Image Processing Reference
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available publications put emphasis on the management of configuration data and on distributed
algorithms for clock synchronization. The MAC and data-link protocols of FIP were not modified.
Since FIP broadcasts the values of process variables periodically, the protocol contains no retransmis-
sion scheme for the time-critical data. Instead, the OLCHFA approach is to enhance the FIP process
variable model with the so-called time-critical variables, which provide freshness information to the
applications. Applications can use this to handle cases of repeated losses.
24.6.3 PROFIBUS
The R-FIELDBUS project (www.rfieldbus.de) evaluated how IEEE . with DSSS can be used
in a PROFIBUS fieldbus system and how such a system can be used for transmission of IP-based
multimedia data [,]. Two different architectures have been proposed: the single logical ring
and the multiple logical ring solution, discussed below. Both solutions run the (almost) unmodi-
fied PROFIBUS protocol. he PROFIBUS protocol uses token-passing on top of a broadcast medium.
The token is passed between active stations along a logical ring and much of the protocols complexity
deals with ring maintenance. he token itself is a small control frame.
In the single logical ring solution all wired and wireless stations are integrated into a single logical
token-passing ring. he coupling devices between the wired and wireless domain simply forward all
packets.hisapproachiseasytorealizebutsubjectsbothdatapacketsandcontrolpacketslikethe
token-frame to the errors on wireless links. It is shown in [] for the PROFIBUS and in [] for
the similar IEEE . Token Bus protocol that repeated losses of token-frames can create severe
problems with the achievable real-time performance. Since there is only a single logical ring, the
whole network is affected.
In contrast, in the multiple logical ring solution [] wireless and wired stations are separated into
several logical rings. These rings are coupled by intelligent devices called brouters (a merger from
bridge and router). In this solution, transmission problems distort only one ring, the other logical
rings remain operational. A second benefit of having multiple rings is traffic segmentation. If the
segments are chosen carefully most of the traffic will be intra-segment and thus the overall traffic
capacity can be increased. A drawback of the multiple logical ring solution, however, is that inter-
segment traffic is not natively supported by the PROFIBUS protocol and extensions are required.
In [,] a system following the wireless MAC-aware bridging approach is proposed. On the
wireless side specifically tailored polling-based protocols are used, whereas wired stations run the
unmodified PROFIBUS protocol stack. The goal is to avoid token-passing on wireless segments.
It is shown that for bursty channel errors the polling-based protocols achieve substantially better
performance in terms of stochastic hard real-time behavior than the PROFIBUS token-passing pro-
tocol;forcertainkindsofchannelsthe%quantileofthedelayneededtosuccessfullytransmit
a high-priority packet is up to an order of magnitude smaller than for the PROFIBUS protocol.
To integrate both protocols, the coupling device between wired and wireless media provides a
virtual ring extension []. In this scheme the coupling device acts on the wired side on behalf
of the wireless stations. For example, it creates token-frames and executes the ring maintenance
mechanisms.
Finally, in [] a scheme for integration of wireless nodes into a PROFIBUS-DP network (single
master, many slaves, no token-passing) is described. An application layer gateway is integrated with
a“virtualmaster”station.hevirtualmasteractsasaproxyforthewirelessstations,itpollsthem
using standard IP and IEEE . distributed coordination function (DCF) protocols.
24.6.4 Other Fieldbus Technologies
For the IEC FieldBus [] (which uses a centralized, polling-based access protocol for periodic data
and a token-passing protocol for asynchronous data) in [], an architecture which allows coupling
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