Image Processing Reference
In-Depth Information
times faster for Ethernet, the frame time is only % lower than CAN. his example shows that the
network speed is only one factor that must be considered when computing the effective bandwidth
of a network.
23.2.2 Delay and Jitter
The “time delay” on a network is the total time between the data being available at the source node
(e.g., sampled from the environment or computed at the controller) and it being available at the des-
tination node (received and decoded, where the decode level depends on where the delay is evaluated
within the end-to-end communication). he “jitter” is the variability in the delay. Many control tech-
niques have been developed for systems with constant time delays [,], but variable time delays can
be much more difficult to compensate for, especially if the variability is large. Although time delay is
an important factor to consider for control systems implemented over industrial networks, it has not
been well-defined or studied by standards organizations defining network protocols [].
To further explain the different components that go into the time delay and jitter on a network,
consider the timing diagram in Figure . showing how messages are sent across a network. The
source node A captures (or computes) the data of interest. here is some preprocessing that must be
done to encapsulate the data into a message packet and encode it for sending over the network; this
time is denoted T pre .Ifthenetworkisbusy,thenodemayneedtowaitforsometime T Twait for the
network to become available. his waiting time is a function of the MAC mechanism of the protocol,
which is categorized as part of layer  of the OSI model. hen, the message is sent across the network,
taking time T tx as described in Section ... Finally, when the message is received at the destination
node B, it must be decoded and postprocessed, taking time T post .hus,thetotaltimedelaycanbe
expressed by the following equation:
T delay
=
T pre
+
T Twait
+
T tx
+
T post
(.)
The waiting time T Twait can be computed based on the network traffic, how many nodes there are,
the relative priority of these nodes and the messages they are sending, and how much data they send.
The pre- and postprocessing times T pre and T post depend on the devices. Oten the network encoding
and decoding are implemented in software or irmware. hese times are rarely given as part of device
specifications. As they can be the major sources of delay and jitter in a network, a more detailed
discussion of these delays is given here.
Node A
Node B
T pre
T pre
Application layer
Application layer
T post
T post
T Twait
T Twait
Data link layer
Data link layer
Physical layer
Physical layer
T tx
T tx
FIGURE .
Timing diagram showing time spent sending a message from a source node to a destination node.
 
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