Processing at the Edge: A Case Study with an Ultrasound Sensor-Based Embedded Smart Device

Abstract

In the current context of the Internet of Things, embedded devices can have some intelligence and distribute both data and processed information. This article presents the paradigm shift from a hierarchical pyramid to an inverted pyramid that is the basis for edge, fog, and cloud-based architectures. To support the new paradigm, the article presents a distributed modular architecture. The devices are made up of essential elements, called control nodes, which can communicate to enhance their functionality without sending raw data to the cloud. To validate the architecture, identical control nodes equipped with a distance sensor have been implemented. Each module can read the distance to each vehicle and process these data to provide the vehicle’s speed and length. In addition, the article describes how connecting two or more CNs, forming an intelligent device, can increase the accuracy of the parameters measured. Results show that it is possible to reduce the processing load up to 22% in the case of sharing processed information instead of raw data. In addition, when the control nodes collaborate at the edge level, the relative error obtained when measuring the speed and length of a vehicle is reduced by one percentage point.

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Citation: Poza-Lujan, J.-L.;
Uribe-Chavert, P.; Sáenz-Peñafiel,
J.-J.; Posadas-Yagüe, J.-L. Processing
at the Edge: A Case Study with an
Ultrasound Sensor-Based Embedded
Smart Device. Electronics 2022,11,
550. https://doi.org/10.3390/
electronics11040550
Academic Editor: Vijayakumar
Varadarajan
Received: 31 December 2021
Accepted: 9 February 2022
Published: 11 February 2022
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4.0/).
electronics
Article
Processing at the Edge: A Case Study with an Ultrasound
Sensor-Based Embedded Smart Device
Jose-Luis Poza-Lujan 1,*,† , Pedro Uribe-Chavert 2,† , Juan-José Sáenz-Peñafiel 3,†
and Juan-Luis Posadas-Yagüe 1,†
1Research Institute of Industrial Computing and Automatics, Universitat Politècnica de València,
Camino de Vera, s/n, 46022 Valencia, Spain; jposadas@upv.es
2Doctoral School, Universitat Politècnica de València, Camino de Vera, s/n, 46022 Valencia, Spain;
pedurcha@doctor.upv.es
3Dirección de Investigación, Universidad de Cuenca, Av. 12 de Abril, Cuenca 010107, Ecuador;
juan.saenz@ucuenca.edu.ec
*Correspondence: jopolu@upv.es; Tel.: +34-963-87-70-00
† These authors contributed equally to this work.
Abstract:
In the current context of the Internet of Things, embedded devices can have some intelli-
gence and distribute both data and processed information. This article presents the paradigm shift
from a hierarchical pyramid to an inverted pyramid that is the basis for edge, fog, and cloud-based
architectures. To support the new paradigm, the article presents a distributed modular architecture.
The devices are made up of essential elements, called control nodes, which can communicate to
enhance their functionality without sending raw data to the cloud. To validate the architecture,
identical control nodes equipped with a distance sensor have been implemented. Each module can
read the distance to each vehicle and process these data to provide the vehicle’s speed and length.
In addition, the article describes how connecting two or more CNs, forming an intelligent device,
can increase the accuracy of the parameters measured. Results show that it is possible to reduce
the processing load up to 22% in the case of sharing processed information instead of raw data. In
addition, when the control nodes collaborate at the edge level, the relative error obtained when
measuring the speed and length of a vehicle is reduced by one percentage point.
Keywords: embedded device; edge and fog computing; smart cities; ambient intelligence
1. Introduction
Intelligent systems, based on embedded devices, have been the focus of attention in
intelligent city environments. The use of cheap and efficient micro-controllers with high
connectivity features allows the devices’ integration into almost all types of urban elements.
These interconnected elements have given rise to the concept of the Internet of Things
(IoT) [
1
]. Having many distributed devices implies a large amount of data to manage
to obtain information to make some decisions. This distributed chain, sensor-decision-
act, is aimed to provide an optimisation of system performance and to provide optimal
services [
2
]. This optimisation has given rise to the concept of distributed intelligence,
usually based on distributed knowledge [3].
Among the fields of application of distributed intelligence, mobility environments,
both in cities and on roads, are one of the most widely used. Environments can apply
intelligence in everyday aspects such as optimising traffic or managing the power con-
sumption of road lighting [
4
]. Using embedded systems with distributed intelligence to
coordinate non-daily aspects, such as accident prevention, detection or management, is also
a challenge. In all cases, intelligent environment management requires devices to detect,
characterise, predict, or act on the behaviour of both elements—vehicles and pedestrians.
Beyond intelligent devices appears the concept of collaborative intelligence in the edge
Electronics 2022,11, 550. https://doi.org/10.3390/electronics11040550 https://www.mdpi.com/journal/electronics

