HARE: Supporting efficient uplink multi-hop communications in self-organizing LPWANs

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by  Toni Adame

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HARE: Supporting efficient uplink multi-hop
communications in self-organizing LPWANs
Toni Adame, Sergio Barrachina, Boris Bellalta, Albert Bel
Department of Information and Communication Technologies,
Universitat Pompeu Fabra, Barcelona
Email: {toni.adame, sergio.barrachina, boris.bellalta, albert.bel}@upf.edu
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1 Abstract—The emergence of low-power wide area networks
0 (LPWANs) as a new agent in the Internet of Things (IoT) will
2 resultintheincorporationintothedigitalworldoflow-automated High BW VHT 802.11 5G
n processes from a widevariety of sectors. Thesingle-hopconcep-
4G
a tion of typical LPWAN deployments, though simple and robust,
J overlooks the self-organization capabilities of network devices, 802.11
suffers from lack of scalability in crowded scenarios, and pays
7 Medium BW
little attention to energy consumption.
1
Aimedtotakethemostoutofdevices’capabilities,theHARE
protocolstackisproposedinthispaperasanewLPWANtechnol- Bluetooth ZigBee /
] BLE 802.15.4
I ogy flexible enough to adopt uplink multi-hop communications
N when proving energetically more efficient. In this way, results Low BW 8W02B.1A5N.6 WPAN LPWAN
. from a real testbed show energy savings of up to 15% when RFID / 802.15.3
s using a multi-hop approach while keeping the same network NFC
c
reliability.System’sself-organizingcapabilityandresiliencehave
[ Short range Medium range Long range
been also validated after performing numerous iterations of the

1 association mechanism and deliberately switching off network
Fig.1. Localization ofLPWANtechnologies according torangecapability
v devices.
andbandwidth required.
3
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6 I. INTRODUCTION
4 In the coming years, electronic equipment will be inter- 2) EvolutionaryLPWANscoveringthealternativesthathave
0 beendevelopedasupgradestowell-establishedprotocols
connected and consequently every person and every industry
. like IEEE 802.11ah (also known as Wi-Fi HaLow) [9],
1 will become simultaneously data generators and consumers.
0 EC-GSM-IoT [10], LTE-M [11], and NB-IoT [12].
Internet of Things (IoT) paradigm is a key enabler of this
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visionbydeliveringmachine-to-machine(M2M)andmachine- LPWAN architecture is characteristically single-hop,where
1
: to-person communications on a massive scale. end devices are connected directly to the base station, greatly
v As more and more things are connected to the Internet, simplifying the network and endowing it with robustness and
i
X low-cost and low-traffic devices are starting to be demanded. centralized control. And yet this single-hop massive channel
r However, traditional cellular networks do not deliver a good accesssetsoutsomeinherentchallenges:reliability,scalability,
a combination of technical features and operational cost for flexibility, and quality of service (QoS). In fact, the channel
thoseIoTapplicationsthatneedwide-areacoveragecombined accessmechanismofsomeLPWANtechnologiesresortstothe
with relativelylow bandwidth,longbatterylife, lowhardware use of ALOHA [13] [14], a random medium access control
and operating cost, and high connection density [1]. (MAC) protocol in which end devices transmit without doing
Low-power wide area networks (LPWANs) are intended to any carrier sensing to check the channel state in advance.
becometheengineoflong-range,low-bandwidthIoTapplica- Although simple, this uncontrolled medium access leads to
tions(seeFigure1),whichuntilnowhavebeenconstrainedby interferenceorpacketcollisionsamonguncoordinateddevices,
deploymentcostsandpowerissues.Thegoalofthesenetworks acutely affecting reliability in dense networks. In addition,
is to deliver small amounts of data over long ranges, at rates LPWAN devices located far away from the base station must
of up to tens of kilobits per second (kbps), with a battery makeuseofhightransmissionpowerlevels,resultinginsevere
lifetimeofuptoseveralyears,supportingthousandsofdevices energy consumption and reduced battery lifetime [15].
connectedto a basestation,andfacilitatingonlineintegration. In this article, the HARE protocol stack is proposed as
Existing LPWAN technologies can be categorized into two a new LPWAN technology flexible enough to adopt uplink
types [2]: multi-hop communications when proving energetically more
1) Dedicated LPWANs consisting of the purposely de- efficient than single-hop. A full set of advanced techniques
signedtechnologiessuchasLoRaTM [3],SIGFOXTM [4], belongingtodifferentcommunicationlayershasbeendesigned
IngenuTM [5], WeightlessTM [6], DASH7 [7], and ETSI- for this purpose, while ensuring data transmission reliability:
LTN [8]. • Inherentclocksynchronization,with nodesbeingperiod-
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TABLEI
ically set in time by means of beacons.
COMMONREQUIREMENTSOFHAREUSECASES
• TDMA-likechannelaccessforgroupsofcontenderswith
multiple transmission opportunities. Requirement Value
• Adaptive transmission power level. Coverage range Uptoseveral km.
Geographic coverage Excellent eveninremoteandruralareas
• Flexible and scalable network association process.
Penetration Goodin-building andin-groundpenetration
• Energy-aware, adaptive and resilient routing protocol. Device density
High(uptothousand)
• Regular use of deep-sleep states. (perbasestation)
Powerprofile Unassisted, battery-powered devices
Furthermore, HARE protocol stack has been implemented Battery lifetime Fromsomemonthsuptoseveral years
andtestedinrealhardwareplatforms.Resultsevaluationfrom Throughput <100bits/s
differentnetworkconfigurations(single-hopvs.multi-hop,use Latency Non-delay sensitive
Mobility Static devices
of different MAC layers within the TDMA slots, channel
Cost Lowhardware andoperating cost
error injection) show very high reliability while maintaining Maintenance Unassistedandself-organizing network
lowenergyconsumption(particularlyinmulti-hoptopologies). Delivery model Continuous datadelivery model2
Lastly, we have observed a better overall system behaviour
whenusingmulti-hoptopologiesinnonerror-pronescenarios.
