From IEEE 802.15.4 to IEEE 802.15.4e
Another Step towards theInternet of (Important) Things

Giuseppe Anastasi
Pervasive Computing & Networking Lab (PerLab)
Dept. of Information Engineering, University of Pisa
E-mail: giuseppe.anastasi@iet.unipi.it
Website: www.iet.unipi.it/~anastasi/
PerLabSun Yat-Sen University
Guangzhou, China, May 16, 2014

Internet of Things
PerLab“The next logical step in the technological revolution connecting people
anytime, anywhere is to connect inanimate objects. This is the vision
underlying the Internet of things: anytime, anywhere, by anyone andanything”(ITU, Nov. 2005)
More than 26 billions devices
will be wirelessly connected to
the Internet of Things by 2020
computers and communication devices
cars, robots, machine tools
persons, animals, and plants
garments, food, drugs, etc.

From IEEE 802.15.4 to IEEE 802.15.4e2

Wireless Sensor & Actuator Networks (WSANS)
PerLabWSANs will behave as a digital skin for the IoT
providing a virtual layer through which any
computational system can interact with the physicalworld

From IEEE 802.15.4 to IEEE 802.15.4e

3Standards for WSANs
PerLabIEEE 802.15.4/802.15.4e
ZigBeeBluetoothWirelessHART
ISA-100.11a
6LoWPAN
IPv6 over Low power WPAN

RPL
Routing Protocol for Low powerand Lossy networks

CoAP
Constrained Application Protocol
From IEEE 802.15.4 to IEEE 802.15.4e

4IEEE 802.15.4 Standard
Reference technology for WSANs
Expected to be a major enabling
technology also for IoT

Overview
PerLabIEEE 802.15.4
MAC protocols

Limits of IEEE 802.15.4 MAC
Performance and reliability

Possible enhancements
IEEE 802.15.4e
Time-Slotted Channel Hopping (TSCH) mode

Open Issues

From IEEE 802.15.4 to IEEE 802.15.4e

6IEEE 802.15.4 standard
PerLabStandard for Personal Area Networks (PANs)
low-rate and low-power
PHY and MAC layers

Main featurestransceiver management
channel access
PAN management
From IEEE 802.15.4 to IEEE 802.15.4e

7

802.15.4 Network Topologies
PerLab

Ed Callaway, Paul Gorday, Lance Hester, Jose A. Gutierrez, Marco Naeve, Bob Heile, Home Networking with IEEE 802.15.4:
Developing Standard for Low-Rate Wireless Personal Area Networks, IEEE Communications Magazine, August 2002.
From IEEE 802.15.4 to IEEE 802.15.4e

8Channel frequencies
PerLabEuropeUSA(20 Kbps)(40 Kbps)(250 Kbps)

Ed Callaway, Paul Gorday, Lance Hester, Jose A. Gutierrez, Marco Naeve, Bob Heile, Home Networking with IEEE 802.15.4:
Developing Standard for Low-Rate Wireless Personal Area Networks, IEEE Communications Magazine, August 2002.
From IEEE 802.15.4 to IEEE 802.15.4e

9802.15.4 MAC protocols
PerLabTwo different channel access methods
Beacon-Enabled duty-cycled mode
Non-Beacon Enabled mode (aka Beacon Disabled mode)
MAC

Non-beacon enabled

Beacon enabled
Superframe Structure

Contention based

Contention based

Contention free

Unslotted CSMA-CA

Slotted CSMA-CA

Reserved time slot

From IEEE 802.15.4 to IEEE 802.15.4e

10802.15.4 Beacon Enabled mode
PerLab

ActiveBeaconBeacon

GTS

CAP

GTS

Inactive

CFP

SD = aBaseSuperFrameDuration*2SO sym
BI = aBaseSuperFrameDuration*2BO sym

From IEEE 802.15.4 to IEEE 802.15.4e

11CSMA/CA: Beacon-Enabled mode
PerLabAt each trial the backoffwindow size is doubled
Yes

Only a limited number ofattempts is permitted(macMaxCSMABackoffs)

No

No

No

Yes

From IEEE 802.15.4 to IEEE 802.15.4e

Yes

12

CSMA/CA: Non-Beacon Enabled mode
PerLab

Un-slotted CSMA

NB = 0,BE = macMinBE

Delay forrandom(2BE - 1) unit
backoff periods

At each trial the backoffwindow size is doubled

Perform CCA

Only a limited number ofattempts is permitted(macMaxCSMABackoffs)

Y
Channel idle?
N
NB = NB+1,
BE = min(BE+1, aMaxBE)

N

NB>
macMaxCSMABackoffs?

