IEEE Consumer Electronics Magazine - October 2016 - 99

higher frequency (2.4 GHz versus 433 MHz), coupled with the
fact that to transmit data, the ESP-03 has to create the whole
ISO stack (while the AM system can transmit on the equivalent
to the physical layer), the energy requirements for the Wi-Fi
system is always going to be higher. But the Wi-Fi system has
significantly higher data rates; so, if a system required a lot of
bandwidth to be transmitted, it would be more efficient to use
the Wi-Fi system. Furthermore, since authentication is required
when connecting to a secured wireless network, this means that
only sensors that have been authorized to access and transmit
on the network can provide information, and this provides more
security. Whereas within a basic AM system, any received signal will be interrupted allowing the possibility of false signals.

IP ADDReSS ALLOCATION
Home networks and Wi-Fi networks often use dynamic IP
address allocation for ease of management. IP addresses are
allocated using DHCP. In order scenario, since the ESP-03 is
switched off to save power between transmissions, an address
has to be allocated whenever the device is switched on, i.e.,
for each transmission. Since the transmissions are short, the
DHCP exchange, although short, has an impact on battery life.
DHCP is a very flexible protocol and could be used to
send measurements without an overlaid Internet service, but
the DHCP server would have to be configured to allow this.
Since we want to use standard Internet protocols and reduce
deployment issues, we would not want to use DHCP in this
manner. However, in a home network, IP addresses are normally allocated for an extended period of time (for example,
24 h). For the MSP430-based device, the MSP430 can be
used to store the allocated IP address between transmissions,
so when the ESP-03 is switched on, it can use a static address
from the MSP430. We have investigated the power consumption with static IP to investigate the reduction in power consumption through the saved DHCP exchange.
A graph comparing static and DHCP IP address allocation
on both the standalone ESP-03 and the ESP-03 + MSP430 is

The scenario that we have
considered is for unidirectional
communications from the sensor
node to the Internet, which fits
well with a sensor gathering
readings or a smart button
triggering an alert.
shown in Figure 9. As shown in the graph, there is roughly
30% reduction in energy consumption when using static IP
address allocation compared to DHCP on each transmission.
This experiment showed that it was possible to further reduce
the energy consumption of the device by using long DHCP
leases but at some reduction in adaptability; so, it was decided
to continue using DHCP for the remainder of the experiments.

SeNSOR ACTIVATION
The sensor used for this prototype had a standby current resulting in power used up by both the sensor and ESP during sleep
mode. For the temperature sensor with the shutdown feature, the
standby current of 750 nA, for the DS18s20, could be saved. In
the long run, this would result in more power savings. Replacing
the sensor with a no-standby current would be an option for the
future to reduce power usage. The temperature sensor would be
high while taking the reading and then turned to low within the
code before it goes into sleep mode. This would reduce the
power, and only the standby current would be used up.

BIDIReCTIONAL COMMUNICATIONS
The scenario that we have considered is for unidirectional communications from the sensor node to the Internet, which fits
well with a sensor gathering readings or a smart button triggering

1.2

MSP430 ESP-03
MSP430 433 MHz

1
Power (mWh)

0.4
Power (mWh)

Static MSP430 ESP-03
Static ESP-03
DHCP ESP-03
DHCP ESP-03 + MSP430

0.2

0.8
0.6
0.4

0

0.2
0

2
4
6
Transmission Period (s)

8
⋅104

FIgURe 8. A comparison of Wi-Fi versus 433 MHz using MSP430.

0

2
4
6
Transmission Period (s)

8
⋅104

FIgURe 9. A comparison of static IP assignment versus DHCP.

OCTOBER 2016

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IEEE Consumer Electronics Magazine

99
Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - October 2016