Electronics 2022,11, 550 2 of 12
based on the connection between close devices. For example, if some streetlights can detect
vehicles, they can alert traffic lights about their characteristics such as size or speed. With
this information, traffic lights can adjust the time they will stay green or red to minimise
the waiting time for vehicles. For example, several streets can send enough information
to provide a picture of the traffic in an area and to be able to predict traffic congestion
situations intelligently.
There are many methods of detecting and characterising vehicles on roads. The most
efficient methods use complex devices, such as cameras [
5
] or even drones [
6
]. These
devices can be tempting for vandalism, in addition to not having the availability of a high
processing or communicating capacity [
7
]. As an alternative to the previous systems, cheap
solutions have been proposed. The use of simple sensors implies that the information
provided by complex sensors, such as cameras, must be supplied through intelligence.
In [
8
], two classes of neural networks are considered, multi-layer perceptron (MLP) and
convolutional neural network (CNN), to analyse the audio signal.
To implement intelligence within embedded devices, we use a minimum element
called a ’control node’ (CN). A control node is an element that can read from sensors
and write to actuators, or both. The core of a control node is a micro-controller with
communication capabilities that must perform basic processing of the data obtained from
the sensors. Consequently, the computational and communication capabilities embedded
into the CNs implies that a set of CNs can provide an intelligent distributed system.
In this context, an interesting question emerges: Is it worth distributing if a single CN
can provide an acceptable result? If CNs distribute their raw data, the messages load the
communications system. A high load of messages implies a high probability of errors such
as high latency or variable jitter, among others. Moreover, an increased communication
load implies that the CN must handle more incoming and outgoing messages. Due to
the limited computational resources of the CN, an increase in communication activity can
limit the micro-controller resources dedicated to computational algorithms. To answer
the previous question, it is necessary to know the processing response time that a CN
could provide, considering both control and communication processing requirements.
The measurement of this response time will involve knowing the reaction times and how
the action or information provided is adequate to the requirements. For example, in the
experiment presented in this article, a concrete CN is used to measure the speed of a vehicle
using an ultrasound sensor. A single sensor has a considerable error; however, if the
system has multiple sensors, it is possible to improve the accuracy of the measurement by
distributing the data from each CN.
This article proposes a distributed paradigm where the control nodes (CN) have intel-
ligence dedicated to a specific functionality. Only the relevant data are distributed instead
of classical hierarchical models where the CN are only raw data providers. The article
depicts how the CN works as the primary element based on this paradigm. Additionally,
the article describes how connecting two or more CN, forming an intelligent device, can
improve the accuracy of the parameters measured. We developed a smart device that
detects vehicles’ speed and length. Different CNs compound the intelligent devices. Each
CN has an ultrasonic (US) sensor that detects distance. Changing the ultrasound signal’s
emission angle makes it possible to accurately detect the vehicle speed or length.
The experiments measured the data processing time (to obtain distances) and the
processing time to generate the information (speed and length). The results show that
reducing the overall processing time is possible if the modules undergo previous processing
and provide information to other modules instead of providing the raw data. This article
has been organised as follows. The following section shows the proposed paradigm from
the classical pyramid to the inverted pyramid. Additionally, as the CN is the basis of the
architecture, we provide a CN description and characterisation to measure the optimisation
experiment. Section 3presents the case study consisting of a device composed of three CN
modules with an ultrasound sensor at each one. In addition to acquiring distances, these
CN modules can process and calculate both vehicle speed and length. Section 4presents a