Y
FailureFrom IEEE 802.15.4 to IEEE 802.15.4e

Success13

Acknowledgement Mechanism
PerLab

Optional mechanism
Destination SideACK sent upon successful reception of a data frame

Sender side
Retransmission if ACK not (correctly) received within thetimeout
At each retransmission attempt the backoff window size
is re-initialized
Only a maximum number of retransmissions allowed(macMaxFrameRetries)

From IEEE 802.15.4 to IEEE 802.15.4e

14Limits of IEEE 802.15.4 MAC
PerLab

MAC Unreliability
Unbounded latency
No guaranteed bandwidth
Unless GTS is used
GTS only provides a limited service (7 slots)

No built-in frequency hopping technique
Prone to failures due to interferences and multi-pathfadingG. Anastasi, D. De Guglielmo, A Seghetti, From IEEE 802.15.4 to IEEE 802.15.4e: a Step towards the Internet of Things,
Chapter 10 in Advances onto the Internet of Things (S. Gaglio, G. Lo Re, Editors), pp. 135-152. January 2014. Springer
From IEEE 802.15.4 to IEEE 802.15.4e

15802.15.4 CSMA/CA Performance
PerLab

G. Anastasi, M. Conti, M. Di Francesco, A Comprehensive Analysis of the MAC Unreliability Problem in IEEE 802.15.4
Wireless Sensor Networks, IEEE Transactions in Industrial Informatics, Vol. 7, N. 1, Feb 2011.
From IEEE 802.15.4 to IEEE 802.15.4e

16Key Question
Why the 802.15.4 MAC
Unreliability Problem?

Possible Answers
PerLab

The access method is CSMA-CA
Contention increases with the # of active nodes

The periodic Beacon synchronizes nodes’
accesses
All sensor nodes contend for channel access uponreceiving a beacon

How about other CSMA-based MAC protocols
operating in same conditions?

From IEEE 802.15.4 to IEEE 802.15.4e

18

802.15.4 MAC vs. S-MAC
PerLab

G. Anastasi, M. Conti, M. Di Francesco, A Comprehensive Analysis of the MAC Unreliability Problem in IEEE 802.15.4
Wireless Sensor Networks, IEEE Transactions in Industrial Informatics, Vol. 7, N. 1, Feb 2011.
From IEEE 802.15.4 to IEEE 802.15.4e

19CSMA/CA parameters
PerLabParameter

2003 release2006 release

Notes

3

0÷7

Max number of retransmissions

macMaxFrameRetries(aMaxFrameRetries)

macMaxCSMABackoff

0÷5
Default: 4

0÷5Default: 4

Max number ofbackoff stages

3÷8

macMaxBE

5(aMaxBE)

Maximum Backoff
Window Exp.

macMinBE

0÷3
Default: 3

From IEEE 802.15.4 to IEEE 802.15.4e

Default: 3

Default: 50÷7Default: 3

Minimum Backoff
Window Exp.

20

CSMA parameter values
PerLabDPS (Default Parameter Set)
Default parameter values defined in the standard

SPS (Standard Parameter Set)
Larger parameter values allowed by the standard

NPS (Non-standard Parameter Set):
Larger parameter values not allowed by the standard

macMinBE

macMaxBE

macMaxCSMABackoff

macMaxFrameRetries

DPS

3

5

4

3

SPS

7

8

5

7

NPS

8

10

10

10

From IEEE 802.15.4 to IEEE 802.15.4e

21

Delivery Ratio vs. Packet Error Rate
PerLab

G. Anastasi, M. Conti, M. Di Francesco, A Comprehensive Analysis of the MAC Unreliability Problem in IEEE 802.15.4
Wireless Sensor Networks, IEEE Transactions in Industrial Informatics, Vol. 7, N. 1, Feb 2011.
From IEEE 802.15.4 to IEEE 802.15.4e

