Image Processing Reference
In-Depth Information
of “process image” that can be viewed from outside. While the basic design of access points in the
tunneling case does not allow many degrees of freedom, the operations performed by a gateway are
on application level and can thus be much more versatile (see Ref. [] for a discussion of design con-
siderations). Services offered by the gateway can include autonomous data-processing features sup-
porting low-level SCADA functionalities (like threshold monitoring), automatic update of the list of
connected nodes and data points [,], or notifications of a client in case predefined events occur.
A large variety of possibilities exist with regard to the way data points can be represented by the
gateway. If specific profiles are available for the field devices, they can be used [,]. Another way
is to simply provide a list of data points, organized either following a function-oriented approach or a
structure-oriented one based on the topology of the fieldbus and its nodes []. While the structure
ofthedatabaseholdingthedatainsidethegatewayisnaturallyfullyfree,thewayofaccessingthedata
from outside, i.e., from the IP-based network, may impose stringent limitations on the organization
of the data because the protocol used must support the data structures implemented on the gateway.
On the lower side of the gateway, the connection to the fieldbus provides again many degrees
of freedom in that there is no general guideline on how to implement it. The fieldbus driver has
to be developed anew for each case and has to exploit the data transfer mechanisms supported by
the ieldbus. his may also affect the overall traffic flow handling implemented on the gateway. One
possibility is to forward incoming requests to the respective field device and wait for the response to
send it back to the client. he alternative is to use a caching mechanism and to answer requests with
data point values from the cache, accompanied by a timestamp indicating the age of the value [].
Which strategy should be preferred depends on the capabilities of the driver interface as well as the
desiredscopeofdatatobehandledbythegateway,i.e.,whetherthegatewayneedstohaveaccessto
process or configuration data.
With regard to possible real-time applications in automation, the situation is considerably differ-
ent to the tunneling approach. The protocol conversion in the gateway necessarily terminates QoS
features on either side because the translation between the two networks is accomplished on appli-
cationlevel.hisisthepurposeofagateway:boththeieldbusandtheIP-basednetworkareableto
operate asynchronously, the gateway serves as buffer in between. Unless the gateway has means to
synchronize the data transfer on both sides (e.g., by acting as master on the fieldbus), no end-to-end
timingguaranteescanbegiven,evenifbothnetworksdohavereal-timecapabilities,becausethe
protocol conversion does not allow for an end-to-end communication relation across the gateway.
Consequently, gateways are typically used for process monitoring or management purposes, where
stringent real-time requirements do not exist.
For the sake of completeness, it must be noted that gateways can also “horizontally” interconnect
different fieldbus systems. Although appealing in principle, this topic has never reached practical
relevant because of the extreme heterogeneity of the fieldbus solutions which prohibits suitable one-
to-one mapping of data and services. One approach introduces an interoperability layer based on
the User Layer philosophy providing a common object model and messaging domain for distributed
applications []. The fieldbus drivers are implemented with OPC. Another experimental solution
employs function blocks according to IEC  to provide the necessary abstraction layer between
the fieldbusses [].
20.7 Industrial Ethernet—The New Fieldbus
As stated before, Ethernet has become increasingly popular in automation. And like in the early days
of fieldbus systems, this boom is driven mainly by the industry—on an academic level, the use of
Ethernet had been discussed decades ago. At that time, Ethernet was considered inappropriate
because of its lack of real-time capabilities. With the introduction of switched Ethernet and certain
modifications of the protocol, however, these problems have been alleviated. And even if there are
 
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