Image Processing Reference
In-Depth Information
a lower level via CIP routers. This means that in hybrid networks based on DeviceNet all devices
(both wired and wireless) can be seen and accessed as conventional CIP nodes, whereas in the case a
gateway/proxy is adopted (as it happens in Profibus DP) wireless nodes are no longer effectively part
of the Profibus network.
When real-time (industrial) Ethernet networks have to be provided with a wireless extension, more
alternatives are possible, in principle, including carrying out interconnection at the data-link layer.
However, as these networks often rely on peculiar protocols to ensure predictable communications
over conventional Ethernet, the integration with WLANs is not usually straightforward. As a conse-
quence, a loss of performance and determinism in hybrid networks is expected to occur also in this
case. Moreover, when IEEE . is selected for the wireless extension, the native randomness of
the DCF medium access technique has to be taken into account carefully in designing/deploying the
hybrid network, as it likely leads to additional (and unwanted) delays. Significant improvements can
be achieved by using deterministic techniques, such as, for example, PCF in WLANs (which, how-
ever, is not so frequently available in real devices) or GTSs in IEEE .. networks (that, however,
suferfromanoticeablylowerbitrate).
Finally, an interesting option for achieving quasi-real-time communications over wireless exten-
sions is provided by the IEEE .e standard, which supports the concept of QoS. It has been shown
that, for example, traffic prioritization could be used to deal with different types of connections (and,
hence, of timing requirements) defined and handled by the CIP protocol, to transparently enable
control applications to meet real-time constraints, even when hybrid network configurations are
adopted.
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