Electronics 2022,11, 550 3 of 12
CN module simulation to have a proof of concept. Section 5presents the experiments made
with a device with two different CN modules configurations. In the first case experiment,
two CN modules send the raw distance data to a third CN module dedicated to calculating
the vehicle speed and length in the first case. In the second case, each CN module processes
its data and produces a candidate speed value. The speed value is transmitted to the third
CN module that obtains the vehicle length. Finally, the article ends with the conclusions
and possible studies to be carried out in future research.
2. Background and Related Work
2.1. Placing Elements at Different Levels
This subsection shows the shift between the intelligent hierarchical control pyramid
and how embedded devices generate a new inverted pyramid model [
9
]. The vision of the
’pyramid of knowledge’ in intelligent control systems implies an analogy with the CN of
each level (Figure 1). A description and review of such a pyramid can be obtained in [
10
].
The data source (pyramid base) is the sensors that provide raw data representing physical
values measured by parameters. Raw data are processed to obtain basic information. For
example, a series of isolated temperature measurements can provide an average value and
error or a trend in temperature change. The processed information can produce knowledge.
For example, different temperature oscillations over several days can estimate the existing
climate. Finally, the knowledge is used to generate intelligence. For example, knowledge
of the climate in a location allows predicting changes and taking preventive action. This
process is known as learning and is the basis of advanced machine learning methods.
Figure 1.
The different intelligence levels are related to the classic vision of the pyramid of knowledge
(left side of the figure) and the relationship with intelligent control (right side of the figure).
At the low level of the pyramid, the primary data characteristic is the short period in
which the data are valid. For example, a vehicle detection sensor will report the vehicle
presence at a specific time moment and in a particular place. As the elements of the
system grow, specifically in lower layers of the classical pyramid (i.e., a large number of
CNs), a considerable amount of data is available. Many connected elements that provide
this massive data have led to the emergence of the Internet of Things (IoT) or Industry
4.0 paradigm. Including the IoT and Industry 4.0 paradigm in distributed intelligent
systems implies reviewing the knowledge pyramid, such as the one proposed in [
11
].
Currently, there is a consensus to divide distributed systems in a layer close to the physical
environment (fog, edge in the hardware) and cloud to provide massive data processing,
advanced computing or machine learning. These layers (or ’areas’) have changed the
design of system architectures, forcing a turn away from the hierarchical models towards
highly connected horizontal models. In these new models, intelligence becomes distributed
and not exclusively at the top of the classic pyramid of knowledge model (Figure 1).
Consequently, a system architecture must support intelligence at the edge level but provide
all available data to the cloud level. Edge elements, as CNs, can provide some intelligent

Electronics 2022,11, 550 4 of 12
processing that helps fog and cloud obtain the data processed and avoid the fog and cloud
elements from making decisions that a CN can make.
This process is known as edge computing and is a concept linked to the emergence
of the Internet of Things [
12
]. Edge computing is performed by devices in direct contact
with the data source or sensors allowing more direct communication between devices. This
article is about devices where sensors that require simple computation, such as infrared
or ultrasonic, can provide relevant information. The use of these devices is useful in a lot
of scenarios, mainly dedicated to road safety [
13
]. To achieve this security, it is necessary
to be able to detect and characterise aspects such as speed or type of vehicle through the
use of sensors. Most detection and characterisation systems use a combination of different
types of sensors. For example, infrared and ultrasonic are used in [
14
]. It is also possible
to use only one type of sensor for speed detection. In the case of [
15
], speed detection is
based on ultrasonic sensors to detect speed violations. Based on the previous work, the
next subsection presents the analysis of these types of devices.
2.2. Control Node Characterisation at the Edge Level
It is necessary to measure the time spent to control processing and communication
tasks to compare the performance of a single CN and a set of CNs working collaboratively.
Figure 2broadly shows the times involved in control and communication actions inside a
single CN. The inputs of a CN are the communications ’Comm’ that receive service requests
from other CN or upper elements and the sensor data provided by the corresponding
hardware elements. In turn, the outputs of the CN are the communications ’Comm’ services
to other edge nodes or system elements (cloud or fog) and the actuators ’Hw (actuators)’.
Figure 2. Times related to communications and processing of a single control node (CN).
In this context, a CN can have different configurations. A CN without incoming or
outgoing communications is an autonomous reactive node; it is not usual, but there are, for
example, irrigation systems without remote monitoring or configuration. A CN that only
senses is the basis of distributed wireless sensor networks (WSN), systems widely used in
human environments [
16
]. In the same way, a CN without sensors and actuators also does
not make sense in the context of distributed control.
Regardless of the configuration used in a CN, one must measure its performance
to understand its ability to assimilate some intelligent algorithms. Absolute and relative
errors measure the efficiency of the control action. Low error values imply effective control,
even though the processing and the communications load can amount to 100% of micro-
controller use. Consequently, high efficiency could mean high electricity consumption [
17
].
Therefore, the cost of such efficiency may be higher than the efficiency achieved. However,
for a CN, or set of CNs, the goal is to obtain a low response time and low micro-controller
processing load, in addition to common error values. Thanks to data processing, the control
error can be decreased from other CN. In that case, it is convenient to evaluate whether it is

