From IEEE 802.15.4 to IEEE 802.15.4e: A step towards the Internet of Things

Abstract

Wireless Sensor and Actuator Networks (WSANs) are expected to have a key role in the realization of the future Internet of Things that will connect to the Internet any kind of devices, living beings, and things. A number of standards have been released over the last years to support their development and encourage interoperability. In addition IETF has defined a set of protocols to allow the integration of sensor and actuator devices into the Internet. In this chapterwefocus on the 802.15.4e, released by IEEE in 2012 to enhance and add functionality to the previous 802.15.4 standard, so as to address the emerging needs of embedded industrial applications. We describe how the limitations of the 802.15.4 standard have been overcome by the new standard, and we also show some simulation results to better highlight this point.

Full text

UNCORRECTED PROOF
From IEEE 802.15.4 to IEEE 802.15.4e: A Step
Towards the Internet of Things
Domenico De Guglielmo, Giuseppe Anastasi and Alessio Seghetti
Abstract Wireless Sensor and Actuator Networks (WSANs) are expected to have
1
a key role in the realization of the future Internet of Things that will connect to the2
Internet any kind of devices, living beings, and things. A number of standards have3
been released over the last years to support their development and encourage inter-4
operability. In addition IETF has defined a set of protocols to allow the integration of5
sensor and actuator devices into the Internet. In this chapter we focus on the 802.15.4e,6
released by IEEE in 2012 to enhance and add functionality to the previous 802.15.47
standard, so as to address the emerging needs of embedded industrial applications.8
We describe how the limitations of the 802.15.4 standard have been overcome by9
the new standard, and we also show some simulation results to better highlight this10
point.11
1 Introduction12
In the future Internet of Things (IoT) a very large number of real-life objects will be13
connected to the Internet, generating and consuming information. IoT elements will14
no longer be only computers and personal communication devices, as in the current15
Internet, but all kinds of devices (e.g., cars, robots, machine tools), living beings16
(persons, animals, and plants) and things (e.g., garments, food, drugs, etc.). A key17
role in the realization of the IoT paradigm will be played by wireless sensor/actuator18
D. De Guglielmo (B
)·G. Anastasi ·A. Seghetti
Department of Information Engineering,University of Pisa, Pisa, Italy
e-mail: domenico.deguglielmo@iet.unipi.it
G. Anastasi
e-mail: giuseppe.anastasi@iet.unipi.it
A. Seghetti
e-mail: seghetti@iet.unipi.it
S.GaglioandG.LoRe(eds.),Advances onto the Internet of Things,1
Advances in Intelligent Systems and Computing 260, DOI: 10.1007/978-3-319-03992-3_10,
© Springer International Publishing Switzerland 2014
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2 D. De Guglielmo et al.
networks (WSANs) that will behave as a sort of digital skin, providing a virtual layer19
through which any computational system can interact with the physical world [1,2].20
A WSAN consists of a number of sensor and actuator devices deployed over a21
geographical area and interconnected through wireless links. Sensor devices gather22
information from the physical environment or a monitored system (e.g., temperature,23
pressure, vibrations), optionally perform a preliminary local processing of acquired24
information, and send (raw or processed) data to a controller. Based on the received25
information, the controller performs appropriate actions, through actuator devices,26
to change the behavior of the physical environment or the monitored system.27
WSANs are already used in many application domains, ranging from traditional28
environmental monitoring and location/tracking applications to more constrained29
applications such as those in the industrial [3] and healthcare domain [4]. In the30
industrial field WSAN applications include factory automation [5], distributed and31
process control [6–8], real-time monitoring of machinery health, detection of liq-32
uid/gas leakage, radiation check [9] and so on. In the healthcare domain WSANs33
have been considered for the monitoring of physiological data in chronicle patients34
and transparent interaction with the healthcare system.35
In many application domains energy efficiency is usually the main concern in the36
design of a WSAN. This is because sensor/actuator devices are typically powered by37
batteries with a limited energy budget and their replacement can be expensive or, even,38
impossible [10]. However, in some relevant application domains additional require-39
ments need to be considered, such as timeliness, reliability, robustness, scalability,40
and flexibility [3,11]. Reliability and timeliness are very critical issues for industrial41
