Image Processing Reference
In-Depth Information
Application
program(s)
Application
program(s)
Application protocol converter
Application
Application
Services
Application
Application
(caching)
Wireless
extension
…
…
…
…
Transport
Transport
Transport
Transport
TCP/UDP
Network
Network
Network
Network
IP
Data-link
Data-link
Data-link
Data-link
Controllers
Physical
Physical
Physical
Physical
Transceivers
Gateway/Proxy
Wired
network
Wired
device
Wired
device
Wireless
device
FIGURE .
Interconnection of networks via a gateway.
of data caching is provided as well to save bandwidth. For instance, proxies are used in fieldbus/IP
configurations to connect legacy fieldbus segments. In this case, they integrate devices connected
to the underlying fieldbus into the overlying Ethernet system. In this way, several advantages of
fieldbus networks, such as the high responsiveness, powerful diagnostic functions, and automatic
configuration can be maintained, and at the same time planning is simplified (because workflows are
well-known) as well as commissioning and operation (thanks the diagnostic capabilities offered by
the fieldbus).
An example of wireless extension implemented at the application layer is shown in Figure .. ..The
gateway in the picture includes two interface boards that are fully compliant with the overall proto-
col stack of the relevant subnetworks, and a protocol converter, usually implemented as a software
module.
26.3.4 Wireless Extensions of Industrial Networks
Although technically feasible, wireless extensions of industrial networks to be used at the shop-
floor (either fieldbuses or RTE networks) are not so straightforward. This is basically due to three
main reasons: first, the transmission support (wireless medium) is shared among all nodes (possi-
bly including those associated to nearby independent wireless subnetworks). Even operating at the
maximum allowable speed (e.g., Mb/s for the currently available IEEE .a/g/e compliant net-
works), this may result in a low per-station net throughput—compared to that achieved in many
wired networks—when the number of connected devices grows higher (this is particularly true when
either switched Ethernet or industrial Ethernet solutions based on a combined message are taken
into account). Furthermore, the nonnegligible overhead introduced by the communication protocol
(larger protocol control fields, acknowledgment, and reservation mechanisms, no provision for full-
duplex operations, and so on) has to be considered as well. For example, the minimum time needed
to reliably exchange bytes of user data over a WLAN—i.e., bit rate equal to Mb/s, no RTS/CTS
mechanism, no TCP/IP encapsulation,
ad hoc
network mode, but including interframe spaces as
they effectively waste network bandwidth—is about µs. Such a value is only slightly shorter than
the worst-case time taken to send the same information over a CAN network operating at Mb/s
(i.e., µs).
The second reason is that random medium access techniques (e.g., CSMA/CA) are often
employed by wireless networks. his means that, on the one hand, unpredictable—and unbounded—
transmission delays might occur (because of the occurrence of repeated collisions) while, on the other