Electronics 2022,11, 550 5 of 12
efficient to wait for the remote data or to act locally with a more significant error. When a
CN must communicate through a shared communications system, you have a distributed
system. In this case, in addition to the times considered in Figure 2, the times invested in
the communication tasks between nodes should be considered as an integral part of the CN
response time. Figure 3shows all times involved due to the use of connected CN devices.
Figure 3.
Times related to communications between two different control nodes (CN) connected in
the fog or the cloud.
The times involved in this case depend on the system architecture. When nodes are
on the edge or fog, in other words, they share a communication medium, these times are
usually shorter than the times involved in cloud communication. From times outlined
in Figure 3, depending on the system errors, it is possible to customise from a CN to a
distributed intelligent system to spend optimal time to pre-process data. The following
section will use these times to characterise a simple system and check which formulas can
answer whether it is better to act fast with a specific error or wait a while to act but with a
minor absolute or relative error.
3. The Proposed Solution
This section presents a case study based on an intelligent device consisting of a smart
device with some CN modules. The aim of the device tested is to measure the speed and
the length of a vehicle based on the work presented in [18].
3.1. Vehicle Detection and Characterisation Method
The way to measure vehicle speed and length is to compare different distance mea-
surements of a vehicle as it approaches the US sensor. Figure 4a shows how a sensor
with an inclination of 45
°
to the road axis can detect the front and side of a vehicle. These
distance measurements are the maximum possible when no vehicle is present. The distance
measurement becomes a downward ramp when the front of a vehicle is detected. Next, the
measurement becomes a constant distance as soon as the side is detected. Finally, when the
vehicle disappears, the measurement becomes the maximum possible again.
The device consists of three interconnected CNs. The CN has been built from the
JSN-SRT04 ultrasonic (US) sensor module. This US sensor is waterproof and widely used
in industry, mainly for measuring the liquid tank level or the distance between elements in
outdoor environments [
19
]. The sensor has a detection range from 0.20 to 6 m. This range
makes it suitable for covering both road and street lane vehicle profiles. The resolution of
the distance is 0.01 m, so it allows relevant variations of vehicle distance so that the detection
algorithm can work efficiently. The relative error detected in the measurements is 0.74% on
average [
20
] with a linear relation between distance and error [
21
]. The sensor sampling
rate used in the device was 500 KHz in the experiments. The sensors have been connected
to an Arduino Nano, which communicates with the device’s CNs via an Inter-Integrated
Circuit (I2C). This channel allows serial communication between a master and several
slaves at speeds between 100 Kbits/s and 3.4 Mbits/s [
22
]. I2C is a channel widely used in
embedded systems due to its simplicity to manage it. Figure 4b shows the experimental

Electronics 2022,11, 550 6 of 12
device. This experimental device has three modules or CNs. Each of them has a specific
orientation. Depending on the orientation angle in which the ultrasound sensor is tilted
concerning the road’s longitudinal axis, there is a specific distance profile along the time for
each detected vehicle. The different orientations can obtain a different measurement profile
over time. Figure 5shows the three measurement profiles with US sensors oriented to 30
º
,
45
º
and 90
º
. Below each module orientation, the signal profile obtained from distance over
time is shown. With the module oriented at 30
°
degrees, the number of samples in which
the vehicle’s front cuts the US ray is greater than with the module oriented at 45
°
degrees.
Comparing the 30
º
and 45
º
degree modules, the 30
º
modules can obtain a more accurate
calculation of speed. As the 90
°
module cannot detect the front of the vehicle, it can only
estimate the length of the vehicle measuring its side. Therefore, the 90
º
module cannot
calculate the vehicle speed directly. As long as the 90
º
module has a speed value from
another CN, it can calculate the length of the vehicle crossing in front of its US sensors.
Figure 4.
Method to detect the vehicle’s front and side using US rays (
a
), and the device implemented
with US modules (b).
Figure 5.
Selected configurations that provide a reasonably accurate estimation of vehicle characteris-
tics: 30° (a), 45° (b), and 90° (c).
Each of the CNs for the device shown in Figure 5is similar and depend on their
orientation to provide speed or distance with concrete accuracy. When a vehicle has been
detected, the module calculates the vehicle’s speed based on the change between the front
and side of the vehicle. When the sensor returns to provide the maximum distance, it is
considered that the transition of the vehicle has already been completed.