and healthcare applications. If data packets are not delivered to the final destination,42
correctly and within a pre-defined deadline, the correct behavior of the system (e.g.,43
the timely detection of a critical event) may be compromised. The maximum allowed44
latency depends on the specific application. Typical values ranges from tens of mil-45
liseconds (e.g., for discrete manufacturing and factory automation), to seconds (e.g.,46
for process control), and even minutes (e.g., for asset monitoring) [11].47
In recent years many standards have been issued by international bodies to support48
the development of WSANs in different application domains. They include IEEE49
802.15.4 [12], ZigBee [13], Bluetooth [14], WirelessHART [15] and ISA-100.11a50
[16]. At the same time, the Internet Engineering Task Force (IETF) has defined51
a number of protocols to facilitate the integration of smart objects (i.e., sensor and52
actuator devices) into the Internet. The most important of them are the IPv6 over Low53
power WPAN (6LoWPAN) [17] adaptation layer protocol that allows the integration54
of smart objects into the Internet, the Routing Protocol for Low power and Lossy55
networks (RPL) [18], and the Constrained Application Protocol (CoAP) [19] that56
enables web applications on smart objects.57
In this chapter we focus on the IEEE 802.15.4 standard [12] that defines the58
physical and Medium Access Control (MAC) layers of the OSI reference model59
and is complemented by the ZigBee specifications [13] covering the networking and60
application layers. The 802.15.4 standard was originally conceived for applications61
without special requirements in terms of latency, reliability and scalability. In order62
to overcome these limitations, in 2008 the IEEE set up a Working Group (named63
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802.15e WG) with the aim of enhancing and adding functionality to the 802.15.464
MAC, so as to address the emerging needs of embedded industrial applications [20].65
The final result was the release of the 802.15.4e standard in 2012. In the following66
sections, after emphasizing the limitations and deficiencies of the 802.15.4 standard,67
we will show how they have been overcome in the new standard. Specifically, we will68
describe the new access modes defined by 802.15.4e, with special emphasis on the69
Time Slotted Channel Hopping (TSCH) mode. We will also present some simulation70
results to better highlight the performance limitations of 802.15.4 and show that they71
are overcome by 802.15.4e.72
The remainder of this chapter is organized as follows. Section 2describes73
the 802.15.4 standard. Section 3highlights its main limitations and deficiencies.74
Section 4describes the new functionalities provided by the 802.15.4e standard.75
Section 5compares the performance of 802.15.4 and 802.15.4e in a simple scenario76
through simulation. Finally, Sect. 6concludes the chapter.77
2 IEEE 802.15.4 Standard78
IEEE 802.15.4 [12] is a standard for low-rate, low-power, and low-cost Personal Area79
Networks (PANs). A PAN is formed by one PAN coordinator which is in charge of80
managing the whole network, and, optionally, by one or more coordinators that are81
responsible for a subset of nodes in the network. Regular nodes must associate with82
a (PAN) coordinator in order to communicate. The supported network topologies are83
star (single-hop), cluster-tree and mesh (multi-hop).84
The standard defines two different channel access methods: a beacon enabled85
mode and a non-beacon enabled mode. The beacon enabled mode provides a power86
management mechanism based on a duty cycle. It uses a superframe structure (see87
Fig. 1) which is bounded by beacons, i.e., special synchronization frames generated88
periodically by the coordinator node(s). The time between two consecutive beacons is89
called Beacon Interval (BI), and is defined through the Beacon Order (BO) parameter90
(BI =15.36 ·2BO ms, with 0≤BO≤14).1Each superframe consists of an active91
period and an inactive period. In the active period nodes communicate with the92
coordinator they are associated with, while during the inactive period they enter a low93
power state to save energy. The active period is denoted as Superframe Duration (SD)94
and its size is defined by the Superframe Order (SO) parameter (SD =15.36·2SO ms,95
with 0≤SO≤BO≤14). It can be further divided into a Contention Access Period96
(CAP) and a Contention Free Period (CFP). During the CAP a slotted CSMA-CA97
algorithm is used for channel access, while in the CFP communication occurs in98
a Time Division Multiple Access (TDMA) style by using a number of Guaranteed99
Time Slots (GTSs), pre-assigned to individual nodes. In the non-beacon enabled mode100
there is no superframe, nodes are always active (energy conservation is delegated to101
1Throughout the chapter we assume that the sensor network operates in the 2.4 GHz frequency
band.
