Image Processing Reference
In-Depth Information
Bridge
IEEE
802.11
Wireless
device
Profinet IO
interface
AP
(with PCF)
Wired
device
Wired
device
Profinet IO
Wireless
device
FIGURE .
Extension of Profinet IO via a bridge.
Bridge
IEEE
802.11e
AP
(QoS-enabled)
Wireless
device
Wired
device
Wired
device
EtherNet/IP
Wireless
device
FIGURE .
Extension of EtherNet/IP via an AP.
26.5.2 Extensions of EtherNet/IP
As long as EtherNet/IP networks [] are taken into account, there is no need for any change to adapt
the CIP protocol to IEEE . networks, at least from a theoretical point of view. APs can be used to
forward each message to the intended destination(s) by means of its Ethernet address (which, in turn,
is derived from the IP address used at the application level), as shown in Figure .. In fact, both
Ethernet and WiFi networks share the same address space, which means that transparent bridging
at the data-link layer can be easily achieved. he only issue concerns the MTU on these networks, as
the maximum payload allowed on WLANs is greater than that of Ethernet. In particular, the size of
the frames sent by stations on wireless extensions has to be limited to the maximum value achieved
on the wired segment.
While this is certainly the most appealing (and inexpensive) solution, it might lead to sub-optimal
results when real-time process data have to be exchanged (i.e., for I/O connections). his is because,
in the existing IEEE .b/g WLANs, differentiated services cannot be offered. In other words,
priorities cannot be assigned to process data higher than those envisaged for background traffic,
unless special techniques, such as, for example, those provided by IEEE .e, are adopted.
In the latter case, however, existing implementations of CIP have likely to be modified, to map
the timing constraints of each message (I/O vs. explicit connection, expected packet rate, and so on)
that are known at the application level, onto the most suitable QoS as provided by the underlying
wireless communication system. From this point of view, Ethernet is not the same as IEEE .e
WLANs. In fact, while IEEE .Q [] foresees up to “traffic classes” to encode frame priority in
switched Ethernet networks, IEEE .e defines four different “access categories” (AC) to provide
traffic prioritization. Moreover, while strict priority order is enforced among frames by switches (i.e.,
the highest priority message is selected for transmission on a given output port among all the queued