Electronics 2022,11, 550 7 of 12
3.2. Vehicle Detection and Characterisation Process
The processes that a module performs to detect and characterise a vehicle are shown
in Figure 6. As shown in this figure, a module that acts as a CN can detect a vehicle and
determine its speed, as well as the length of it.
Figure 6. Phases performed by a module to detect and characterise a vehicle.
As previously discussed, Figure 4illustrates how vehicle detection is initiated when the
sensor starts to detect a distance less than the maximum distance determined. Consequently,
the first step is to sample and filter the data, because the ultrasound signal is subject to
many problems, including echoes and material-dependent responses. This first step (1 in
Figure 4) consists of sampling five distances and calculating their average to obtain the
main data, distance
d(t)
. Not all samples are correct, for example, echoes can produce
false measurements. The raw data are filtered to identify any values that could lead to
erroneous measurements. Filtering is performed by discarding the samples that do not
comply with the sensor minimum and maximum constraints or that differ by 10% from
a window of five previous samples. Additionally, if the filter detects two continuous
erroneous samples, five-window samples are discarded. In the second phase (2 in Figure 4),
the vehicle is detected. Vehicle detection occurs when the values
d(t)>d(t+
1
)
over
N
consecutive operation cycles. Initially,
N
is set to two. In order to characterise both the
spatial and kinematic properties of a vehicle during its detection phase, a non-parametric
method called frame differences [
23
] is required. The spatial characteristics of the vehicle
are its length, while the kinematic characteristic is its speed. The module changes to
instantaneous speed detection when an approaching vehicle has been detected (phase 3
in Figure 4). During instantaneous detection, the speed is calculated by comparing the
two distances obtained by consecutive measurements of the vehicle’s front. Since the
difference of distances detected and time between these distances is available, the vehicle
speed calculation is immediate. From the instantaneous speed detected, the fourth phase
(4, in Figure 4) updates the speed value. As a result of this update, an average speed and
standard deviation can be detected. Additionally, this phase can recognise specific patterns,
such as acceleration and deceleration of the vehicle. When the difference between two
consecutive distances is less than a certain threshold, 5% in the experiments, the side of
the vehicle is considered detected. From this point on-wards, the length update phase, 5
in Figure 4, starts. Based on the speed calculated in the previous phase, this phase works
while the vehicle is being detected to determine the vehicle length. According to the needs
of other control nodes, the control node can send them raw or processed data. This aspect
allows CNs to decrease their process load but increases the network throughput. Due to
this, all phases can offer their outputs to other control nodes (phase 6 in Figure 4).
4. Experiments and Results
As explained in the previous sections, one of the goals of edge computing is to
determine whether the ratio of processing load to the accuracy of the result is acceptable.
In light of this, checking the processing and communication load for different device

Electronics 2022,11, 550 8 of 12
configurations is essential. To test all previous concepts, we have developed a device
with several modules, each of which is a CN. The experimentation has been carried out
in two phases: simulation and prototyping. The simulation has been performed on the
measurement accuracy of a single CN with a US sensor. The simulation aimed to determine
which angles are the most appropriate to be implemented in a prototype. Based on the
simulation results, the prototyping performed consists of a single device, using three
different CNs with the US sensor oriented with the angles: 30
º
, 45
º
, and 90
º
, as described in
Figure 5.
4.1. Simulations
The simulation has been carried out using the simulator presented in [
24
] and coded in
Python. Based on the Pygame environment [
25
], we extend the simulator code to introduce
the CN with a US sensor. The experimental environment consists of an entry vehicle with a
fixed speed that can be varied to generate different cases. A US sensor that allows the entry
angle to be varied is also included. This US sensor detects the distance between the sensor
exit ray and the collision point in the vehicle. The vehicle simulated has a size of 100 cm
for the front and 180 for the side. A constant speed of 2.25 m/s is applied to the vehicle.
Figure 7shows the sensor with different angles and the corresponding data collected of
the distances.
Figure 7.
Selected configurations that provide a reasonably accurate estimation of vehicle characteris-
tics: 90º, 75º, 60º, 45º, and 30º, and corresponding distances obtained (bottom).
As can be seen in Figure 7, the angles closer to the road axis obtain a more significant
amount of data from the front of the vehicle. This amount of data means that more possible
speeds are available, which generates a minor relative error in the calculation. Table 1