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B&W IN PRINT
Fig. 1 IEEE 802.15.4 Superframe Structure
the layers above the MAC protocol) and use an unslotted CSMA-CA algorithm for102
channel access.103
2.1 CSMA-CA Algorithm104
The CSMA-CA algorithm is used in both the beacon enabled mode (during the105
CAP portion of the active period) and the non-beacon enabled mode. In the beacon-106
enabled mode a slotted scheme is used—i.e., all operations are aligned to backoff107
period slots (whose duration is 320 µs)—while in the non-beacon enabled mode108
there is no such alignment.109
Upon receiving a data frame to be transmitted, the CSMA-CA algorithm performs110
the following steps.111
1. A set of state variables is initialized, i.e., the contention window size (CW =112
2, only for the slotted variant), the number of backoff stages carried out for the113
on-going transmission (NB =0), and the backoff exponent (BE =macMinBE).114
2. A random backoff time, uniformly distributed in the range [0,2BE−1]·320 µs, is115
generated and used to initialize a backoff timer. In the beacon-enabled mode, the116
starting time of the backoff timer is aligned with the beginning of the next backoff117
slot. In addition, if the backoff time is larger than the residual CAP duration, the118
backoff timer is stopped at the end of the CAP and resumed at the beginning of119
the next superframe. When the backoff timer expires, the algorithm proceeds to120
step 3.121
3. A Clear Channel Assessment (CCA) is performed to check the state of the wireless122
medium.123
(a) If the medium is busy, the state variables are updated as follows: NB =124
NB +1,BE =min(BE +1,macMaxBE)and CW =2 (only for the slotted125
variant). If the number of backoff stages has exceeded the maximum admis-126
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sible value (i.e. NB>mac MaxCS M ABackof f s), the frame is dropped.127
Otherwise, the algorithm falls back to step 2.128
(b) If the medium is free and the access mode is unslotted, the frame is imme-129
diately transmitted.130
(c) If the medium is free and the access mode is slotted, then CW =CW −1. If131
CW =0 then the frame is transmitted.2Otherwise the algorithm falls back132
to step 3 to perform a second CCA.133
It should be noted that, unlike the algorithm used in 802.11 WLANs, the 802.15.4134
slotted CSMA-CA does not guarantee a transmission at the end of the backoff time135
after the channel is found clear. Instead, transmission occurs only if the wireless136
medium is found free for two consecutive CCAs. The complete CSMA-CA algo-137
rithm, both in the slotted and unslotted version, is depicted in Fig. 2.138
The 802.15.4 CSMA-CA algorithm also includes an optional retransmission139
mechanisms for improving reliability. When retransmissions are enabled, the des-140
tination node must send an acknowledgement whenever it correctly receives a data141
frame (the acknowledgement is not sent in case of collision and corrupted frame142
reception). On the sender side, if the acknowledgment is not (correctly) received143
within the pre-defined timeout, a retransmission is scheduled. The frame can be144
re-transmitted up to a maximum number of times, specified by the MAC parameter145
macMaxFrameRetries. Upon exceeding these value, the data frame is rejected and a146
failure notification is sent by the MAC sublayer to the upper layers.147
3 Limitations of IEEE 802.15.4 MAC148
The performance of the 802.15.4 MAC protocol, both in BE mode and NBE mode,149
have been thoroughly investigated in the past. As a result of this extended study, a150
number of limitations and deficiencies have been identified, the main of which are151
discussed below.152
•Unbounded Delay. Since the 802.15.4 MAC protocol, both in BE mode and NBE153
mode, is based on the CSMA-CA algorithm it cannot guarantee any bound on the154
maximum delay experienced by data to reach the final destination. This makes155
802.15.4 unsuitable for time-critical application scenarios where a low and deter-156
ministic delay is required (e.g., industrial and medical applications).157
•Limited communication reliability. The 802.15.4 MAC in BE mode provides a158
very low delivery ratio, even when the number of nodes is not so high which make159
it unsuitable for critical application scenarios. This is mainly due to the random-160
access method (i.e., CSMA-CA algorithm) and the synchronization introduced161
by the periodic Beacon. A similar behavior also occurs in the NBE mode when162
2In the beacon-enabled mode, before starting the frame transmission, the algorithm calculates
whether it is able to complete the operation within the current CAP. If there is not enough time, the
transmission is deferred to the next superframe.