Theremainderofthispaperisorganizedasfollows:Section 1) Node initiated network connection: Once installed for
II introduces the main requirements of feasible use cases for the first time or relocated, any node shall initiate its
HARE.Next,SectionIIIdescribesthegeneraloperationofthe associationprocessthroughasimpleaction(forinstance,
protocol stack and Section IV provides detailed information pressing a button).
of the developed mechanisms. Section V describes the pro- 2) Self-configuration and management: With the aim of
posed testbed and Section VI compiles the obtained results building a robust network, it shall adapt itself to envi-
from different experiments. Lastly, Section VII presents the ronmentaland/ortopologychangeswithouthumaninter-
conclusions and discusses open challenges. vention.
3) Battery lifetime maximization: LPWANs replace old
monitoring systems consisting of assigning human re-
II. SCENARIOS AND REQUIREMENTS
sourcestostudyinsituthebehaviorofoneormorephysi-
According to their own characteristic range and bandwidth calparameters.Therefore,maximizingbatterylifetimein
capabilities with respect to other technologies, the main use such systemsis vital in orderto justify their usageahead
casestowhichLPWANsareaddressedincludesecurityalarms, of other methods.
car park spaces, agricultural applications, smart metering, 4) Firmware distribution: Any change in the network con-
consumer electronics, and intelligent buildings. By way of figurationor in the applicationpurposeshall be remotely
illustration, in the ENTOMATIC EU-project1 a network of and easily distributed by the GW.
wireless sensor nodes [16] periodically report information on
Lastly, the operating system of embedded sensor nodes is
pestpopulationdensityandenvironmentalparameters,suchas
typicallylesscomplexthangeneral-purposeoperatingsystems
temperature and relative humidity.
[18]. However, the high variety of resources to manage in
HARE is clearly aligned with typical LPWAN applications
this kind of devices (processors, memories, clocks, network
andcircumscribesitssuitabilitytothosescenarioswithspecial
interfaces, etc.) and the demand of support for concurrent
concern for energy efficiency, where device batteries are so
executionofprocesses(timesynchronization,dataacquisition,
limitedthattheestablishmentovertime ofadirectconnection
taskscheduling,channelaccess,routingparameters,etc.)make
to the base station, or gateway (GW), would greatly affect
essential the use of a real time operating system (RTOS).
their lifetime.
Under these premises, Table II compiles some of the main
Inthissense,TableIoffersacomprehensivelistofcommon
use cases supported by the HARE protocol stack in five IoT
requirements from use cases to which HARE protocol stack
representativesectors: homeand industrialautomation,public
gives response in combination with the appropriatehardware.
infrastructure, natural resources, and smart agriculture and
Assumingthatthishardwareprovidesgoodsignalpenetration,
farming.
asingleGWcanserveuptothousanddeviceswithinitsgiven
coverage range. Applications executed by stations (STAs), in
turn, follow a continuous data delivery model [17] for their III. HARE OPERATION
sensed information, periodically delivering small amounts of The HARE protocol stack conceives end devices as ele-
non-delay sensitive data. ments controlled by the GW by means of beacons. This cen-
As sensor nodes are scattered over large areas, sometimes tralizedapproachallowsSTAstoremainasleepthemajorityof
with problematicaccess, self-maintenance of the system shall thetime,sothattheirsingleconcernistobeawakeenoughin
be a priority, capable of giving response to the following advancetolistentothenextbeacon.Networksynchronization
challenges: is thus achieved and allows the GW to ask for specific data
and/or distribute configuration changes in just one hop.
1ENTOMATICmainwebpage: https://entomatic.upf.edu/ The GW is considered to be appropriately placed close
2Althoughnotconsideredinthecurrentarticle,futureHAREdevelopments
to a power source. Thus it may always stay in an active
willconsiderofferingQoSinscenarios withmiscellaneous sensors(continu-
ous,event-driven, query-driven, andhybrid) state and is providedwith the ability to directly communicate
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TABLEII
USECASESSUPPORTEDBYHAREPROTOCOLSTACK N
30 N
19
Sector Usecases N20
Home automation DChoimldo/etilcdserly tracking N29 N18 N
10
Smartmetering
Remotemaintenance/control N N
4 21
Industrial automation LLoogcaisltiacsssettracking management N28 N9 N5
DSmisatrritbmutieotenrianugtomation (smartgrid) N17 N3 N1 N11
N
Citysmartlighting 27 N GW
Smartparking 8
Public infrastructure R R R R
Intelligent buildings 1 2 3 4
Predictive maintenance N7 N2
Natural resources ENnavtuirroanlmdiesnatsatlermsodneitteocrtiinogn N26 N16 N6 N12
Sanmdarfatramgrinicgulture AAgnrimicualltumroenmitoorniintgoring N25 N15 N N13 N22
Silostockmonitoring 14
N
N 23
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(i.e., via single-hop communications) with any node of the
network through unicast and/or broadcast messages as well (a) Multi-hop LPWANwith agateway (GW)and30stations (N1−
as to redirect gathered data from the WSN to other networks N30)deployed in4rings(R1−R4).
or the Internet.
