Image Processing Reference
In-Depth Information
mechanisms, for example, retransmissions, packet duplications, or error-correcting codes. On the
other hand, systems like FIP/WorldFIP [] and CAN [] implement the model of a
real-time
database
where
data
is identified instead of stations. A piece of data has one
producer
and potentially
many
consumers
. he producer broadcasts the data and all interested consumers copy the data packet
into an internal buffer. This broadcast approach prohibits the use of acknowledgments and packet
retransmissions, but error-correcting codes can be still used to increase transmission reliability. Oten
the data is transmitted periodically and (repeated) packet losses can be detected by comparing the
known period and the time of the last arrival of a data packet. This “freshness” information can be
used by the application to react properly.
24.2.3 Integration of Wired and Wireless Stations/Hybrid Systems
There are a huge number of existing and productive fieldbus installations and it is best if wireless
stations can be integrated into them. Such a network with both wireless stations (stations with a
wireless transceiver) and wired stations are called
hybrid systems
. he most important requirements
for hybrid systems are
•
Transparency
:hereshouldbenoneedtomodifytheprotocolstackofwiredstations.
•
Using specifically tailored protocols
: Most fieldbus systems are specified on the layers
one (physical), two (MAC and link layer), and seven (application layer). The introduc-
tion of a wireless physical layer affects the behavior and performance of both the MAC
and link layer. The existing protocols for wired fieldbus systems are not designed for
a wireless environment and should be replaced by protocols specifically tailored for
the wireless link. However, this comes at the cost of
protocol conversion
between wired
and wireless protocols.
•
Portability of higher-layer software:
: If the link layer interface is the same for both the wire-
less and wired protocol stacks, implementations of higher-layer protocols and application
software can be used in the same way on both types of stations.
The different approaches to integrate wireless stations into wired fieldbus LANs can be classified
according to the layer of the OSI reference model where the integration actually happens [,].
Almost all fieldbus systems are restricted to the physical, data-link, and application layers []. [].The
classification is as follows:
•
Wireless cable-replacement approach
: All stations are wired stations and thus attached to
a cable. A piece of cable can be replaced by a wireless link and special bridge-like devices
translatetheframingrulesusedonthewirelessandwiredmedium,respectively.Inthis
approach, no station is aware of the wireless link. A typical application scenario is the
wireless interconnection of two fieldbus segments.
•
Wireless MAC-unaware bridging approach
:henetworkcomprisesofbothwiredand
wireless stations, but integration happens solely at the physical layer. Again, a bridge-like
device translates the framing rules between wired and wireless media. The wireless sta-
tions use merely an alternative physical layer (PHY), but the MAC and link layer protocols
remain the same as for wired stations.
•
Wireless MAC-aware bridging approach
: he LAN comprises of both wired and wireless
stations and integration happens at the MAC and data-link layer. here are two different
MAC and link layer protocol stacks for wired and wireless stations, but both offer the same
link layer interface. he wireless MAC and link layer protocols should be (a) specifically
tailoredtothewirelessmediumand(b)easilyintegrablewiththewiredMACandlink
