Image Processing Reference
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different levels of control in the factory such as machine level, cell level, and system level. Within
this section, an example is presented of a multilevel factory networking solution that supports net-
worked control, diagnostics, and safety. his chapter concludes with a discussion of future trends in
industrial networks with a focus on the move to wireless networking technology.
23.2 Parameterization of Industrial Networks
The function of a network is to transmit data from one node to another. Different types of industrial
networks use different mechanisms for allocating the bandwidth on the network to individual nodes,
and for resolving contentions among nodes. Briefly, there are three common mechanisms for allocat-
ing bandwidth: time-division multiplexing, random access (RA) with collision detection (CD), and
RA with collision avoidance (CA). In time-division multiplexing, the access time to the network is
allocated in a round-robin fashion among the nodes, either by passing a token (e.g., ControlNet) or
having a master poll the slaves (e.g., AS-I). Because the bandwidth is carefully allocated, no collisions
will occur. If RA to the network is allowed, collisions can occur if two nodes try to access the network
at the same time. he collision can be destructive or nondestructive. With a destructive collision, the
data are corrupted and both nodes must retransmit (e.g., Ethernet). With a nondestructive collision,
one node keeps transmitting and the other backs off (e.g., CAN); in this case, the data are not cor-
rupted. CA mechanisms (e.g., WiFi) use random delay times to minimize the probability that two
nodes will try to transmit at the same time, but collisions can still occur. hese mechanisms and the
most common network protocols that use them will be discussed in detail in Section .
Although any network protocol can be used to send data, each network protocol has its pros and
cons. In addition to the protocol, the type and amount of data to be transmitted are also important
when analyzing the network performance: will the network carry many small packets of data fre-
quently, or large packets of data infrequently? Must the data arrive before a given deadline? How
many nodes will be competing for the bandwidth, and how will the contention be handled?
he QoS of a network is a multidimensional parameterized measure of how well the network per-
forms its function; the parameter measures include the speed and bandwidth of a network (how much
data can be transmitted in a time interval), the delay and jitter associated with data transmission (time
for a message to reach its destination, and repeatability of this time), and the reliability and security
of the network infrastructure []. When using networks for control, it is often important to assess
determinism as a QoS parameter, specifically evaluating whether message end-to-end communica-
tion times can be predicted exactly or approximately, and whether these times can be bounded.
In this section, we will review the basic QoS measures of industrial networks, with a focus on time
delays, as they are typically the most important element determining the capabilities of an industrial
control system. In Section ., more detailed analysis of the delays for speciic networks will be given.
In Section ., a methodology will be presented for evaluating the many dimensions of QoS along
with other factors as they relate to the particular application environment.
23.2.1 Speed and Bandwidth
he“bandwidth”ofanindustrialnetworkisgivenintermsofthenumberofbitsthatcanbetrans-
mitted per second. Industrial networks vary widely in bandwidth, including CAN-based networks,
which have a maximum data rate of Mb/s, and Ethernet-based networks, which can support data
rates upwards of Gb/s,
∗
although most networks currently used in the manufacturing industry
are based on or Mb/s Ethernet. DeviceNet, a commonly used network in the manufacturing
industry,isbasedonCANandhasamaximumdatarateofkb/s.he“speed”istheinverseofthe
