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

39-8 Industrial Communication Systems
transmitted within two subframes. The CRC type used (either CRC8 or CRC16) depends on the size of
the user data. All systematic failures will be recognized by the POWERLINK safety protocol including
delay, repetition, wrong sequence, insertion, loss, and masquerade [11].
39.7 Performance Analysis
While the performance of real-time networks can be assessed according to a multitude of performance
criteria, in the context of EPL the most common metrics are jitter, turn-around time (TAT), and cycle
time. Other parameters, such as the latency of the asynchronous messages, are also commonly used to
assess the protocol performance. This section addresses the performance analysis of the EPL protocol,
illustrating how these parameters can be estimated and how key network design options affect them.
39.7.1 Jitter
Jitter, defined as the maximum time deviation that may occur on cyclic events, is defined in the EPL
standard as being in the submicrosecond range. The jitter in EPL networks is determined by the MN,
which is responsible for correct sending of the SoC message, as well as by the network components.
While the MN jitter is intrinsic to the hardware/software implementation characteristics, the jitter due
to the network components depends on the nature of the components themselves and of the network
topology. Although the jitter induced by a single hub (or by a limited number of such devices) may seem
negligible, as stated above EPL permits the violation of the IEEE 802.3 topology guidelines, opening the
way for the use of tree or linear topologies with several cascaded hubs. Despite their convenience, such
topologies have a negative impact on the global network jitter, which depends on the highest number of
hubs that exist between the MN and any CN. The jitter upper bound can be computed as
where
J mn is the MN jitter
J hub is the hub-induced jitter (homogeneous hubs assumed)
N hub is the number of hubs between the MN and any CN
J NET = Jmn + Nhub × Jhub
(39.1)
For illustrative purposes, consider a MN and a CN interconnected by a single hub. In this configuration,
considering, for example, the POWERLINK interface described in [3], J mn = 50.ns, whereas a
typical value for the hub-induced jitter is J hub = 70.ns. Thus, the jitter upper bound becomes, trivially,
J mn = 120.ns. For a two-level tree topology, the jitter upper bound becomes J mn = 190.ns, with an
increase of 58.3%. In these conditions, a linear topology has to be limited to 13 devices in order to
guarantee a jitter value below 1.μs.
39.7.2 turn-Around Time
Another important performance criterion is the TAT, defined as the time elapsed between the transmission
of a request and the reception of the corresponding response. The TAT has a noticeable impact on the
bandwidth utilization efficiency since this corresponds to network idle time.
As for jitter, the TAT depends on constructive aspects of the network components and devices as well
as on the network topology. In software-based implementations, the EPL stack is entirely executed in a
microprocessor, requiring several processing steps to decode the requests and issue the corresponding
replies. The associated latency depends on the delays introduced by several factors such as code execution
times, DMA techniques, interrupt handling procedures and bus protocols that may lead to response times
© 2011 by Taylor and Francis Group, LLC