Image Processing Reference
In-Depth Information
IP cross traffic
sources
IP-based network
IP channel
Encaps.
Encaps.
Encaps.
CN/IP
router
Fieldbus
IP device
FIGURE .
Structure of an IP channel for fieldbus data tunneling over the Internet.
the transmission inside the tunnel is much faster than in the fieldbus. As this is not normally the case,
the tunnel endpoints rather act as remote bridges separating the two segments (in the sense of two
timingdomains).Tothatend,theieldbusmusteithersupportmultiplesegmentsandthussomesort
of routing, or traffic forwarding from one segment to the other needs to be introduced “manually”
on a higher protocol layer (or a respective application).
In recent years, the EIA- standard has been established to cope with some of these prob-
lems [,]. It introduces means to define and manage IP channels for a peer-to-peer transmission
of fieldbus data packets. Basically, the methods defined in the standard are generic. In practice,
implementations are currently only defined for EIA- (LonWorks) and EIA- (CEBus). It is not
surprising that the standard evolved in the building automation domain. In this environment, large
IT networks are common as well as localized control networks which might benefit from an inter-
connection over IP. Consequently, the devices in an EIA- system can either be purely IP-based or
regular control network devices which are connected to the IP network through the access points (in
EIA- terminology called control network/IP tunneling routers). hese devices connected over the
IP-based network form together the IP channel (Figure .). .).The native PDUs are encapsulated in
UDP frames and routed to the respective recipients on the IP channel. he tunneling routers take care
of appropriate data flow control mechanisms. Sequence numbers are assigned to the packets to allow
for correct reassembly of the packet order even if the packets used different paths to reach the receiver.
If the receiver recognizes that packets are missing, it can—depending on the configuration—either
continue and drop them when they finally arrive (late packets are useless in control applications)
or store the subsequent for a given time (the escrow time in EIA- terminology), hoping that the
missing packet(s) still arrive. To increase the efficiency of the fieldbus packet encapsulation in the
UDP frames, there is the possibility to bundle subsequent packets into one single UDP frame (packet
bunching). his introduces extra delay, but reduces the encapsulation overhead (especially the UDP,
IP, and media-speciic headers). To cope with excessive transmission delays on the IP channel, times-
tamps can be issued for the frames to measure the delay. Frames exceeding a predefined transmission
time will then be discarded as stale packets.
20.6.2 Gateways
The alternative approach to implement a connection between a fieldbus and an IP-based network is
to design the central access point as a gateway. Like with the tunneling approach, the gateway is a full
member of the fieldbus on the one side and can be accessed through IP-based mechanisms via the
