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Abstract— The current tendency to embed computational
resources on quotidian objects transforms them into smart
objects. This is the vision of Internet of things, where many
different devices collect and process information from different
sources to both control physical processes and interact with
human users. Initially wireless sensor networks and Internet of
things concepts were used with the same meaning. The same
happened with wireless sensor networks and the protocol IEEE
802.15.4. It is now accepted that the Internet of tings can
comprise more than one type of networks and therefore several
layer two protocols. In such scenarios the IP protocol it is used,
like as before, to make this heterogeneity interoperable. This
paper presents a demonstrator for power energy monitoring and
actuating system and it was developed for home environments.
This system is based on 6LoWPAN to connect to the Internet a
smart object network with two layer 2 technologies: IEEE
802.156.4 and power line communication. IPv4 to IPv6 transition
mechanisms were included to provide connectivity to both IPv4
and IPv6 Internet end systems.
INTRODUCTION
Kevin Ashton proposed the term of Internet of Things (IoT)
before 2000 in the context of supply chain management. Now
the IoT it is more inclusive and includes a wide range of
applications. Also the concept of IoT network nodes, known as
Things, was evolved as a consequence of technology evolution,
in particular due the MEMS devices advances.
The IoT it is the driver to change the traditional Internet
into a network of interconnected objects with resources to
harvest information from the surrounding environment (i.e.
sensing) and to interact with the physical world (i.e. actuation
and control). In this scenario, the existing Internet protocols are
required to support information transfer, analytics, applications
and communications. At the beginning, the IoT and the
wireless sensor networks concepts were used interchangeably.
The same happened with wireless sensor networks and IEEE
802.15.4 layer two protocol. Now IoT it is more embracing thus, it is not restricted to only one device type neither to only
one particular layer two technology.
In the IoT networks, some of the devices are embed on
quotidian objects and therefore they must have small size,
restricted computational resources and energy constraints. In
such situations, the IEEE 802.15.4 [1] support may be a
requirement because it is a wireless communication protocol
and it was designed to operate on computational and energy
resource constrained devices. The power line communications
(PLC) [2] solution it is the natural layer two protocol to
connect power energy monitoring and actuation devices. In
such situations, the energy wires are used also for
communication purposes and there is no severe restriction
about energy consumption because the energy on the wires can
be used to feed the monitoring devices. However, the
computational resource constrains remains valid because the
size and the price of the devices. In home power energy
monitoring and actuation solutions IEEE 802.15.4 and PLC are
the most promising layer two technologies to provide wireless
and wired connectivity respectively [3].
Although initially IP was not considered for IoT
environments, the scientific community and the industry have
rethink many misconceptions about the use of IP protocol in all
nodes [4]. First, the IP protocol can be used as a standard
solution for the interconnectivity between incompatible lower
layer protocols. Second, it provides an application developing
environment, which is open and royalty free. Finally, if all
nodes are compatible with IP protocol the use of complex and
hard to manage proxies and gateways necessary to connect
non-IP nodes to the Internet can be avoided. While the IPv6
protocol has enough address space to accommodate also the
IoT devices, it was not originally designed to be used on power
and resource constrained nodes. The 6LoWPAN adaptation
layer [5] was proposed to be used on such devices between
data link and the network layers to make the IEEE 802.15.4
layer two standard protocol compatible with IPv6. It provides
new routing approaches, header compression and
fragmentation support above 1280 bytes. It also includes auto
configuration and neighbor discovery mechanisms, which are
more adapted to energy and resource constrained devices.
This paper presents a solution based on 6LoWPAN to
provide connectivity between incompatible layer 2 IoT devices
and also to provide the interaction between this devices and the
Internet. Both the IoT nodes and the gateway were developed
in house and are based on COTS chipsets. At the application
layer, two main applications were developed. The first one is
running in the gateway and it is used to manage the IoT
connected devices providing it with a resource discovery
mechanism. The second one implements the home power
energy monitoring and actuating system. The second
application can be used to interact with IoT devices, to store
and analyse harvested data and to provide a web interface to
the users. The last application it is installed on the IoT devices,
on the gateway and on the application server. A demonstrator
has been constructed to evaluate the proposed solution and to
prove their capabilities.
The remainder of this paper is organized as follows. Section
II presents the related technologies, while Section III focuses
on the system architecture. Section IV presents the system
evaluation and demonstration. Finally, Section V concludes the
paper and pinpoints future research work.
