Image Processing Reference
In-Depth Information
frames), EDCA only provides a mean to offer higher-priority access classes an increased likelihood
to win the contention (by decreasing the related back off times).
It is worth noting that developing WLAN-enabled EtherNet/IP devices, such as the target node on
the right side in Figure ., is not really a difficult task. he advantage of adopting this kind of solu-
tion is that devices can exploit the knowledge about the timing requirements of every CIP connection
to select the most suitable QoS. A viable choice might be, for example, to map I/O connections with
tight timing requirements (i.e., high expected packet rate) on voice-grade (AC_VO) traffic, whereas
those having less severe constraints on transmission times can be mapped on video-grade (AC_VI)
transmissions. Finally, explicit connections devoted to parameterization activities may rely on either
best-effort (AC_BE) or background (AC_BK) traffic.
In the case conventional APs are used to interconnect wired EtherNet/IP segments with wireless
extensions, exploiting the IEEE .e QoS property is a non-trivial task. Indeed, a conventional AP
cannot determine the exact timing requirements of each frame, as this information is known at the
application layer, whereas the AP operates at the data-link layer. A possible solution is to rely on APs
that are able to map IEEE .Q traffic classes directly onto IEEE .e ACs (and vice versa). In
this way, the hybrid communication system does its best in trying to fit the timing requirements of
the different connections.
Besides traffic prioritization, another important issue concerns mechanisms for broad-
cast/multicast traffic coninement. his is needed as process data, in modern industrial communica-
tion systems, are often exchanged according to the producer/consumer paradigm. In wired networks
such as EtherNet/IP, traffic confinement is dealt with by means of IGMP snooping or VLANs.
Although the latter mechanism is not available in IEEE . wireless networks, some devices are
currently available off-the-shelf that are able to map VLANs to different service set identifiers [].
Such a feature is quite interesting, as it can be used to ease the integration of WLAN extensions in
the existing EtherNet/IP networks.
26.6 IEEE 802.15.4-Based Extensions
The IEEE .. specification, as all networks defined by the IEEE committee, is only concerned
with the two lowest layers of the communication stack and does not specify any type of upper protocol
stack. hus, the access to IEEE .. networks by higher layers may be achieved in two different
alternative ways, as recommended by the standard, that is:
. Via logical link control (LLC) Type services []
. Directly, by means of the MAC services
Both techniques are effective, even if it is worth mentioning that IEEE .. boards manufacturers
rarely include LLC services in their products.
IEEE .. has a low transmission speed and, hence, wireless extensions are not able to provide
fast communications in this case. Nonetheless, the high efficiency of the protocol makes it particularly
appealing for applications where limited amount of data have to be transmitted. Moreover, the GTSs
available in the beacon-enabled mode allow for deterministic access to the shared medium by wireless
nodes, with a consequent reduction of the uncertainty typical of CSMA/CA-based protocols.
26.6.1 Extensions of Fieldbuses
he wireless extension of ieldbuses by means of IEEE .. suffers from the same problems already
discussed for IEEE .. Specifically, differences between the MAC protocols of the two kinds of
networks discourage interconnections at the lower layers. Hence, wireless fieldbus extensions based
on IEEE .. networks can only be achieved through a gateway that maps services foreseen
