The PublishingInterval, the SamplingOffset the PublishingOffset and the timestamp in the NetworkMessage header shall use the same time base.

If an underlying network provides a synchronized global clock, this clock shall be used as the time base for the Publisher and Subscriber.

The beginning of a PublishingInterval shall be a multiple of the PublishingInterval relative to the start of the time base. The reference start time of the PublishsingInterval can be calculated by using the following formula:

Start of periodic execution =

current time + PublishingInterval – (current time % PublishingInterval)

Current time is the number of nanoseconds since the start of epoch used by the reference clock.

PublishingInterval is the duration in nanoseconds.

Start of periodic execution is the number of nanoseconds since the start of epoch which is the next possible start of a PublishingInterval.

Figure 23 shows an example how to select the possible start of a PublishingInterval.

Figure 23 – Start of the periodic publisher execution

The different timing offsets inside a PublishingInterval cycle on Publisher and Subscriber side are shown in Figure 24. The SamplingOffset and PublishingOffset are defined as parameters of the UADP WriterGroup. The ReceiveOffset and the ProcessingOffset are defined as parameters of the UADP DataSetReader in 6.3.1.3.

Figure 24 – Timing offsets in a PublishingInterval

The GroupVersion with DataType VersionTime reflects the time of the last layout change of the content of the NetworkMessages published by the WriterGroup. The VersionTime DataType is defined in OPC 10000-4. The GroupVersion changes when one of the following parameters is modified:

• NetworkMessageContentMask of this WriterGroup
• Offset of any DataSetWriter in this WriterGroup
• MinorVersion of the DataSet of any DataSetWriter in this WriterGroup
• DataSetFieldContentMask of any DataSetWriter in this WriterGroup
• DataSetMessageContentMask of any DataSetWriter in this WriterGroup
• DataSetWriterId of any DataSetWriter in this WriterGroup

The GroupVersion is valid for all NetworkMessages resulting from this WriterGroup.

The DataSetOrdering defines the ordering of the DataSetMessages in the NetworkMessages. Possible values for DataSetOrdering are described in Table 51. The default value is Undefined_0.

The DataSetOrderingType is an enumeration that specifies the possible options for the ordering of DataSetMessages inside NetworkMessages. The possible enumeration values are described in Table 51.

Table 51 – DataSetOrderingType Values

Value	Description
Undefined_0	The ordering of DataSetMessages is not specified.
AscendingWriterId_1	DataSetMessages are ordered ascending by the value of their corresponding DataSetWriterIds.
AscendingWriterIdSingle_2	DataSetMessages are ordered ascending by the value of their corresponding DataSetWriterIds and only one DataSetMessage is sent per NetworkMessage.

If DataSetOrdering is Undefined_0 any ordering between DataSets and their distribution into NetworkMessages is allowed. Ordering and distribution even may change between each PublishingInterval. If DataSetOrdering is set to AscendingWriterId_1 the Publisher has to fill up each NetworkMessage with DataSets with an ascending order of the related DataSetWriterIds as long as the accumulated DataSet sizes will not exceed the MaxNetworkMessageSize. The different options are shown in Figure 25.

Figure 25 – DataSetOrdering and MaxNetworkMessageSize

The parameter NetworkMessageContentMask defines the optional header fields to be included in the NetworkMessages produced by the WriterGroup. The DataType for the UADP NetworkMessage mapping is UadpNetworkMessageContentMask.

The DataType UadpNetworkMessageContentMask is formally defined in Table 52.

Table 52 – UadpNetworkMessageContentMask Values

Value	Bit No.	Description
PublisherId	0	The PublisherId is included in the NetworkMessages.
GroupHeader	1	The GroupHeader is included in the NetworkMessages.
WriterGroupId	2	The WriterGroupId field is included in the GroupHeader. The flag is only valid if Bit 1 is set.
GroupVersion	3	The GroupVersion field is included in the GroupHeader. The flag is only valid if Bit 1 is set.
NetworkMessageNumber	4	The NetworkMessageNumber field is included in the GroupHeader. The field is required if more than one NetworkMessage is needed to transfer all DataSets of the group. The flag is only valid if Bit 1 is set.
SequenceNumber	5	The SequenceNumber field is included in the GroupHeader. The flag is only valid if Bit 1 is set.
PayloadHeader	6	The PayloadHeader is included in the NetworkMessages.
Timestamp	7	The sender timestamp is included in the NetworkMessages.
PicoSeconds	8	The sender PicoSeconds portion of the timestamp is included in the NetworkMessages.
DataSetClassId	9	The DataSetClassId is included in the NetworkMessages.
PromotedFields	10	The PromotedFields are included in the NetworkMessages.

The UadpNetworkMessageContentMask representation in the AddressSpace is defined in Table 53.

Table 53 – UadpNetworkMessageContentMask Definition

Attributes	Value
BrowseName	UadpNetworkMessageContentMask
IsAbstract	False
References	NodeClass	BrowseName	DataType
Subtype of UInt32 defined in OPC 10000-5.
HasProperty	Variable	OptionSetValues	LocalizedText [ ]

The SamplingOffset with the DataType Duration defines the time in milliseconds for the offset of creating the NetworkMessage in the PublishingInterval cycle.

Any negative value indicates that the optional parameter is not configured. In this case the Publisher shall calculate the time before the PublishingOffset that is necessary to create the NetworkMessage in time for sending at the PublishingOffset.

The Duration DataType is a subtype of Double and allows configuration of intervals smaller than a millisecond.

The PublishingOffset is an array of DataType Duration that defines the time in milliseconds for the offset in the PublishingInterval cycle of sending the NetworkMessage to the network.

The Duration DataType is a subtype of Double and allows configuration of intervals smaller than a millisecond.

Figure 26 depicts how the different variations of PublishingOffset settings affect sending of multiple NetworkMessages.

Figure 26 – PublishingOffset options for multiple NetworkMessages

If all DataSets of a group are transferred with a single NetworkMessage, the scalar value or the first value in the array defines the offset for sending the NetworkMessage relative to the start of the PublishingInterval cycle. If the DataSets of a group are sent in a series of NetworkMessages, the values in the array define the offsets of sending the NetworkMessages relative to the start of the PublishingInterval cycle. If a scalar value is configured, the first NetworkMessage is sent at the offset and the following NetworkMessages are sent immediately after each other. If more NetworkMessages are available for sending than offset values in the array, the offset for the remaining NetworkMessages are extrapolated from the last two offset values in the array.

The PublishingInterval, the SamplingOffset the PublishingOffset and the timestamp in the NetworkMessage header shall use the same time base.

This Structure DataType is used to represent the UADP NetworkMessage mapping specific WriterGroup parameters. It is a subtype of WriterGroupMessageDataType defined in 6.2.5.6.3.

The UadpWriterGroupMessageDataType is formally defined in Table 54.

Table 54 – UadpWriterGroupMessageDataType Structure

Name	Type	Description
UadpWriterGroupMessageDataType	Structure
groupVersion	UInt32	Defined in 6.3.1.1.2.
dataSetOrdering	DataSetOrderingType	Defined in 6.3.1.1.3.
networkMessageContentMask	UadpNetworkMessageContentMask	Defined in 6.3.1.1.4.
samplingOffset	Duration	Defined in 6.3.1.1.5.
publishingOffset	Duration[]	Defined in 6.3.1.1.6.