wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

39-10 Industrial Communication Systems
Indeed, the size of each communication slot depends on the amount of data transmitted as well as on the
network topology and latency induced by the network components, which therefore also influence the
cycle time. Moreover, the adoption of the multiplexed communication class (either alone or in conjunction
with the continuous one) allows to reduce the maximum number of slots in each cycle and consequently
the cycle time as well.
Finally, it is also important to remember that the CNs have also to perform some protocol-related
tasks like, for example, SoC processing or the maintenance of the EPL cycle state machine. Thus, the time
required to handle the different node activities also has to be considered. In this context, the EPL protocol
provides a (limited) level of control since it allows the MN to control the poll order permitting, for example,
to avoid polling the CNs with highest SoC latency immediately after the sending of the SoC frame.
39.7.4 acyclic Traffic
According to the EPL specification, the asynchronous period aims at the transfer of non time-critical
data, typically related with the network management and supervision (ASnd messages) as well as with
non-EPL traffic (e.g., TCP or UDP/IP messages). Despite not being addressed in the EPL specification,
the use of asynchronous services for carrying infrequent real-time data, e.g., alarms, could also bring
efficiency gains provided that suitable latency values can be guaranteed.
The latency associated with the asynchronous traffic is a relevant performance figure since it is
important to have acceptable response time in the network management services as well as on the user-level
applications supported on IP traffic.
The latency experienced by asynchronous requests depends on the cycle duration, on the period of
the real-time data associated with the CN, and on the particular scheduling algorithm used by the MN.
The cycle duration defines the number of requests that can be handled by time unit. The period of the
real-time data associated with the CN constraints the signaling latency since CNs notify the MN about
the existence of requests via PRes messages. Thus, in the worst-case situation, the CN may have to wait
one entire poll period prior to be able to inform the MN of the existence of requests. Finally, the latency
also depends on the scheduling decisions at the MN. The user can influence these decisions by assigning
higher priorities to the more time-sensitive asynchronous messages.
As an example, an EPL network in which N different asynchronous requests occur at the same time
at one or more CNs can be considered. The CNs can only notify that event to the MN in the following
cycle, and the MN shall schedule at most one request in each following cycle, thus requiring a total of
N + 2 cycles to handle all the requests. It has to be noticed that the number of cycles necessary to process
the pending requests does not depend on the cycle duration; thus decreasing the cycle time (if possible)
leads to shorter response times.
On the other hand, the latency of acyclic messages could be reduced, allowing multiple ASnd transmissions
in the same cycle. This is actually addressed by the latest version of the EPL specification [6],
which makes such an opportunity possible, provided that there is enough remaining time in the cycle.
Another possible way of improving the response time for asynchronous time-sensitive messages would
consist in reserving part of the asynchronous phase exclusively for alarms, as discussed in [13].
References
1. Bernecker & Rainer Industrie-Elektronik GmbH, Eggelsberg, Austria [Online]: http://www.brautomation.com
2. Ethernet POWERLINK Standardization Group. [Online]: http://www.ethernet-powerlink.org
3. IXXAT Automation GmbH: POWERLINK interface board for PCI bus systems PL-IB 200/PCI,
Technical description. [Online]: http://www.ixxat.com/powerlink_pci_interface_en.html
4. OpenPOWERLINK: Open source solution for POWERLINK MN and CN. [Online]: www.sourceforge.
net/projects/openPOWERLINK
© 2011 by Taylor and Francis Group, LLC