Electronics 2022,11, 550 9 of 12
shows the parameters obtained from simulating five vehicles with the same characteristics
for each angle.
From the simulation data, it seems appropriate to use control modules with angles
close to the road axis. Consequently, angles of 30
°
and 45
°
are selected for experimentation
on the prototype. The length calculation is based on the lateral samples. In all cases, the
number of samples is similar, although the shortest distances are obtained with the angles
perpendicular to the road axis, i.e., with the angles of 75
º
and 90
º
. Since the 75
°
speed error
is the largest, it seems appropriate to use only one sensor at 90
°
for the calculation of the
vehicle length.
Table 1.
Simulation data obtained. Average of the valid samples in the detection of the front and side
of the vehicle and calculated speed, together with the corresponding errors.
90º 75º 60º 45º 30º
Samples front - 12 25 44 76
Samples side 84 83 83 82 81
Calculated Speed - 2.06 m/s 2.16 m/s 2.20 m/s 2.22 m/s
Relative Error - 8.44% 4.00% 2.22% 1.33%
4.2. Prototyping
A vehicle with a length of 3.7 m was used for the experiment. The vehicle has been
passed through the measuring module ten times for angles of 30
º
, 45
º
and 90
º
. For each
angle, the vehicle speed was 10 m/s (36 km/h). Measured times are all in milliseconds
(ms). The device aims to measure the vehicle speed and length accurately. To achieve this
accuracy, the CNs must collaborate among them. The modules configured with 30
º
and 45
º
can obtain vehicle speed accurately. The module configured with 90
º
can obtain the vehicle
length using the speed obtained from the previous angled CNs. Two different cases have
been tested. The first case represents central processing and the second case represents
distributed processing. In the first case, the 30
º
and 45
º
modules transmit the raw data of
the measured distances to the 90
º
module. This case corresponds to a hierarchical model in
which the angled CNs are dedicated to data collection. In contrast, the 90
º
node is dedicated
to collecting data and calculating the resulting speed and length. Results are shown in
Table 2.
Table 2.
Results obtained from two control nodes (CN30 and CN45) sending raw data to the third
module (CN90).
CN30 CN45 CN90
tControl(AVG) 32.34 ms 28.97 ms 235.73 ms
tControl (STD) 3.85 ms 2.15 ms 18.93 ms
tResponse (AVG) 51.38 ms 31.38 ms 276.54 ms
tResponse (STD) 3.18 ms 2.18 ms 20.49 ms
tLatency (AVG) 1340.81 ms 1362.25 ms -
tLatency (STD) 90.90 ms 87.34 ms -
Speed (AVG) - - 10.12 m/s
Speed (STD) - - 1.17 m/s
Speed (Rel.E) - - 5.79%
Length (AVG) - - 3.76 m
Length (STD) - - 0.15 m
Length (Rel.E) - - 1.61%
In the second case, the 30
º
and 45
º
degrees CNs calculate the speed and length average,
in conjunction with the standard deviation, and this information is sent to the 90
º
degree
module. Results are shown in Table 3.