Conversely, STAs can take advantage of their neighbors to
create multi-hop paths over which data is transmitted to the
GW bymeansof lowertransmissionpowerlevels. Depending
N
ontheirpositionwithinthesepaths,STAsareideallyorganized 4
N
intorings,asshowninFigure2.Thenumberofhopstoreach 5
N
the GW determine the ring number (i.e., STAs from ring 2 1 N2 N
6
need two hops to reach the GW).
Each uplink data transmission phase (consisting of one or GW N3
R
1
moretransmissionwindows)beginswithabeaconsignalfrom obstacle R2
the GW. Transmission windowsare in turn virtually split into N7 R3 R
4
as many TDMA slots as networkrings, so that STAs are only N9
N
active during their own slot (for transmitting data) and the 8
previousone belongingto their children3 (for receivingdata). N
10
The first slot is allocated to the highest ring and the
rest are scheduled consecutively. Data received by STAs is
aggregated to that generated by themselves, and finally sent
(b) Multi-hopLPWANaffectedbyanobstacle,withagateway(GW)
tothecorrespondingparentattheminimumpowerlevelwhich and10stations (N1−N10)deployed in4rings(R1−R4).
ensures reliable communications. This process is repeated as
Fig.2. Networktopologyoftypical LPWANs
many times as rings the network has.
The correct reception of data transmissions at the GW is
acknowledgedwith abroadcastmessage,sothatSTAsarenot way in order to reduce the energy consumption.
onlyawareoftheirownend-to-endreliability,butalsoofthose
STAs in the same path to the GW. These acknowledgment
IV. PROTOCOL STACK
beacons, together with the information obtained from their
The main features of the HARE protocol stack are shown
adjacent nodes, allow STAs to decide whether they should
in Table III; a complete description of them is offered next.
remain awake to perform retransmissions of lost network
packets.
Network association (also started by a beacon) remains A. PHY layer
stable until a change in the topology is detected or the HARE protocol stack is intended to be used over any
mechanism is reset by the GW. Nevertheless, the agreed wireless PHY layer fulfilling a minimum set of functions;
transmission power between adjacentnodesin the association namely, availability of different operational states both in
phase is constantly monitored and adjusted in a decentralized the microprocessor (processing, low power mode) and in the
radiomodule(receiving,transmitting,and sleeping),selection
3ChildrenreferstoallSTAsofanadjacenthigherringfromwhichanSTA of different transmission levels in the radio transceiver, and
receives packets. Similarly,parentreferstothatSTAfromanadjacent lower
ability to execute low level tasks required by typical shared
ring to which an STA transmits its own packets (after aggregating the ones
fromitschildren) initswaytotheGW. medium access techniques.
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TABLEIII
data aggregation mechanisms.
MAINFEATURESOFHAREPROTOCOLSTACK
Even though STAs have predeterminedactive periods, they
Layer Features can go to sleep even earlier in the transmitting (TX) time
End-to-endACK
period if their parent has acknowledged all their data, or in
Poisoningmechanism
Transport Transmissionwindows the receiving (RX) time period after having received all data
Distributed caching from their children.
Addressingsystem
Network Association
Routing transmission window
Beaconing system
Wakeuppatterns RX Time
Link Datatransmission,aggregation &segmentation awake Period awake
Powerregulation mechanism GW
Physical Hardwaredependant
RX Time TX Time
sleep Period Period sleep
STAs from ring 1
B. Link layer RX Time TX Time
sleep Period Period sleep
TheMAClayerisacombinationofatimedivisionmultiple
STAs from ring 2
access (TDMA) scheme, where time slot durationis managed
TX Time
by the GW, and an underlying carrier sense multiple access sleep Period sleep
with collision avoidance (CSMA/CA) technique with packet STAs from ring 3
acknowledgment (ACK), performed by the group of STAs
allocated into each generated time slot. At this point it is Fig. 4. Example of a staggered wakeup pattern in a 3-ring LPWAN
performinguplinkcommunications.
worth noting that HARE is not only limited to CSMA-based
accesstechniques,butalsocanproperlyworkwithotherMAC
3) Data transmission, aggregation, and segmentation:
protocols for WSNs [19].
Downlink communications are generally executed through
1) Beaconing system: The designed beaconing system has
broadcast messages from the GW. Conversely, uplink com-
a double function: synchronizing the network devices and
munications are unicast and follow a multi-hop route.
scheduling the different actions to be performed. Two types
The staggered wakeup pattern fits here perfectly with the
of beacons are used for this purpose: primary and secondary
approach of data aggregation in WSN. Thus nodes attach
beacons (see Figure 3).
their own data to that received from their children and all
Both beacons include a timestamp, the time until the
the information is jointly sent to the next hop (i.e., parent). If
next primary beacon, and the next action to be taken by
the total amount of data aggregated by an STA exceeds the
the network: for instance, an association phase (network
maximum payload supported by the hardware, it is split into
association primary beacon), or an uplink data transmission
segments4 sent consecutively.
phase (data primary beacon). Secondary beacons include the
A selective ACK mechanism has been developed, so that
same information as primary ones, and are used to guarantee
beforetheendoftheallocatedtimeslot,thereceiverexplicitly
information redundancy for already associated STAs as well
lists which segments in a stream coming from the same child
as to accelerate network discovery for non-associated ones.
are acknowledged. Upper layers are therefore responsible for
However, no action is performed by STAs after a secondary
making the sender retransmit only the missing segments in
beacon.
successive transmission windows.
