Image Processing Reference
In-Depth Information
Traffic
classes
Automation-related
traffic
Parallel traffic
Management
data
Tunneled over
the fieldbus
Sharing same
backbone
Process data
Real-time or non-real-time
(irrelevant for fieldbus application)
Periodic
Aperiodic
Periodic
Aperiodic
Bandwidth
limitation on
fieldbus
Bandwidth
limitation on
backbone
Real-time
Non-real-time
Unconfirmed
Buffered
Time-
triggered
Confirmed
Buffered
Event-
triggered
Confirmed
Queued
Guaranteed
delivery
FIGURE .
Traffic classes and typical properties.
20.5.1.1 Process Data
This type is sometimes also called “cyclic or identified” traffic because the communication relations
must be known once the application is specified. Depending on the characteristics of the process,
such data can be periodic or aperiodic. Periodic traffic mostly relates to the state of a process and
is typically handled by some sort of time-slot-based communication strategy, where each variable is
assigned a dedicated share of the network bandwidth based on the a priori known sampling time
or generation rate of the data source. The frequency or data update rate may also be adaptive, to
dynamically change according to the current state of the process (e.g., alarm conditions may require
a more frequent sampling of a state variable).
“Aperiodic, acyclic, or spontaneous” traffic is generated on demand in an event-based manner and
transmitted according to the availability of free communication bandwidth. If the predominating
data transfer scheme is time-slot-based, there may be spare slots or some idle time in the periodic
traffic explicitly reserved for this purpose.
From an application point of view, process data typically are real-time data and require timely
delivery. For both periodic and aperiodic data, this means that they should be transmitted (and
received) within a limited time window to be meaningful for the purpose of process control. For
aperiodic data, there is the additional restriction that the service ought to be reliable, i.e., data loss
should not occur or at least be detected by some appropriate mechanism.
20.5.1.2 Management Data
This type is also called “parameterization or configuration” data and generally refers to all data that are
needed to set up and adjust the operation of the automation system as such. Settings of distributed