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B&W IN PRINT
Fig. 2 CSMA-CA algorithm
a large number of nodes start transmitting simultaneously (e.g., in event-driven163
applications).164
•No protection against interferences/fading. Interferences and multi-path fading165
are very common phenomena, especially in application scenarios where sen-166
sor/actuator networks are expected to be used. Unlike other wireless network167
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technologies (e.g., Bluetooth [14], ISA 100.11a [16] and WirelessHART [15]),168
the 802.15.4 MAC takes a single-channel approach and has no built-in frequency169
hopping mechanism to protect against interferences and multi-path fading. Hence,170
the network is subject to frequent instabilities and may also collapse. This make171
802.15.4 unsuitable to be used in critical application scenarios (e.g., industrial or172
healthcare applications).173
•Powered relay nodes. The 802.15.4 support both single-hop (star) and multi-hop174
(peer-to-peer) topologies. In principle, the BE mode could be used to form multi-175
hop PAN with a tree topologies where intermediate node do not need to stay active176
all the time. In practice, intermediate relay nodes in 802.15.4 networks (both with177
tree and mesh topologies) need to keep their radio on all the time, which leads to178
a large energy consumption.179
4 IEEE 802.15.4e Standard180
To overcome the limitations of the 802.15.4 standard, emphasized in the previous181
section, the 802.15e Working Group was created by IEEE in 2008 to redesign the182
existing 802.15.4 MAC protocol. The goal was to define a low-power multi-hop183
MAC protocol, capable of addressing the emerging needs of embedded industrial184
applications. The final result was the IEEE 802.15.4e MAC Enhancement Standard185
document [20], approved in 2012. Specifically, the 802.15.4e standard extends the186
previous 802.15.4 standard by introducing two different categories of MAC enhance-187
ments, namely MAC behaviors to support specific application domains and general188
functional improvements that are not tied to any specific application domain. In189
practice, 802.1.5.4e borrows many ideas from existing standards for industrial appli-190
cations (i.e., WirelessHART [15] and ISA 100.11.a [16]), including slotted access,191
shared and dedicated slots, multi-channel communication, and frequency hopping.192
The MAC behavior modes defined by the 802.1.5.4e standard are listed below.193
They will be described in the next section.194
•Radio Frequency Identification Blink (BLINK). intended for applications such as195
item and people identification, location, and tracking;196
•Asynchronous multi-channel adaptation (AMCA). targeted to application domains197
where large deployments are required (e.g., process automation/control, infrastruc-198
ture monitoring, etc.);199
•Deterministic and Synchronous Multi-channel Extension (DSME). aimed to sup-200
port industrial and commercial applications with stringent timeliness and reliability201
requirements;202
•Low Latency Deterministic Network (LLDN). intended for applications requiring203
very low latency requirement (e.g., factory automation, robot control)204
•Time Slotted Channel Hopping (TSCH). targeted to application domains such as205
process automation.206
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The general functional enhancements, not specifically tied to a particular207
application domain, are as follows.208
•Low Energy (LE). This mechanism is intended for applications that can trade209
latency for energy efficiency. It allows a device to operate with a very low duty210
cycle (e.g., 1 % or below), while appearing to be always on to the upper layers. This211
mechanism is extremely important for enabling the Internet of Things paradigms as212
Internet protocols have been designed assuming that hosts are always on. However,213
it may be useful also in other applications scenarios (e.g., event-driven and/or214
infrequent communications, networks with mobile nodes).215
•Information Elements (IE). The concept of IEs was already present in the 802.15.4216
standard. It is an extensible mechanism to exchange information at the MAC217
sublayer.218
•Enhanced Beacons (EB). Extended Beacons are an extension of the 802.15.4219
beacon frames and provide a greater flexibility. They allow to create application-220
specific beacons, by including relevant IEs, and are used in the DSME and TSCH221
modes.222
•Multipurpose Frame. This mechanism provides a flexible frame format that can223
address a number of MAC operations. It is based on IEs.224
•MAC Performance Metrics are a mechanism to provide appropriate feedback on the225
channel quality to the networking and upper layers, so that appropriate decision226
can be taken. For instance the IP protocol running on top of 802.15.4e MAC227
may implement dynamic fragmentation of datagrams depending on the channel228
conditions.229
•Fast Association (FastA). The 802.15.4 association procedure introduces a230
significant delay in order to save energy. For time-critical application latency has231
priority over energy efficiency. The FastA mechanism allows a device to associate232
in a reduced amount of time.233
4.1 802.15.4e MAC Behavior Modes234
In this section we describe the MAC behavior modes that have been introduced in235
the previous section. The description is necessarily brief for the sake of space. The236
reader can refer to [20] for details.237
The Radio Frequency Identification Blink (BLINK) mode is intended for238