Time between two consecutive primary beacons and two
consecutive secondary beacons is defined as T and T , 4) Power regulationmechanism: The selection of the min-
p s
respectively. Where T = (k +1)·T , being k the number imum suitable transmission power level for outgoing packets
p s s s
ofsecondarybeaconstransmittedaftereveryprimary beacon. ismanagedthroughamechanismbasedonthereceivedsignal
2) Wakeup patterns: A wakeup pattern is a set of instruc- strengthindicator(RSSI).Forthispurpose,asafetymarginfor
tions generated by the GW which define the wakeup plan reliablecommunicationsis definedby RSSImin andRSSImax.
of its associated STAs over time periods. With the goal of If a node is transmitting data packets (ACKs) to its parent
minimizing the time STAs remain active (and, consequently, (child) at a power level making the received RSSI higher
their energy consumption), two different wakeup patterns than RSSImax, it will be asked to decrease it for the next
controlledbytheGWareproposedaccordingtothenetwork’s transmission. Similarly, if the received RSSI is lower than
traffic flow [20]. RSSImin, it will be asked to increase it.
The periodic wakeup pattern is suitable for listening to Power regulation requests are included in an RSSI control
broadcastdownlinkcommunicationsfromtheGW,asitmakes field of data packet and ACK headers. Possible values of this
all STAs wake up at the same time. On the other hand, field are: increase, keep, and decrease. Once computed the
uplink communications follow a staggered wakeup pattern, requestsfromparentandchildren,theSTAdetermineswhether
which allocates differentactive periods to nodes belonging to
adjacent rings with partial overlapping (as shown in Figure 4The amount of data aggregated by an STA (from itself and from its
children) is called packet. If this packet is split into different parts, each
4). Apart from reducing time STAs are awake during uplink
one of these parts is called segment. In case both terms can be indistinctly
communications,thismethodfacilitatestheimplementationof used,thecurrentarticle willusepacket.
5
T = (k+1)·T
p s s
T
#1 s #2 #3 #4
Network association phase Secondary beacons STA association phase UL data transmission phase Secondary beacons STA association phase UL data transmission phase Secondary beacons Network association phase ...
Network association Data Data Network association
primary beacon primary beacon primary beacon primary beacon
Fig.3. HAREbeaconing systemconsisting ofnetworkassociation primarybeacons,dataprimarybeacons,andsecondarybeacons.
and how to regulate its own power level depending on the in paths autonomously, but all subsequent data transmissions
following considerations: are addressed to the GW, directly or through other STAs.
• If one or more STAs ask for a higher value, increase the Conversely, the GW can make use of its greater transmission
power level. power to periodically send broadcast messages to all network
• If allSTAs ask for a reduction,decreasethe powerlevel. STAs, or send unicast messages to single STAs.
• Otherwise, keep the current power level. 1) Addressing system: The addressing system is managed
In addition, if an STA needs to retransmit a packet to by the GW, which allocates a uniquenetwork addressto each
its parent, it will also increase the power level in each node during the association process. Nodes will maintain the
new transmission window. Regarding the association process, samenetworkaddressaslongastheydonotleavethenetwork.
wheneveranSTAlistenstoadiscoveryrequest,itwillanswer AdynamicrecordmatchingtheMACandthenetworkaddress
at maximum power. The STA selected as parent will keep of all STAs is stored in the GW. The size of the network
the maximum power level at the beginning and regulate it addressisconfigurableanditsvaluedeterminestheaddressing
following the previously described procedure. Instead, those range.
STAs not selected as parents will set their power back to the 2) Association: To cope with multiple association requests
level they had before answering to the discovery request. in a short period of time, the system is able to admit new
STAs through two different mechanisms: an active, global,
Consequently, the main advantages of using such a MAC scheduled one, called network association mechanism; and a
layer scheme are: passive, singular one, called STA association mechanism.
• Clock synchronization is inherent to TDMA, with nodes • Network association mechanism
being periodically set in time by means of beacons.
The network association mechanism allows a large
• Groups of nodes have their time slots clearly allocated, amount of STAs to associate to the network in a short
andcollisionswithingroupsaresensiblyreducedoreven
period of time. Once the GW is activated, or after a pre-
avoided by using CSMA/CA.
determined number of primary beacons (N ), the GW
pr
• Network overall lifetime is increased by putting nodes broadcasts a network association primary beacon.
in non-active modes for most of the time and only
Depending on the RSSI value received in the network
periodically waking up to check for activity.
associationprimarybeaconaswellassomeotherconfig-
• Association and routing mechanisms are also fit for this uration parameters, STAs determine their turn to initiate
scheme,sothatintermediateandalreadyassociatednodes
the association process (generally, the greater the RSSI
do not have to constantly listen to hypothetical network
received, the earlier association turn is selected).
discovery requests.
STAs then follow with a discovery message sent via
• The scheme is also suitable for uplink data aggregation. broadcast, which is responded by the GW and all the
• Changes in the network configuration or even new already associated STAs, provided they are within the
firmware can be easily distributed in a coordinated man-
coveragerange. The process of selecting the best path to
ner.
reach the GW is detailed in the Routing subsection.
Once the routing mechanism is completed, the GW
C. Network layer notifiesthejoiningofnewSTAsbymeansofa summary
Network communications follow a centralized scheme, broadcast message sent immediately after every associa-
where the GW adopts the main role and assumes the respon- tion turn.
sibility of managing network associations, delivering network • STA association mechanism
addresses, and periodically notifying the start of new routing The STA association mechanism provides a solution to
processes. those specific nodes that (i) have not found a path to
STAs adopt a subordinated role waiting for orders coming the GW during the network association mechanism, (ii)
fromtheGW.Intheroutingprocess,theyorganizethemselves have been powered on between two consecutive network
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associationprimarybeacons,or(iii)havesimplysuffered GW
routing problems in their path to the GW.
