Image Processing Reference
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be developed, in this case, to enable the exchange of data between the two segments. Wireless nodes
belonging to the extension are organized in a BSS, which represents the building block of a WLAN.
A similar solution was proposed in Ref. [] for the previous version of Profibus, namely, Profibus
FMS. he practical application described in such a paper allowed to achieve some interesting results
derived from a set of experiments. In particular, the stations of the BSS were queried according to
a virtual polling algorithm; each station exchanged -byte with the gateway, resulting in a polling
time for wireless stations as low as  ms.
Clearly, the use of a gateway introduces additional delays. However, their effects may be consider-
ably lessened by the adoption of a proxy. Such a device maintains an image of the process input/output
data (decoupled from data exchanges, which take place actually on wireless segments) accessible from
the wired segment at any time, without restrictions introduced by the wireless communication proto-
col.heexampleinFigure.showsthatthismaybeaccomplishedbymeansofasetofinput/output
buffers. Stations on the wireless segment access these buffers to store input data acquired from the
controlled process. Similarly, output data stored into the proxy output buffers may be subsequently
retrieved by wireless stations.
heproxycodeallowsinterfacingtheProibusDPprotocoltotheinput/outputbufers.Specif-
ically, when a data exchange request, that carries output data, is issued by the Profibus DP master
protocol, the proxy extracts such data from the PDU and writes them into the related output buffers.
At the same time, input data are picked up from the input buffers and encapsulated in the acknowl-
edgment frame, which is returned to the Profibus DP master in accordance with the protocol rules.
It is worth noting that the above technique does not impose any specific restriction on the protocols
actually used on both the wired segment and the wireless extension. Indeed, data exchanges take
place via input/output buffers so that they are effectively decoupled. For example, the virtual polling
algorithm described in Ref. [] could be replaced by an implicit token-passing technique, which does
not require specific queries of the wireless nodes.
26.4.2 Extensions of DeviceNet
DeviceNet was designed bearing in mind the option of having several subnetworks interconnected
by means of routers. his mechanism was conceived initially to overcome the drawbacks of the CAN
protocol, which limit every DeviceNet segment (that coincides with one arbitration domain) to 
nodes at most and no more than few hundred meter extension. However, it proves to be useful for
interconnecting other kinds of networks as well. Seamless routing capabilities set DeviceNet apart
from most of the other fieldbus solutions. In particular, a set of suitable objects have been included in
the specification that defines the mechanisms a router can use to forward messages from one network
port to another.
Besides making the interconnection with other ODVA networks (e.g., ControlNet and Ether-
Net/IP) simpler, such a feature turns out to be helpful when implementing wireless extensions.
Routing in heterogeneous CIP networks is achieved by means of the so called “port segments,” which
are added to messages and explicitly describe the route they should follow (this is often referred to as
“source routing”). he route is specified through a list of items. Each item, in turn, describes a “hop”
bymeansoftwopiecesofinformation:theoutputportoftherouterusedtoforwardthemessageand
the address of the intended destination in the next subnetwork (that can be either the target node or
another router). As the frame travels along its route, the port segment is shortened by routers: in par-
ticular,eachoneofthemremovestheitemconcerningthehopithasprocessed.Obviously,therouting
process takes some time, and hence response times over the interconnected network increase. his
aspect has to be taken into account carefully when control applications with demanding real-time
requirements have to be designed (for example, timing constraints might need to be relaxed).
Using WLANs together with conventional DeviceNet devices can be accomplished in several ways.
The first, simpler, solution is to rely on an existing DeviceNet to EtherNet/IP router that, in turn, is
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