application domains such as item/people identification, location, and tracking and239
is, thus, very relevant in the perspective of Internet of Things. Specifically, it allows240
a device to communicate its ID (e.g., a 64-bit source address) to other devices. The241
device can also transmit its alternate address and, optionally, additional data in the242
payload. No prior association is required and no acknowledgement is provided to the243
sending device. The BLINK mode is based on a minimal frame consisting only of244
the header fields that are necessary for its operations. The BLINK frame can be used245
by “transmit only” devices to co-exist within a network, utilizing an Aloha protocol.246
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The Asynchronous multi-channel adaptation (AMCA) mode is targeted to247
application domains where large deployments are required, such as smart utility248
networks, infrastructure monitoring networks, and process control networks. In such249
networks using a single, common, channel for communication may not allow to con-250
nect all the devices in the same PAN. In addition, the variance of channel quality251
is typically large, and link asymmetry may occur between two neighboring devices252
(i.e., a device may be able to transmit to a neighbor but unable to receive from it).253
The AMCA mode relies on asynchronous multi-channel adaptation and can be used254
only in non Beacon-Enabled PANs.255
The Deterministic and Synchronous Multi-channel Extension (DSME) mode is256
intended for the support of industrial applications (e.g., process automation, factory257
automation, smart metering), commercial applications (such as home automation,258
smart building, entertainment) and healthcare applications (e.g., patient monitoring,259
telemedicine). This kind of applications requires low and deterministic latency, high260
reliability, energy efficiency, scalability, flexibility, and robustness [20]. As men-261
tioned in Sect. 2, the 802.15.4 standard provides Guaranteed Time Slots (GTSs).262
However, the GTS mode has a number of limitations. It only includes up to seven263
slots and, thus, it is not able to support large networks. In addition, it relies on a single264
frequency channel. DSME enhances GTS by grouping multiple superframes to form265
a multi-superframe and using multi-channel operation. Like GTS, DSME runs on266
Beacon-enabled PANs. All the devices in the PAN synchronize to multi-superframes267
via beacon frames. A multi-superframe is a cycle of superframes, where each super-268
frame includes the beacon frame, the Contention Access Period, and Contention Free269
Period (i.e., GTS slot). A pair of nodes wakes up at a reserved GTS slot to exchange270
a data frame and an ACK frame. In order to save energy, DSME uses CAP reduc-271
tion, i.e., the Contention Access Period (CAP) is only in the first superframe of the272
multi-superframe, while it is suppressed in subsequent superframes.273
The Low Latency Deterministic Network (LLDN) mode is mainly targeted to274
industrial and commercial applications requiring low and deterministic latency. Typ-275
ical application domains addressed by LLDN include factory automation (e.g., auto-276
motive manufacturing), robots, overhead cranes, portable machine tools, milling277
machines, computer-operated lathes, automated dispensers, cargo, airport logistics,278
automated packaging, conveyors. In this kind of applications typically there are a279
large number of sensors/actuators observing and controlling a system, e.g., a pro-280
duction line or a conveyor belt. In addition, applications have very low requirements281
in terms of latency (transmission of sensor data in 5–50 ms, and low round-trip282
time) [20]. To guarantee stringent latency requirements of target applications LLDN283
only supports the star (i.e., single hop) topology, and uses a superframe, based on284
timeslots, with small packets. Keeping the size of packets (and, hence, timeslots)285
short leads to superframes with short duration (e.g., 10 ms). Obviously, the number286
of timeslots in a superframe determines the number of devices that can access the287
channel. Since the number of devices may very large (there may be more than 100288
devices per PAN coordinator) LLDN allows the PAN coordinator to use multiple289
transceivers on different channels. In the LLDN mode each superframe consists of a290
beacon timeslot,management timeslots (if present), and a number of base timeslots of291
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equal size. Base timeslots include uplink timeslots and bidirectional timeslots. There292
are two categories of base timeslot, namely dedicated and shared group timeslots.293
Dedicated timeslots are assigned to a specific node (owner) that has the exclusive294
access on them, while shared group timeslots are assigned to more than one device.295
The devices use the slotted CSMA-CA algorithm described in Sect.2to contend for296
shared group timeslots. In addition, they use a simple addressing scheme with 8-bit297
addresses in. The LLDN mode includes a Group ACK (GACK) function to reduce298
the bandwidth overhead. GACK is sent by the PAN coordinator in a superframe to299
stimulate the retransmission of failed transmission in uplink timeslots.300
The Time Slotted Channel Hopping (TSCH) mode is mainly intended for the301
support of process automation applications with a particular focus on equipment and302
process monitoring. Typical segments of the TSCH application domain include oil303
and gas industry, food and beverage products, chemical products, pharmaceutical304