R R R R
aTshsioscmiaeticohnanoinsme,fwoliltohwtshethseinsgamleeepxactetpertinonasththaet ntheetwreoriks N2 N1 1 2 3 4
N N N
only one association turn located immediately after each 9 4 3 N
8
dataprimarybeaconto beusedbynon-associatedSTAs.
Inactive or erratic STAs are removedfrom the network and N10 N5 N X N7 N14
the GW’s routing table to create, if necessary, new routing 6
paths that ensure correct packet reception from remaining
N
network STAs. Disassociations can be controlled by the GW 11 N N13
12
through the disassociation mechanism or by the STAs them-
selves through the self-disassociation mechanism:
Fig.6. Network topology ofthe multi-hop LPWANfrom Figure 5,witha
• Disassociation mechanism gateway(GW)and14stations (N1−N14)deployed in4rings(R1−R4).
The GW removes an STA from the network if not re-
ceiving any data packet during a pre-determined number
of consecutive primary beacons(Npd). A roster with the GW. As long as the STA is associated to the network, it
latest disassociated STAs is included in every primary uses the same routing path, which is only recomputed after
beacon.This informationis not only useful for malfunc- an internal or external (i.e., from its parent) failure. Indeed,
tioningSTAs, whichcanmakeimmediateuseofthe STA no new routing process is initiated unless it is part of a new
associationmechanism,butalsofortheirparents,asthey network association mechanism.
cancheckthecurrentstate oftheirchildren.Hence,ifall
its children became disassociated, a parent would go to
D. Transport layer
sleep during the RX time period allocated to its ring.
• Self-disassociation mechanism Reliable end-to-end communications from the STAs to the
Thegoalofthismechanismistoavoidrepetitiveassocia- GW, where retransmissions are only executed when needed
tionrequestsandotherenergyconsumingproceduresthat and by the minimum number of involved STAs, are achieved
could make STAs run outof batterywhen no connection in HARE by using the following mechanisms:
with the GW is possible. All STAs have a timer that is 1) End-to-end ACK: According to the staggered wakeup
activatedafterbeingswitchedonorwhenreceivingapri- pattern, STAs from ring 1 are the last ones to access to the
mary beacon. From that moment on, if an STA does not channel and transmit their information. Once compared the
receive any other beacon during a predetermined period data sources with the expected uplink traffic, the GW emits
(T ), it turnsitself off. Thus the STA is considereddead a broadcast message called end-to-end ACK (e2e ACK) with
d
and it will need to be reactivated by manual procedures. a list of acknowledged STAs. Figure 5 shows the e2e ACK
3) Routing: The routing protocol has been designed as an operationattheendofeverytransmissionwindow.Apartfrom
intrinsic part of the association process. Thus, according to beingsimple,quickandsimultaneouslylistenedbyallnetwork
the responses to the discovery message coming from other elements, end-to-end ACKs allow STAs to evaluate the state
nodes, each STA determines which candidate is the best one of their path to the GW and act consequently.
to become its parent; i.e., the one with the minimum S value 2) Poisoningmechanism: Thepoisoningmechanismidenti-
from: fieswhichspecificnodesexperiencecommunicationproblems
in their path to the GW, so that they can perform subsequent
S =a1·(PTXmax −RSSITX)+a2·(PTXmax −RSSIRX)+a3·rr+etraa4ns·mc,iss(i1o)ns. Nodes having problems with their children
transmit packets with the poison flag activated. An STA is
where P is the maximum transmission power of the
TXmax considered poisoned if, before transmitting an outgoing data
transceiver (in dBm), RSSI is the RSSI received at the
TX
packet, one of the following conditions is satisfied:
candidate (in dBm), RSSI is the RSSI received at the STA
RX
itself (in dBm), r is the ring to which the candidate belongs, • The STA is part of a poisoned path; i.e., it has received
and c is the current number of candidate’s children. The a oneormorepacketswiththepoisonflagactivatedduring
weights are attached to every primary beacon, and can be the current transmission window.
tuned by the GW according to environmentrequirements. • The STA has not received any data packet from one or
Once computed the best parent, the STA sends it a specific more of its children.
request. This request will be forwarded by the parent through • TheSTAhasnotreceivedalltheexpectedsegmentsfrom
its own path until reaching the GW, which will send a packet one or more of its children.
via broadcast confirming the association and providing the In Figure 6, node N3 activates its poison flag after not
STAwithitsnewaddress.Thisway,boththenewlyassociated receivingdata fromits child N6. In its way to the GW, a data
STA and its parent are informed of the establishment of the packet from N3 poisons its next hop: N1. Therefore, nodes
new path. N6, N3, and N1 form a poisoned path, as shown in Figure 7.
When the association process is finished, the STA exactly 3) Transmission windows: A number of transmission win-
knows the next hop its messages must follow to reach the dows (w) with their corresponding e2e ACK are included in
7
transmission window #1 transmission window #2
RX A TX RX A TX
Time C e2e Time C e2e
awake Slot K ACK awake Slot K ACK awake
GW
RX A TX A RX RX A TX A RX
Time C Time C e2e Time C Time C e2e
sleep Slot K Slot K ACK sleep Slot K Slot K ACK sleep
N
1 (ring 1)
RX A TX A RX RX A TX A RX
Time C Time C e2e Time C Time C e2e
sleep Slot K Slot K sleep ACK sleep Slot K Slot K sleep ACK sleep
X N
3 (ring 2)
RX A TX A RX TX A RX
Time C Time C e2e Time C e2e
sleep Slot K Slot K sleep ACK sleep Slot K sleep ACK sleep
N
6 (ring 3)
TX A RX
Time C e2e
sleep Slot K sleep ACK sleep
N
correct transmission Unicast transmission Broadcast transmission 12 (ring 4)
Xincorrect transmission Unicast reception Broadcast reception
poisoned transmission
Fig.5. Uplinkdatatransmissionphaseinamulti-hopLPWANrunningHAREprotocolstackwiththenetworktopologyfromFigure6.Notethecommunication
problemsinthefirsttransmissionwindow between nodesN6 andN3.
