented by a PTP Instance which can have an associated ClockSource or ClockTarget. A PTP Instance with an associated ClockSource is a Grandmaster PTP Instance. A ClockSource and ClockTarget can serve as a provider of time for a UAFX Application (e.g., sequence of events).
Figure 17 and Figure 18 show examples of a UAFX Station consisting of a single End Station Component supporting one or two gPTP domains. Each gPTP domain has an associated PTP End Instance, which connects via a PTP Port to an Ethernet port.
Figure 19 shows an example of a UAFX Station consisting of a single End Station Component and Bridge Component, each supporting two gPTP domains. This example includes Bridge Component ClockTargets and End Station Component PTP End Instances and ClockTargets for both domains. ClockTargets and PTP End Instances may be omitted in an implementation depending on the expected usage of time.
Engineered time-synchronization spanning tree (sync tree) for a given gPTP domain refers to the usage of external port configuration instead of BMCA to construct a desired sync tree with the Grandmaster PTP Instance as the root (see IEEE Std 802.1AS2020, 10.3.1).
One of the advantages of engineered sync trees is to enable a planned, deterministic, and stable configuration of the IEEE Std 802.1AS2020 sync tree for a given gPTP domain, e.g., prevent sync tree changes in case of UAFX Station addition or removal from the network. Working Clock is a use case of the engineered sync tree.
An external sync tree management entity should use the remote management interface described in clause 188.8.131.52.4 to set up an engineered sync tree.
- The externalPortConfigurationEnabled parameter is set to TRUE;
- The ptpPortEnabled parameter is set to TRUE.
For validation that the computed sync tree configuration can be applied to all PTP Ports intended to participate in the given gPTP domain, the management entity configuring the sync tree will, for example, verifies that these ports are up, IEEE Std 802.1AS2020capable, and satisfy topology constraints by checking the following parameters:
- The status of oper-status parameter is up (see IETF RFC 8343) for all participating Ethernet links;
- The status of isMeasuringDelay (see IEEE Std 802.1AS2020, 14.16.4) parameter is TRUE;
- The status of asCapable (see IEEE Std 802.1AS2020, 14.8.7) is TRUE;
- The status of asCapableAcrossDomains (see IEEE Std 802.1AS2020, 14.16.5);
- The status of gmCapable (see IEEE Std 802.1AS2020, 14.2.7) is TRUE, only applicable to the Grandmaster PTP Instance;
- Verify that the number of PTP Relay Instances (hops) between the Grandmaster PTP Instance and any given PTP End Instance is within the limit prescribed by an external sync tree management entity;
- Verify per PTP link that the value of meanLinkDelay (see IEEE Std 802.1AS2020, 14.16.6) is less than or equal to meanLinkDelayThresh (see IEEE Std 802.1AS2020, 14.16.7 and IEEE Std 802.1AS2020 Table 11-1) value to detect, e.g., an anomaly in propagation delay;
NOTEEven if neighbouring PTP Instances do report asCapable, it can be that the link between asCapable neighbouring PTP Instances is not asCapable due to, for example, the wrong setting of meanLinkDelayThresh value. The meanLinkDelayThresh value reflects the estimated propagation delay of the installed link.
The sync tree must have the following properties to ensure consistent protocol behaviour and time synchronization:
- The desiredState of all PTP Ports of the Grandmaster PTP Instance is set to MasterPort;
- The desiredState of exactly one PTP Port of all the other PTP Instances is set to SlavePort;
- The desiredState of remaining PTP Ports that are part of sync tree in non-Grandmaster PTP Relay Instances is set to MasterPort.
- The desiredState of all other PTP Ports is set to PassivePort.
After synthesis, the configuration of the gPTP domain and its engineered sync tree may then be applied and validated, for example:
NOTEClocks are currently not modelled in OPC UA, and it is a vendor decision to select a gPTP domain for use at the application layer if multiple gPTP domains are supported. Clock representation in OPC UA needs to be added in a future release for UAFX.
A UAFX Station supporting gPTP time synchronization shall support at least two gPTP domains in a Bridge Component for Working Clock and Global Time and at least one gPTP domain in an End Station Component for Working Clock or Global Time.
The gPTP message priority is defined in IEEE Std 802.1AS2020, 8.4.4.
NOTE IEEE Std 802.1AS2020 defines gPTP messages to be transmitted untagged, i.e., frames do not carry priority information for traffic class selection. At the same time, IEEE Std 802.1AS2020 neither specifies a well-defined device-internal priority nor management capabilities for the configuration of the traffic class to be used for the transmission of gPTP messages.
IETF RFC 8343 – A YANG Data Model for Interface Management, March 2018
IEEE Std 802.1AS-2011 – IEEE Standard for Local and Metropolitan Area Networks – Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks
IEEE Draft Std P802.1ASdn – Draft Standard for Local and Metropolitan Area Networks: Timing and Synchronization for Time-Sensitive Applications – Amendment: YANG Data Model