products, water/waste water treatments, green energy production, climate control305
[20]. TSCH combines time slotted access, already defined in the IEEE 802.15.4306
MAC protocol, with multi-channel and channel hopping capabilities. Time slotted307
access increases the potential throughput that can be achieved, by eliminating col-308
lision among competing nodes, and provides deterministic latency to applications.309
Multi-channel allows more nodes to exchange their frames at the same time (i.e.,310
in the same time slot), by using different channel offsets. Hence, it increases the311
network capacity. In addition, channel hopping mitigates the effects of interference312
and multipath fading, thus improving the communication reliability. Hence, TSCH313
provides increased network capacity, high reliability and predictable latency, while314
maintaining very low duty cycles (i.e., energy efficiency) thanks to the time slot-315
ted access mode. TSCH is also topology independent as it can be used to form any316
network topology (e.g., star, tree, partial mesh or full mesh). It is particularly well-317
suited for multi-hop networks where frequency hopping allows for efficient use of318
the available resources.319
4.2 Time Slotted Channel Hopping (TSCH) Mode320
Among the various access modes defined by the 802.15.4e standard, TSCH is321
certainly the most complex and interesting one. Hence, in the following we will322
provide a more detailed description of it.323
In the TSCH mode nodes synchronize on a periodic slotframe consisting of a324
number of timeslots. Figure 3shows a slotframe with 4 timeslots. Each timeslot325
allows a node to send a maximum-size data frame and receive the related acknowl-326
edgement (Fig. 4). If the acknowledgement is not received within a predefined time-327
out, the retransmission of the data frame is deferred to the next time slot assigned to328
the same (sender-destination) couple of nodes.329
One of the main characteristics of TSCH is the multi-channel support, based on330
channel hopping. In principle 16 different channels are available for communication.331
Each channel is identified by a channelOffset i.e., an integer value in the range.332
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B&W IN PRINT
Fig. 3 Slotframe
B&W IN PRINT
Fig. 4 Timeslot
However, some of these frequencies could be blacklisted (because of low quality333
channel) and, hence, the total number of channels Nchannels available for channel334
hopping may be lower than 16. In TSCH a link is defined as the pairwise assignment335
of a directed communication between devices in a given timeslot on a given channel336
offset [20]. Hence, a link between communicating devices can be represented by a337
couple specifying the timeslot in the slotframe and the channel offset used by the338
devices in that timeslot. Let denote a link between two devices. Then, the frequency339
fto be used for communication in timeslot of the slotframe is derived as follows.340
f=F[(ASN +channelof f set)%Nchannels](1)341
where is the Absolute Slot Number, defined as the total number of timeslots elapsed342
since the start of the network (or an arbitrary start time determined by the PAN343
coordinator). It increments globally in the network, at every timeslots, and is thus344
used by devices as timeslot counter. Function Fcan be implemented as a lookup table.345
Thanks to the multi-channel mechanism several simultaneous communications can346
take place in the same timeslot, provided that different communications use different347
channel offsets. Also, Eq. 1implements the channel hopping mechanism by returning348
a different frequency for the same link at different timeslots.349
Figure 5shows a possible link schedule for data collection in a simple sensor350
network with a tree topology. We have assumed that the slotframe consists of four351
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12 D. De Guglielmo et al.
B&W IN PRINT
A
C
D
B
E
G
H
I
L
timeslot
Channel Offset
0
1
2
3
4
12 4
3
(a) (b)
Fig. 5 A sensor network with a tree-topology (a) with a possible link schedule for data-collection
(b)
timeslots and there are only five channel offsets available. We can see that, thanks to352
the multi-channel approach used by TSCH, eight transmissions have been accom-353
modated in a time interval corresponding to four timeslots. In the allocation shown354
in Fig. 5all links but one are dedicated links, i.e., allocated to a single device for355
communication. The 802.15.4e standard also allows shared links, i.e., links inten-356
tionally allocated to more than one device for transmission. This is the case of the357
link [1,0] allocated to both nodes E and G.358
Since shared links can be accessed by more than one transmitter, collisions may359
occur that result in a transmission failure. To reduce the probability of repeated360
collisions, the standard defines a retransmission backoff algorithm. The latter is361
invoked by a sending device whenever a data frame is transmitted on a shared link362
and the related acknowledgment is not received. The data frame will be retransmitted363
in the next link assigned to the sending device and with the same destination, which364
may be either a shared link or a dedicated link. The retransmission algorithm relies365
on a backoff delay and works as follows. The retransmission backoff only applies366
to the transmission on shared links, whereas dedicated links are accessed without367
any delay. The retransmission backoff is calculated using an exponential algorithm368