GW
YES NO
R R R R Poisoned?
1 2 3 4
N2 N1 Stay awake
N N N
9 4 poisoned path 3 N8 YES abAcylk lm nsoyew gpmlaeredengntets?d NO
N10 N5 N N7 N14 Go to sleep YES Appeared in the NO
6 e2e ACK?
N Stay awake
11 N
N12 13 YES one sMegomree tnhta sne nt in NO
the last transmission
window?
Stay awake Go to sleep
Fig.7. StateofthenetworkfromFigure6afterthecorrespondinge2eACK.
Note the poisoned path passing through nodes N6, N3, and N1. Together *Retransmit segments not yet
acknowledged by my parent
with the GW, these nodes (colored in red) stay awake during the second
transmissionwindow.Therestofnodes(coloredingreen)gotosleepasthey
Fig.8. STA’sdecisionflowcharttostayawakeorgotosleepbeforethestart
arenotinvolved inthenewtransmissionprocess.
ofanewtransmissionwindow.
each uplink data transmission phase to ensure correct packet Toalleviatethisproblem,a distributedcachingsystem isused
reception. Within these windows, not all STAs remain awake, inHARE,sothatparentsacknowledgethecorrectreceptionof
butonlytheonesdirectlyinvolvedinthetransmissionprocess. packets from children and cache their data until it is properly
Beforethestartof anew transmissionwindow,STAs evaluate received in the GW.
whether they shall stay awake or go to sleep. As it can be seen in Figure 7, nodes N12 and N13 can
This decision takes into account if the STA has been go to sleep after the first transmission window, because their
previously poisoned by one of its children as well as several datapacketshavebeenacknowledgedbynodeN6,whichwill
other conditions according to the decision flowchart from cache them in memory together with its own data to be sent
Figure 8. Whenever an STA decides to go to sleep, it will in the next transmission window.
remain in this state until the next primary beacon.
4) Distributed caching: Due to the structure of multi-hop V. TESTBED
networks, lost packets cause expensive retransmissions along Contiki 3.0 OS [22] was the selected RTOS to validate
everyhopofthepathbetweenthesenderandthereceiver[21]. the HARE protocol stack, mainly due to its ability to eas-
8
TABLEV
ily execute multiple processes concurrently and its powerful
CURRENTVALUESOFTHEZOLERTIARE-MOTEDIFFERENT
COOJA network simulator [23]. Apart from simulations, two OPERATIONALSTATES(FROM[26])
different real platforms5 were used for preliminary testing
andoperationalvalidation:MEMSICTMTelosB2.4GHznodes Operational state Current
[24] and ZolertiaTMRE-Mote 868 MHz nodes [25], whose MARicMropCroorcteexs-sMor3 LowPrpoocwesesrinmgo(dCeP(UL)PM) IILCPPMU==103.4mµAA
mmaoHidnAulfReeEaftouprrreoCstooacnroetilkdsiteap3ci.kc0theOdaSsibnweTehnaibcphleroiIngVtrea.rmacmtsedwaisthanthaeddailtrieoandayl RTIadCiCo1M20o0dule TRrSaenlceseemipviiitnntiggng((SR(LTX)X)) ITXIISR=LX=3=901−.192m6µ1AAmA
available upper communication layers of the system (MAC
and Network), regardless the employed hardware. Specific
Secondly, and always over the same node deployment,two
interactionsof HARE with PHY layersof the aforementioned
differentnetworktopologiesweretested:single-hopandmulti-
hardware were separately programmed.
hop.In the first case, all nodeswere directly connectedto the
GW,whileinthesecondcase,STAswerefreetoestablishtheir
TABLEIV
MAINFEATURESOFTHEHARDWAREEMPLOYEDINTHEHARE own routes to the GW with the single limitation of having 5
OPERATIONALVALIDATION children per STA.
Andthirdly,thewholesystemwasalteredwith thearbitrar-
Platform MEMSIC TelosB Zolertia RE-Mote
Microprocessor TIMSP430 ARMCortex-M3 ily introduction of a certain error probability when sending
RadioModule TICC2420 TICC1200 both application packets and their corresponding ACKs (it
FrequencyBand 2.4GHz 868/915MHz is worth noting here that neither messages implied in the
association process nor statistics packets were affected by
Performance evaluation of HARE protocol stack was per-
arbitrary generated errors). Errors were generated through
formedina testbedlocatedonthe2ndfloor,rightwingofthe
a uniformly distributed random variable according to mean
TangerbuildingatUPFfacilities6.Thetestbedconsistedof13
error values from Table VI. Before sending a message, STAs
ZolertiaTM RE-Mote nodes (one of them acting as a gateway
computed this value and discarded messages accordingly.For
and connected to a PC) running the HARE protocol stack.
this purpose, four different error configurations were defined.