II. RELATED TECNOLOGIES
A. IEEE 802.15.4
The IEEE 802.15.4 protocol defines the physical and the
media access control (MAC) layers to address the low-power
and low-rate wireless personal area networks requirements
(WPAN). The PHY layer defines three physical operation
modes, 20 kbps at 868 MHz, 40 kbps at 915 MHz, and 250
kbps at 2.4 GHz (DSSS). The MAC layer provides two
operational modes, the synchronous beacon-enabled mode and
the asynchronous beaconless. The beacon-enabled mode is
designed to support the transmission of beacon packets
between transmitter and receiver, providing synchronization
among nodes. In the beacon-enabled mode the period between
two consecutives beacons defines a superframe structure that is
divided into 16 slots. Beacons always occupy the first slot,
while the other slots are used for data communications. In
order to support low-latency applications, the PAN coordinator
can reserve one or more slots, designated by guaranteed time
slots, avoiding the use of MAC mechanisms. In the beaconless
mode, there is no superframe structure and no guaranteed time
slots. As a consequence, only random access methods, such as
unslotted CSMA/CA can be used to medium access. The frame
length is limited to 127 bytes because unreliable and error
prone wireless links are used and the devices have limited
buffering capabilities.
B. Power line communication
The idea of using power lines, not only as electricity
conductors, but also for communications purposes was
proposed at the beginning of the last century [4]. This
technology is known as power line communication (PLC). The
main advantage is the wide spread availability of power
distribution infrastructure and therefore the theoretical
deployment costs are limited to connecting the communication
devices to the existing electrical grid. Power line
communication technologies can be grouped into narrow-band
PLC (NB-PLC) and broadband PLC [7].
The narrow-band PLC systems operate in the frequency
range between 3 kHz and 500 kHz, regulated by CENELEC,
ARIB and FCC organizations. PRIME, G3-PLC and IEEE
1901.2 are examples of PLC narrow band standards. The
PRIME was developed within PRIME alliance and uses up to
96 OFDM subcarriers over the frequencies from 42kHz to 89
kHz and it is able to achieve a bit rate of 128.6 Kbit/s. In order
to deal with unpredictable impulsive noise, the PRIME
standard includes an automatic repeat request (ARQ)
mechanism, based on selective repeat retransmission. PRIME
has de ability to form sub-networks, each one has one base
node and several service nodes. The base node manages the
sub-network’s resources, such as the PLC channel access
arbitration. The medium access control can be based on
contention free and contention-based access mechanisms. The
contention free mechanism relies on time division multiplex
channel access period, where the base node assigns the channel
to only one node at a time. The CSMA/CA it is used on
contention-based. The G3-PLC can operate from 10 kHz to 490
kHz and it reaches peak bit rates up to 300 kbits/s. The MAC
layer it is based on IEEE 802.15.4-2006 and therefore
6LoWPAN can be used to fulfil the IPv6 requirements. When
PRIME and G3-PLC are compared, the first allows cheaper
implementation because it is less complex, while the second it
is more robust under interference conditions. The PRIME and
G3-PLC form the IEEE 1901.2 protocol baseline.
The broadband power line communication (BB-PLC) is
widely used for broadband networking applications, such as
Internet access, gaming and high definition and 3D video. It
operates in the band between 1MHz to 250MHz and having a
bitrate ranging from several Mbps to several hundred Mbps.
The BB-PLC is mainly based on four protocols: IEEE P1901,
HomePlug, universal power line association and high definition
power line communication [8]. The HomePlug alliance it is
responsible for three protocols compatible with IEEE P1901.
The HomePlug AV uses physical and MAC technology that
provides 10Mbps for ROBO mode and up to 200Mbps on the
adaptive bit-loading mode. On the physical layer, it operates in
the frequency range of 2–28MHz, uses windowed orthogonal
frequency division multiplexing and turbo convolutional code.
On the MAC layer, HomePlug AV provides a quality of
service connection oriented, contention-free service on a
periodic Time division multiple access (TDMA) allocation, and
a connectionless, prioritized contention-based service based on
CSMA/CA). The HomePlug AV2 was developed to support
high definition 3D and video while maintaining full
compatibility with other HomePlug protocols. The HomePlug
Green PHY it is similar to the other HomePlug protocols and
was designed to support smart grid applications. Only ROBO
mode and QPSK modulation it is supported and as a
consequence it only support 4,5 and 10 Mbps. The HomePlug
Green PHY MAC it is a simplified version of the HomePlug
AV MAC. It shares the same CSMA and Priority Resolution
mechanisms as HomePlug AV, but it does not support the
optional TDMA mechanism.
C. 6LoWPAN
Low bandwidth, low-power resources and the maximum
link-layer packet size of 127 bytes are the most relevant
characteristics of the IEEE 802.15.4 standard. Implementing standard IPv6 headers over LoWPAN would result in
extremely small payloads for higher-level protocols, more over
it requires a neighbor discovery protocol too verbosely based
on multicast messages.