analogous to that described in Sect. 2for CSMA-CA (it is still based on macMaxBE369
and macMinBE). However, in TSCH the backoff delay is expressed in terms of370
number of shared links that must be skipped. The backoff window increases for each371
consecutive failed transmission in a shared link, while it remains unchanged when a372
transmission failure occurs in a dedicated link. A successful transmission in a shared373
link resets the backoff window to the minimum value. The backoff window does not374
change when a transmission is successful in a dedicated link but there are still other375
frames to transmit (the transmission queue is not empty). The backoff window is376
reset to the minimum value if the transmission in a dedicated link is successful and377
the transmit queue is then empty.378
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From IEEE 802.15.4 to IEEE 802.15.4e: A Step Towards the Internet of Things 13
A key element in TSCH is the link schedule, i.e., the assignment of links to nodes379
for data transmissions. Of course, neighboring nodes may interfere and, hence, they380
should not be allowed to transmit in the same timeslot and with the same channel381
offset. The multi-channel mechanism makes the link scheduling problem easier with382
respect to the traditional scenario where a single channel is used. However, finding out383
an optimal schedule may not be a trivial task, especially in large networks with multi-384
hop topology. The problem is even more challenging in dynamic networks where385
the topology changes over time (e.g., due to mobile nodes). It may be worthwhile386
emphasizing here that the derivation of an appropriate link schedule is out of the387
scope of the 802.15.4e standard. The latter just defines mechanisms to execute a link388
schedule, however, it does not specify how to derive such a schedule. This is left to389
upper layers.390
A number of link scheduling algorithms have been specifically proposed for TSCH391
[21–23]. Also previous solutions for slotted multi-channel systems can be easily392
adapted to TSCH. Link scheduling algorithms can be broadly classified into two393
different categories, namely centralized and distributed algorithms. In centralized394
solutions [22] there is a specific node in the network (typically, the PAN coordinator)395
that is in charge of creating and updating the link schedule, based on information396
received by network nodes (about neighbors and generated traffic). Since the PAN397
coordinator has a global knowledge of the network status, in terms of network topol-398
ogy and traffic matrix, it can create very efficient link schedules. However, the link399
schedule has to be re-computed each time the network conditions change. Hence,400
the centralized approach is not very appealing for dynamic networks (e.g., networks401
with mobile nodes), where a distributed approach is typically more suitable. In a dis-402
tributed link scheduling algorithm [21,23] each node decide autonomously which403
link to activate with its neighbors, based on local and, hence, partial, information.404
5 Performance Comparison405
To measure the potential performance improvements that can be achieved when using406
IEEE 802.15.4e, instead of IEEE 802.15.4, we performed a set of simulation exper-407
iments using the ns2 simulation tool [24]. Specifically, we considered the 802.15.4408
MAC in Beacon Enabled (BE) mode and Non Beacon Enabled (NBE) mode, and409
compared its performance to that of the 802.15.4e MAC in TSCH mode. To make410
the comparison fair and, also, to better emphasize the performance improvements411
that can be achieved with 802.15.4e, in TSCH we did not consider the multi-channel412
and frequency hopping mechanisms, i.e., we assumed a single channel frequency.413
Under this assumption TSCH reduces to a simple TDMA scheme.414
In our analysis we considered a sensor network with star topology, where the sink415
node acts as the PAN coordinator and sensor nodes are placed in a circle centered416
at the PAN coordinator, 10 m far from it. The transmission range was set to 15 m,417
while the carrier sensing range was set to 30 m (according to the model in [25]). We418
considered a periodic reporting application where data acquired by sensors have to be419
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14 D. De Guglielmo et al.
reported periodically to the PAN coordinator. Time is divided into communication420
periods of duration T and each sensor node generates one data packets every T421
seconds.422
To evaluate the performance of the different access modes, we derived the423
following performance indices.424
•Latency, defined as the average time from when the packet transmission is started425
at the source node to when the same packet is correctly received by the PAN426
coordinator. It characterizes the timeliness of the system.427
•Delivery ratio, defined as the ratio between the number of data packets correctly428
received by the PAN coordinator and the total number of data packets generated by429
all sensor nodes. It measures the network reliability in the data collection process.430
•Energy per packet, defined as the total energy consumed by each sensor node431
divided by the number of data packets correctly delivered to the PAN coordinator.432
It measures the energy efficiency of the system.433
The energy consumed by a sensor node was calculated using the model presented434
in [26], based on the Chipcon CC2420 radio transceiver [27]. This model supports435
the following radio states: transmit,receive,idle (the transceiver is on, but it is not436