Alltestswereexecutedconsideringnomobilityandwiththe
The addressing system followed the Rime format [28]
sameSTAs’placement(seeFigure9).AllSTAswerepowered
consisting of two 8-bit numbers. Similarly to IP addressing,
byan800mAhbatteryexceptthegateway,whichwasperma-
theuseofnetmasksleadsto flexiblesubnettingconfigurations
nentlypoweredbythePC.Resultsweredirectlyobtainedfrom with up to (216−2) STAs. In our particular case, the first 8-
the GW, or thanks to the statistics messages periodically sent
bitnumberidentifiedthenetworkprefixsharedbyalldevices,
by STAs. These messages contain information aboutdifferent
and the second one the host part, whose value for GWs was
metricssuchasthenumberofpacketssentandacknowledged,
0 and for STAs was selected from 1 to 255.
RTT delays, as well as power profiles of microprocessor and
All tests began with a network association primary beacon
radio module.
in which all nodes tried to associate to the network. From
The calculation of total energy consumption (ET) is based then on, the GW emitted a new (network association or data)
on these two power profiles: E and E , for the mi-
µP RADIO primary beacon every T = 3 min. Data primary beacons
p
croprocessor and the radio module, respectively, as shown in
couldaskSTAsforanewapplicationorstatisticspacket.Inall
Equation (2). V is the supply voltage, while t and I are,
DD ourtests,applicationandstatisticspacketsgeneratedbySTAs
respectively, the time and the current corresponding to the contained, respectively, 10 and 20 bytes of net information7.
operationalstates of the microprocessorand the radio module
of the employed hardware, whose values are summarized in
VI. RESULTS
TableV.NoticethattheI valueoftheradiomoduledepends
TX
on the transmission power level. A. Association process
To show the performance and the coherence of the pro-
E = E +E
T µP RADIO
posedassociationprocessanditsunderlyingrouting,allSTAs
E = V ·(t ·I +t ·I )
µP DD CPU CPU LPM LPM were forced to repeatedly renew every two primary beacons
E = V ·(t ·I +t ·I +t ·I ) (2)
RADIO DD RX RX TX TX SL SL
7Implementation ofIEEE802.15.4 in Contiki OS increases the minimum
In addition, different network configurations were applied. length ofanytransmitted packet upto43bytes after including headers and,
Firstly, two different MAC layers inherent to Contiki OS ifnecessary,applying padding
weretested:NULLMACandX-MAC[27].WhileNULLMAC
maintains STAs continuously awake during active periods,
TABLEVI
X-MAC combines the introduction of sleeping periods for DEFINITIONOFERRORCONFIGURATIONSFORTHEPROPOSEDTESTBED
receivers with the use of strobed preambles for senders.
Error Config. DataError ACK Error
5SeeContikimainwebpage(http://www.contiki-os.org/)foracomprehen- E0/0 0% 0%
sivetableofhardwarecompatible withContiki3.0OS E10/5 10% 5%
6UPFcommunication campusmainwebsite: E20/10 20% 10%
https://www.upf.edu/campus/en/comunicacio/tanger.html E30/15 30% 15%
9
Fig.9. Nodes’placement atUPFfacilities andassociation diagram wheneachSTAadmitsupto5children.
(N = 2) their association to the network and compute their for application packets. To send their packets, STAs had 5
pr
best parent according to (1) with the following parameters: available transmission windows (w =5).
a1 = a2 = 10, a3 = 1, and a4 = 5. In addition, the number TheresultswiththeobtainedPDRinalltheseconfigurations
of children per STA was artificially limited to 5 to guarantee are compiled in Figure 11. After 5 transmission windows,
multiple paths towards the GW. Interspersed data primary PDR is in any configuration above 95%, and it even achieves
beaconswereusedtocheckthereliabilityofroutingpathsand values above 90% after 3 and 4 transmission windows when
to allow not yet associated STAs to have another opportunity using X-MAC and NULLMAC, respectively. In this case,
to join the network. NULLMAC specially suffers from the effect of collisions,
The selected underlying MAC for all STAs was X-MAC due to the backoff implementation8 and the higher number
and no error was introduced in the network (i.e., E0/0 error of concurrently active STAs compared to X-MAC.
configuration was used). Under these premises, and after 200 Another insight from obtained results is how multi-hop
repetitions,anaveragenumberof11.97STAswereassociated topologyoutperformssingle-hopinallpossibleconfigurations
to the network after the data primary beacon of the given except when using X-MAC with E30/15. Again, the inherent
sequence (i.e., 99.75%of success). As for the packetdelivery reduction of concurrently active STAs competing for the
ratio (PDR), it achieved 100% in all the associated STAs. channel during the same time period (in this case, due to the
Routing tables compiled by the GW were processed and allocation of STAs to different slots according to their ring)
adapted to graphical representation in Figure 9, where line’s proves beneficial for system’s reliability.
thicknessis proportionalto link’sfrequencyappearance.Pref- The network’s ability to properly deliver data packets to
erenceofSTAsforestablishingpathswithcloserneighboursin its destination was also analyzed by computing the quotient
theirwaytotheGWbecomesevident,justliketheimportance between the total number of packets sent by STAs and
of clear paths (i.e., without obstacles) such as the formed by those properly received by the GW. As shown in Figure 10,
the corridor walls. multi-hop schemes still have better performance than single-
hop in low-error configurations. On the contrary, in highly
The limitation of 5 children can be clearly appreciated in
unfavorable channels, parents usually do not receive all their
STAs #6, #8, #9, #10, and #11 being almost always di-
expected payloads at once, so that they tend to send several
rectlyconnectedtotheGWinring1.TherestofSTAs(princi-
packets in successive transmission windows with only partial
pally#7)couldonlyaccesstothatringwhencircumstantially
information.
havingbetterchannelconditionsthantheaforementionedones.