The IEEE 802.15.4 standard is mostly accepted as the
physical and MAC layer protocol to provide wireless
connectivity between IoT nodes. However the IPv6 protocol
don’t fully match with the IEEE 802.15.4 constraints. For
example the minimum IPv6 MTU is 1500 bytes and the
IEEE802.15.4 MTU is 127 bytes. Beside to this
incompatibility, using standard IPv6 headers would result in
extremely small payload for high protocols. To address these
issues, the IETF 6LoWPAN-working group was formed to
define the support of IPv6 over IEEE 802.15.4 networks. To
support IPv6 over IEEE 802.15.4 an additional adaptation layer
was introduced between data link and network layers. Like on
the IPv6, the 6LoWPAN use stacked headers and therefore
only the necessary header types are used. The 6LoWPAN
standard defines four header types: i) the dispatch header, ii)
the IPv6 header compression header, iii) the fragmentation
header and iv) the mesh header. In the simplest case, only the
dispatch and compression headers are used. At the beginning of
each header, a header type field identifies the header format.
Although the standard IPv6 neighbor discovery (ND) protocol
should work on 6LoWPANs, the node’s resource constraints,
the absence of multicast support at layer two, the low dutycycle
and the use of non-transitive links requires a different
approach for the ND protocol on 6LoWPANs focused on the
efficient use of available energy.
Although 6LoWPAN was originally designed to support
IPv6 over IEEE 802.15.4, it can later be adapted to be used on
other similar link technologies. A typical 6LoWPAN network
it is formed by nodes, routers and edge routers. 6LoWPAN
nodes usually perform only sensing and actuation operations.
They do not forward datagrams originated on other nodes and
destined to other nodes. Routers are intermediate nodes that
can be used to forward datagrams to others nodes or routers in
the same LoWPAN and are present only in route-over
topologies. Edge routers are used to connect the LoWPAN to
others networks, for example, the Internet. Typically, nodes
and routers have energy and computational resources
constraints and only the edge routers are main powered and
have more computational resources.
III. SYSTEM DESCRIPTION
The demonstrator (Fig. 1) it is composed by: (i) IoT nodes
(i.e. the environmental sensor, the panel module and the smart
plug), (ii) a gateway to connect the IoT to the Internet and (iii)
an application server and Internet connected clients.
IoT nodes
Three different IoT nodes developed in house are used to
measure energy, voltage and current and temperature (Fig. 2).
Two of them are compatible with IEEE 802.15.4 and the other
is compatible with PLC HomePlug Green PHY. All nodes are
running Contiki 2.5 and RPL routing protocol [6]. An
application compatible with Contiki 2.5 was developed to
allow data retrieval, node management and actuation. The
smart plug and the environmental sensor compliant with
IEEE802.15.4 (Fig. 2) are based on Texas Instruments CC2530
MCU, which provides 8 KB RAM and 256 KB flash. The
smart plug (Fig. 2, left) was developed to be inserted between
the power outlet and the device to be controlled and it is
equipped with voltage and current meters and with one relay
that can be used to remotely switch on and off the electric
device connected to the power outlet. The environmental
sensor (Fig. 2, right) it is equipped with temperature and light
transducers and can be used to control home environmental
parameters.
The panel module PLC HomePlug Green PHY (Fig. 2,
center) compatible node it is based on Texas Instruments MCU
MSP 430, which provides 16KB RAM and 128KB flash. The
PLC interface is provided by Qualcom QCA 7000 chipset. This
node was developed to measure and control more than one
device connected to the same distribution circuit and therefore
it should be installed near to the utility grid global energy
consumption meter. It is equipped with three voltage and
current meters, one for each circuit, and with two relays that
can be used to control two different circuits.
Gateway
The gateway was also developed in house and uses a LPC
3240 CPU with 64MB RAM and 256MB capacity flash
NAND (Fig. 3). It provides four interfaces compatible with
Fast Ethernet, IEEE 802.11 g/n, HomePlug Green PHY PLC
and IEEE 802.15.4. The Qualcom QCA 7000 and the Texas
Instruments chips CC2530 are used to implement, respectively,
the PLC and the IEEE 802.15.4 interfaces. The gateway is
running embedded Linux and supports both IPv4 and IPv6
stacks. At the application layer three application modules were
developed: i) the sensor discovery and registration, ii) the
command parser and iii) the management. The sensor
discovery and registration it is based on 6LoWPAN neighbor
discovery and RPL messages and it is used to maintain the list
of the available IoT devices updated and to provide seamless
connectivity. The command parser allows the interaction
between the application server and the IoT devices. All the
requests from the application server are validated before being
sent to the IoT nodes. Two main reasons can be evoked to
avoid end-to-end connectivity between the IoT network and the
Internet connected devices. First, end-to-end connectivity
exposes the IoT network to several remote security attacks.
Second, IoT devices should be accessed based on its name,
location and supported functionalities, therefore only the data
and its context is important for the end users not the IoT device
IP address. The command parser also translates the IPv6
requests into IPv4 and vice-versa, if needed.