Image Processing Reference
In-Depth Information
hand, network congestions could be experienced, and this is surely the worst aspect. In other words,
when the network load generated by devices exceeds a given threshold (even for a limited time),
a condition may likely arise—because of the positive feedback due to the increased collision rate—
so that the effective net throughput decreases as the load increases. This means that the network
may become temporarily unavailable to carry out data exchanges timely, which is not compatible
with proper operation of distributed control systems. he number of lost messages (discarded by the
sender because the retry limit is exceeded) may even grow up to a point no longer acceptable for
the control application. Moreover, even in the case of lightly loaded networks, non-negligible jitters
may affect data exchanges and hence the accuracy of the system, unless some form of prioritization
scheme is adopted for frame transmission.
Third and last, wireless channels are much more error-prone than wired cabling [], and this is
a serious drawback when they are used in those industrial environments that are often plagued by
non-negligible electromagnetic interferences. Besides causing higher communication latencies and
jitters, transmission errors directly affect the network reliability and, consequently, the robustness
of the overall system. They may also cause consistency problems—for instance, when a multicast
message is received only by a part of the addressed devices.
Unless acknowledged transmission schemes are adopted (which, incidentally, are not allowed
when broadcast and producer/consumer-like multicast traffic is taken into account), there is a non-
negligible chance that messages sent over the wireless medium never reach the intended destination,
even in non-noisy environments. Unfortunately, if electromagnetic disturbance is anything but low
(as in real plants) acknowledgments are no longer sufficient to ensure reliable connections.
Each one of the three drawbacks mentioned above can be tackled (at least partially) by means
of already existing or soon available technologies. In particular, the throughput problem could be
somehow lessened by the upcoming IEEE .n specification, which is based on the multiple-input
multiple-output technology and is theoretically able to provide a big leap (about one order of mag-
nitude) in the network throughput. Indeed, as process data are usually small-sized, the same level
of improvement cannot be reasonably expected in industrial scenarios, where simple field devices
are interconnected. A more viable choice is relying on several (smaller) separate wireless subnet-
works (e.g., WLANs), possibly operating on different channels, interconnected by means of a wired
backbone to limit the packet rate on each one of them. Keeping the size of each wireless subnetwork
small also helps high speed operation (because, in this case, the automatic rate adaptation mechanism
featured by most APs and WLAN adapters is not activated).
The second problem (determinism) can be tackled through the adoption of enhanced medium
access techniques, such as PCF in the case of IEEE .. Unfortunately, PCF is seldom imple-
mentedincommerciallyavailableAPs.VariantsofPCF,suchastheiPCFmechanismintroduced
by Siemens [], have the drawback of being proprietary solutions, and hence compatibility and
interoperability with standard equipment can hardly be ensured. Alternatively, the new prioritiza-
tion features offered by the EDCA function of the IEEE .e standard (already supported by many
WLAN adapters currently available off-the-shelf) might be exploited []. While not guaranteeing
strict determinism, such a technology (mainly developed for multimedia applications) is able to pro-
vide tangible improvements, such as, for example, guaranteeing shorter transmission times “on the
average” for urgent notifications, which might be often enough for several industrial automation
systems. For example, it is possible to ensure quasi-real-time behavior by assigning higher prior-
ities (i.e., voice and video access classes) to those messages characterized by tight timing/safety
constraints.
Also TDMA techniques may be useful to solve the determinism problem. In such a case, for
example, the GTSs provided by IEEE .. represent an interesting opportunity. Other solutions,
recently appeared in literature [], suggest the adoption of synchronization techniques on WLANs,
to reduce the likelihood of a collision for data exchanges that take place according to predictable (e.g.,
periodic) patterns.