C. Energy consumption
B. Reliability The effect of this interdependence can also be observed
in total energy consumption (Figure 12), computed after 20
OnceallSTAsareassociatedtothenetworkandtheirpaths
transmitted beacons (i.e., a 1-hour test). Important savings
to the GW properly established, the next goal is to analyze
(up to 15%)9 can be achieved when using multi-hop schemes
the reliability and the cost (in terms of energy consumption)
of sending data. To do that, the GW was programmed to 8MainvaluesoftheNULLMACCSMA/CAdefaultbackoffimplementation
send 20 beacons with the following sequence: beacon #1 inContikiOS:minimumvalueofthebackoffexponent(macMinBE=0),
was a network association primary beacon, beacons #10 maximumvalueofthebackoffexponent(macMaxBE=4),andmaximum
numberofbackoff attempts (macMaxCSMABackoffs=5).
and #20 were data primary beacons asking for statistics 9Frompreviousstudies[15],webelievethatinlargernetworks,thesegains
packets;therestofbeaconsweredataprimarybeaconsasking willbemuchhigher.
10
TABLEVII
AVERAGELIFETIMEOFAN800MAHBATTERYINTHEPROPOSEDTESTBED
Batterylifetime(days)
Tp=3min Tp=1h Tp=4h
E0/0 2.37 47.35 187.87
AC Single-hop EE2100//150 22..2118 4443..1496 117752..4513
NULLM Multi-hop EEEE3210000////110550 2222....16424344 45442284....75660106 121160979827....51723797
E30/15 2.07 41.35 164.23
E0/0 4.46 88.75 349.64 (a) Logical network topology after (b) Logical network topology from
Single-hop E10/5 4.27 85.00 335.07 thenetworkassociationprimarybea- beacon #15untilbeacon#50
C E20/10 4.52 89.99 354.46 con
X-MA Multi-hop EEEE3210000////110550 4554....52056983 1199000051....71109074 343351957374....51285615 Fig.13. Networktopologybeforeandaftershutdownofnodes#1and#4.
E30/15 4.39 87.38 344.31
Once finished the initial network association mechanism,
the network was organized in four rings, as shown in Figure
13(a). After beacon #4 (A), STA #1 was switched off, but it
did not imply further problems to the network, as this STA
with respect to single-hop ones in low-error configurations
did not have any children. However, after beacon #12 (B),
(E0/0 − E20/10) and similar or slightly worse values (less
STA #4 was also switched off, and it forced the network to
than 4% of extra consumption) in E30/15.
reconfigureitself. The path to the GW of STAs #2,#3 and #5
Time percentage of STAs’ microprocessor in low power was broken, and they had to look for a new route by using
mode is, in all studied cases, above 97% for X-MAC and the STA association mechanism of successive data primary
99% for NULLMAC, due to the higher number of operations beacons. After beacon #15, all active STAs (i.e., all of them
involvedinthefirstcase.However,theimpactofradiomodule except #1 and #4, which remain off) had a path to the GW
sleeping periods introduced by X-MAC layer reduces total and the network was again stable (see Figure 13(b)).
energyconsumptioninupto50%withrespecttoNULLMAC.
This test was also useful to analyze the performanceof the
In this case, values of energy consumed per bit of payload
proposed power regulation mechanism when setting it with
delivered are confined between 50−65 mJ/bit for X-MAC,
RSSI = −110 dBm and RSSI = −100 dBm. It is
min max
and 105−140 mJ/bit for NULLMAC.
worth noting here that Zolertia RE-Mote devices use up to
As for the battery lifetime, Table VII compilesthe duration 31 different power levels (from −16 dBm to 14 dBm with
in days of the 800mAh battery included in the Zolertia RE- steps of 1 dB [29]) and are programmed by default with the
Mote for the current testbed with T = 3 min, as well as maximum transmission power level.
p
two estimations with T = 1 h and T = 4 h. The temporal Figure 14 shows the clear reduction of transmission power
p p
flexibility of the TDMA-based system employed in HARE in most of the analyzed STAs during 50 primary beacons,
allows this kind of extrapolations, by assuming that, in non- beingthemostsignificantexamplesSTAs#7,#8,#9and#10;
active time periods, both the microprocessor and the radio thenearestonestotheGW. Thisfactresultsinalowerenergy
module remain asleep. consumption,asI =61mA whentransmittingat14dBm,
TX
but almost half (I =39 mA) when doing it at -16 dBm.
TX
The effects of switching off nodes are also visible in the
transmission power, as shown in (A) and (B) from Figure
14. While STA #1 in (A) simply stopped working, nodes
involvedintheshutdownofSTA#4in(B)experiencednotable
D. Resilience against failures changes.Thus,STAs#2,#3and#5disappearedalongwiththe
shutdown of STA #4. However, they became associated again
between beacons #13 and #15 with maximum transmission
To prove the adaptability and resilience of the routing power. For its part, when STA #6 became parent of STA #2,
protocol implemented in HARE, the network was subjected it set the maximum power level to establish connection with
tothedeliberateshutdownoftwoofitsSTAs. Inthisway,the its new child.
GW was programmed to send 50 beacons with the following Lastly, the power regulation mechanism proved its good
sequence: beacon #1 was a network association primary performance against channel alterations as shown in area
beacon, beacons multiple of 10 were data primary beacons (C). In this case, and due to the test execution on a real
asking for statistics packets; the rest of beacons were data scenario, the presence of people in the floor corridor may
primary beacons asking for application packets. In addition, havedisturbedchannelconditions.Toovercomethissituation,
the disassociation mechanism was programmedin the GW to someSTAs(#9,#10,#11and#12)selectedtemporarilygreater
remove an STA from the network if not receiving any data transmissionpowerlevelsthatwerereestablishedoncefinished
packet during one primary beacon (N =1). the detected channel issues.
pd