Image Processing Reference
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solution is able to guarantee deterministic behavior thanks to the bitwise arbitration technique of
CAN. his protocol, in fact, basically implements a non-preemptive priority-based communication
system, which does not suffer from destructive collisions and where the message characterized by the
highest priority is always given precedence for transmission. Besides, providing deterministic behav-
ior and a high level of robustness, this also means that network congestion is implicitly prevented
(hence, almost all the theoretical bandwidth is effectively available to control applications). Moreover,
CANisabletofeatureagooddegreeofeiciencywhensmall-sizedprocessdataareexchanged.
EtherNet/IP is conceived to rely on conventional Ethernet transmissions (theoretically, both
half- and full-duplex network can be employed). Although strictly deterministic operations are not
ensured, at least from a general viewpoint, the Open DeviceNet Vendor Association (ODVA)—the
organization which is in charge of managing CIP—has established a set of design rules [], which
achieve quasi-real-time behavior in EtherNet/IP. Basically, they require that high-speed switched
Ethernet is adopted (i.e., full-duplex  Mb/s equipment) and that unnecessary network traffic is
kept as low as possible. For instance, virtual local area networks (VLANs) or internet group manage-
ment protocol (IGMP) snooping could/should be used to confine multicast traffic that is generated
as a consequence of process data exchanges carried out over I/O connections according to the pro-
ducer/consumer model. As long as the network load is well below the theoretical available bandwidth,
non-blocking switches are able to guarantee that every message is eventually delivered to the intended
destination within a period that, because of the high network bandwidth, is short enough for the vast
majority of control applications in factory automation systems.
26.2.3 IEEE 802.11
The IEEE . family includes several standard specifications for WLAN. All devices belonging to
such a family (also referred to as WiFi) work in specific bands of the radio spectrum, centered around
either . GHz (legacy ., .b, .g, .e) or  GHz (.a). High transmission bit
rates are foreseen in IEEE . standards, ranging from  Mb/s (.b) to  Mb/s (.g/e/a).
Moreover, they are expected to increase further—up to (about) one order of magnitude—with the
next-generation devices (which will be based on .n).
The IEEE . standard specifies a mandatory distributed coordination function (DCF) to access
the physical medium that is based on the carrier sense multiple accesses with collision avoidance
(CSMA/CA) technique. As a further, non-mandatory, option, IEEE . includes a centralized
MAC protocol as well that is the point coordination function (PCF). In this case, access to the network
is regulated by a specific station, namely, the PC that grants exclusive access to one wireless station at
a time thus preventing collisions (the primary cause of nondeterminism in WLANs). Although PCF
is currently not supported by the vast majority of commercial WiFi boards (with a few noticeable
exceptions), it nevertheless represents an interesting option for industrial applications, at least from
a theoretical point of view.
Finally, it is worth mentioning that devices complying with the IEEE .e standard also support
the concept of Quality-of-Service (QoS), in the form of traffic prioritization and/or parameterization.
Despite it was originally conceived for supporting multimedia traffic, such a feature is particularly
appealing for industrial applications, too, as it allows critical and urgent messages to be delivered
in shorter times. In particular, the IEEE .e specification defines an additional “hybrid coordi-
nation function” (HCF), which in turn consists of two distinct mechanisms that are the enhanced
distributed channel access (EDCA) and the HCF controlled channel access (HCCA). Despite HCCA
is able to provide a fairly more deterministic behavior than EDCA, thanks to the presence of a hybrid
coordinator that effectively ensures stations contention-free access to the wireless medium—by allo-
cating them suitable transmission opportunities—compliant devices are, at the time of writing, not
availableyet.EDCA-compliantequipment,instead,iscurrentlyavailableof-the-shelfatlowcost.
EDCA is noticeably simpler and cheaper than HCCA, as it basically relies on a completely distributed
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