Electronics 2022,11, 550 10 of 12
Table 3.
Results were obtained from the three CNs processing data and sharing the information
obtained. The third module (CN90) uses the speeds calculated by the modules CN30 and CN45 to
calculate the vehicle length and the vehicle speed.
CN30 CN45 CN90
tControl(AVG) 45.51 ms 49.65 ms 113.04 ms
tControl (STD) 5.34 ms 1.44 ms 1.06 ms
tResponse (AVG) 105.16 ms 56.67 ms 132.22 ms
tResponse (STD) 1.77 ms 4.77 ms 12.06 ms
tLatency (AVG) 1339.41 ms 1362.65 ms -
tLatency (STD) 92.30 ms 85.94 ms -
Speed (AVG) 10.55 m/s 12.92 m/s 10.08 m/s
Speed (STD) 1.01 m/s 1.18 m/s 0.97 m/s
Speed (Rel.E) 5.79% 17.77% 4.12%
Length (AVG) - - 3.71 m
Length (STD) - - 0.09 m
Length (Rel.E) - - 0.78%
5. Discussion
The results obtained show how the speed and length calculated by the CN90 module
have similar accuracy to the previous case. This result is expected but is not the aim of the
experiment. We need to compare the total time involved in both cases. If we calculate the
overall time that the device dedicates to the process, we obtain 315.04 ms in the first case,
but the total process time is 208.00 ms in the second case.
Indeed, if we consider the response time, thus it is the sum of communications and
process times, the results are 359.3 ms in the first case and 294.05 ms in the second one.
This result means that distributed processing saves around 22% of processing and com-
munications time. The results show how distributing the processing between CNs in the
device decreases the overall processing time. Although the latency time has no significant
changes, since modules work in an I2C network, there is considerable data transferred in
the first case. Due to this, five messages must be sent with the measured distances for each
message with the instantaneous speed transmitted. This difference is even more significant
in the case of transmitting only the final speed or the final transmitted length.
Reducing messages and processing data before sending it has relevant implications,
especially for power consumption due to the processing time and network load. The
power consumption has been measured, at the module level, in order to check the energy
savings. The three modules implied in the first case have a total consumption average of
0.5269 W per second, whereas the second case has a total consumption of 0.5141 W per
second. The difference in consumption is small because communications are a major part
of the time consumed. However, processing consumption is relevant. This indicates the
importance of optimising processing to reduce communications.
6. Conclusions
This article has presented a paradigm that allows modules to share data and process
information. Based on the paradigm presented, a module called a control node (CN) has
been presented and characterised. A CN has been implemented as part of a device that
obtains the speed and the length of vehicles using ultrasonic sensors. These low-cost and
high-error sensors provide data that improve the accuracy of the devices that receive this
information instead of raw data being processed and distributed as a piece of information.
It is possible to prove how the collaboration of modules at the edge level improves the
quality of the information, measured in terms of the relative error.
Experiments have proven that processing data close to the CN reduces the overall
time dedicated to processing in global terms. These results open the door to future experi-
ments where information is shared through the fog between devices in addition to sharing
information within a device. The overall power consumption of the system can be reduced.
This reduction implies that studying how to process data close to the edge level is a good
starting point for new experiments. Additionally, some parameters can be used to tune the

Electronics 2022,11, 550 11 of 12
system. Aspects such as message transmission policy based on the relative error to reduce
non-relevant information can optimise the system performance. As a future research line,
we propose implementing CN as cells of a distributed neural network that, dynamically,
can select which kind of information suits to be distributed.
Author Contributions:
Conceptualisation, J.-L.P.-L. and P.U.-C.; methodology, J.-L.P.-L. and P.U.-C.;
software, P.U.-C.; validation, J.-L.P.-Y.; formal analysis, J.-L.P.-L.; investigation, P.U.-C. and J.-J.S.-
P.; resources, J.-L.P.-L. and J.-L.P.-Y.; data curation, J.-L.P.-L. and P.U.-C.; writing—original draft
preparation, J.-L.P.-L. and P.U.-C.; writing—review and editing, J.-L.P.-Y. and J.-J.S.-P.; visualisation,
P.U.-C. and J.-J.S.-P.; supervision, J.-L.P.-L.; project administration, J.-L.P.-L.; funding acquisition,
J.-L.P.-L. and J.-L.P.-Y. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the Spanish Science and Innovation Ministry MICINN: CICYT
project PRECON-I4: “Predictable and dependable computer systems for Industry 4.0” TIN2017-86520-
C3-1-R.
Data Availability Statement:
Data sets are available on demand from the corresponding author
via email.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
CN control node
CNN convolutional neural network
I2C inter-integrated circuit
IoT Internet of Things
MLP multi-layer perceptron
US ultrasonic
WSN wireless sensor networks
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