Image Processing Reference
In-Depth Information
being replaced by networks that influence the quality of control of the implemented application. QoS
parameters are a reasonable means to quantify this influence. In principle, QoS refers to the capability
to provide resource assurance and service differentiation in a network []. In a narrower sense, time-
liness and reliability properties are often referred to as network QoS. Some typical QoS parameters
relevant for fieldbus systems in an automation context are discussed below.
End-to-End Delay
This is an absolute measure for the total delay a packet experiences on its way from sender to receiver.
A requirement on the end-to-end delay
D
i
of each packet
i
can be defined such that the delay
D
i
of
each packet
i
imust not exceed a maximum end-to-end delay
D
max
with a probability greater or equal
than
Z
min
,
P
(
D
i
≤
D
max
)≥
Z
min
(.)
This statistical bound on the end-to-end delay can be easily converted into a deterministic bound
by defining
Z
min
=
.
Delay Jitter
The delay jitter
J
k
is the variance of the end-to-end delay
D
k
andisameasureforthestochasticpart
of the delay. It stems from variations in data-processing times on the end and intermediate nodes a
packet visits along its way. I may also be influenced by the mechanism used to grant a node access to
the transmission medium or to route traffic in intermediate nodes. Hence, it heavily depends on the
overall traffic situation in the network. he jitter is frequently defined by decomposing the network
delay
D
k
=
D
min
+
J
k
(.)
into a deterministic part
D
min
and a stochastic part
J
k
. Applications may want to bound the delay
jitter
J
i
to a maximum value
J
max
with a probability equal or greater than
U
min
,
P
(
J
i
≤
J
max
)≥
U
min
(.)
Applications requiring jitter bounds are typically highly dynamical control systems needing truly
equidistant sampling and execution of commands, such as electric drive controls.
Lifetime
This parameter indicates how long a given data value is meaningful for the application process. It is
connected to the network delay and depends on the type of data under consideration. For control
operations, there is no point in trying to recover lost or delayed samples of state variables if they
are too old. On the other hand, configuration or management data may be valid for a long time.
In packet-switched networks, the time-to-live parameter indicates the point in time when a packet
becomes obsolete and may be deleted from the network by any node to reduce the network load.
Frequency
The generation rate of the data sources is an important parameter for the planning of the network
traffic and resource allocation. If the frequency is too high and the overall data generation exceeds
thenetworkcapacity,datalossisunavoidable.Ifthefrequencyonlytemporarilyisabovethecapacity,
backlogwilllikelyoccur,andthedelayforindividualpacketswillincrease.
Loss Rate
The drop probabilities for different packets on the individual hops can be subsidized by a global “loss
probability” or average “loss rate.” The most common reason for high loss rates is again network