transmitting nor receiving, i.e., it is monitoring the channel) and sleep (the transceiver437
is off and can be switched back on quickly).438
The operating parameter values used in our experiments are shown in Table 1.The439
acknowledgement mechanism was always enabled in all the considered modes. When440
using the 802.15.4 BE mode the communication period corresponds to the Beacon441
period. We set BO =6, which corresponds to a Beacon period of approximately 1 s442
(0.983 s to be precise). To make the comparison fair we used the same T value also443
for NBE and TSCH. In our experiments, for each simulated scenario, we performed444
10 independent replications, where each replication consists of 1000 communication445
periods. For each replication we discarded the initial transient interval (10% of the446
overall duration) during which nodes associate to the PAN coordinator node and447
start generating data packets. The results shown below are averaged over all the448
different replications. We also derived confidence intervals through the independent449
replication method. However, they are so small that they cannot be appreciated in450
the figures below.451
Figures 6,7and 8show the performance of the different MAC modes, for an452
increasing number of sensor nodes, in terms of delivery ratio, average latency, and453
energy efficiency, respectively. As expected, TSCH outperforms both BE and NBE454
for all the considered indices. Specifically, it performs a 100% delivery ratio, with low455
(and fixed) latency and minimal energy consumption. In addition, its performance456
do not depend on the number of sensor nodes, at least until this number is less than457
or equal to the number of timeslots in the slotframe. Conversely, the 802.15.4 BE458
mode exhibits very poor performance, even when the number of sensor nodes is459
relatively high (e.g., with 20 nodes). This is because in BE mode nodes synchronizes460
to the periodic beacon emitted by the PAN coordinator. Hence, all sensor nodes461
having data to transmit compete for channel access at the beginning of the beacon462
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From IEEE 802.15.4 to IEEE 802.15.4e: A Step Towards the Internet of Things 15
B&W IN PRINT
Fig. 6 Delivery ratio versus
number of nodes
0
20
40
60
80
100
20 40 60 80 100 120
Delivery ratio (%)
Number of nodes
NBE
BE
802.15.4e-TSCH
Table 1 Operating
parameters Parameter Value
Communication Period (T) 0.983 s
Data frame size 127 bytes
ACK frame size 11 bytes
macMaxFrameRetries 3
macMaxCSMABackoffs 4
macMaxBE 5
macMinBE 3
Prx 35.46mW
Ptx 31.32mW
Pidle 0.77 mW
Ps36µW
period. This maximizes the competition among nodes and results in high latencies463
and energy consumption. Also, a large percentage of frames is discarded due to464
exceeded number of backoff trials [28]. The NBE mode performs better than BE465
because, unlike BE, there is no synchronization and sensor nodes access the channel466
asynchronously, when they have a data packet ready for transmission. This reduces467
the competition among nodes even if conflicts can still occur. Hence, NBE performs468
similarly to TSCH when the number of nodes is low and there are no conflicts, while469
the performance gap between NBE and TSCH increases very quickly as the number470
of nodes grows up. It must be emphasized that, while TSCH provides a deterministic471
latency, thanks to its slotted access scheme, NBE is not able to guarantee a bounded472
latency, even when the number of nodes is low, since it implements a contention-473
based access scheme. For the same reasons, it is not able to guarantee a 100 % delivery474
ratio when the number of nodes is large or under high traffic conditions. Hence, NBE475
is not suitable for application scenarios where low and deterministic latency and/or476
high reliability are required. On the other side, being based on contention-based477
access, NBE does not require any preliminary link schedule to work and is, thus,478
more flexible and easy to manage, especially in network with dynamic topology.479
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16 D. De Guglielmo et al.
B&W IN PRINT
Fig. 7 Average latency versus
number of nodes
4
6
8
10
12
14
16
18
20
20 40 60 80 100 120
Average packet latency (ms)
Number of nodes
NBE
BE
802.15.4e -TSCH
B&W IN PRINT
Fig. 8 Energy per packet
versus number of nodes
0
0.5
1
1.5
2
20 40 60 80 100 120
Per-packet energy
Number of nodes
consumption (mJ)
802.15.4e-TSCH
NBE
BE
Therefore, it can be preferred to TSCH in all application scenarios where latency480
and/or reliability requirements are not so stringent.481
AQ1
6 Conclusions482
In this chapter we have focused on the 802.15.4e standard, recently released by483
IEEE to enhance and add functionality to the 802.15.4 standard so as to address484
the emerging needs of embedded industrial applications. The 802.15.4 standard was485
conceived for applications without special requirements in terms of timeliness, relia-486
bility, robustness, and scalability. Therefore, it is unsuitable for application domains487
such as applications in the industrial and healthcare fields. We have highlighted the488
main limitations and deficiencies of the 802.15.4 standard and shown how these489
limitations have been overcome in the new standard. We have also presented some490
simulation results to better highlight the performance improvements allowed by the491
new standard.492
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From IEEE 802.15.4 to IEEE 802.15.4e: A Step Towards the Internet of Things 17
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