22Latency and Energy vs. Packet Error Rate
PerLab

Avg. Latency

Energy/msg

G. Anastasi, M. Conti, M. Di Francesco, A Comprehensive Analysis of the MAC Unreliability Problem in IEEE 802.15.4
Wireless Sensor Networks, IEEE Transactions in Industrial Informatics, Vol. 7, N. 1, Feb 2011.
From IEEE 802.15.4 to IEEE 802.15.4e

23

Learned Lesson
PerLab

MAC unreliability mainly due to CSMA/CA
Periodic Beacons synchronize channel accesses
thus maximizing contention and increasing the collision
probability

Default MAC parameter values not appropriate
More appropriate settings can alleviate the problem
However the optimal setting depends on
Applications requirements
Operating conditions…

From IEEE 802.15.4 to IEEE 802.15.4e

24

Enhancements
How to achieve the desired reliability
level (e.g., 80%) with the minimum energy
expenditure?

Strategies for Optimal Settings
PerLabModel-based offline computation
Model-based online adaptation
Measurement-based adaptation
Learning-based adaptation

[1]
[2]
[3]
[4]

[1]

P. Park, P. Di Marco, P. Soldati, C. Fischione, K. H. Johansson, A Generalized Markov Chain Model for Effective
Analysis of Slotted IEEE 802.15.4, Proc. IEEE International Conference on Mobile Ad Hoc and Sensor Systems(MASS 2009). Macao, China, October 2009[2]

P. Park, P. Di Marco, C. Fischione, K. H. Johansson, Modeling and Optimization of the IEEE 802.15.4 Protocol for
Reliable and Timely Communications, IEEE Transactions on Parallel and Distributed Systems, 2012.

[3]

M. Di Francesco, G. Anastasi, M. Conti, S. K. Das, V. Neri, Reliability and Energy-efficiency in IEEE 802.15.4/ZigBee
Sensor Networks: An Adaptive and Cross-layer Approach, IEEE Journal on Selected Areas in Communications,
2011.

[4]

S. Brienza, D. De Guglielmo, C. Alippi, G. Anastasi, G. Anastasi, M. Roveri, A Learning-based Algorithm for Optimal
MAC Parameters Setting in IEEE 802.15.4 Wireless Sensor Networks, Proc. ACM International Symposium on
Performance Evaluation of Wireless Ad Hoc, Sensor, and Ubiquitous Networks (PE-WASUN 2013), Barcelona, Spain,
November 3-7, 2013

From IEEE 802.15.4 to IEEE 802.15.4e

26

Park’s Model
PerLabMarkov Chain Model
Assumptions
Star network
Ideal channel
Fixed and known # of nodes…

Reliability (R)Latency (D)Energy per packet (Etot)
As functions of the 802.15.4 MAC
parameters
P. Park, P. Di Marco, P. Soldati, C. Fischione, and K. Johansson, A Generalized Markov Chain Model for Effective Analysis of SlottedIEEE 802.15.4, Proc. IEEE International Conference on Mobile Ad hoc and Sensor Systems (MASS 2009), October 2009, pp. 130 –139.
From IEEE 802.15.4 to IEEE 802.15.4e27Model-based Offline Computation
PerLab

Offline Algorithm
Provides the MAC
parameters set thatguarantees(R>Rmin ) & (D<Dmax)

while minimizing
Etot

From IEEE 802.15.4 to IEEE 802.15.4e

28

Limits of Model-based Computation
PerLab

Offline approach
Requires to know the operating conditions of thesensor network
unsuitable for real environments

Requires a significant amount of time
to solve the Park’s model and derive the optimal MAC
parameter setting
About 400s (i.e., around 7 minutes) with a quad-core PC

From IEEE 802.15.4 to IEEE 802.15.4e29Model-based Adaptation
PerLabOnline Algorithm
Estimates α, β, and τ through direct measurements
α: probability to find the channel busy during the first CCAβ: probability to find the channel busy during the second CCA
τ: probability that the node will attempt the first CCA in a genericbackoff slot

Introduces α, β, and τ in the Park’s model
Derives R, D and Etot
for any MAC parameters set

Selects the optimal parameter setting

P. Park, P. Di Marco, C. Fischione, K. H. Johansson, Modeling and Optimization of the IEEE 802.15.4 Protocol for Reliable andTimely Communications, IEEE Transactions on Parallel and Distributed Systems, 2012.
From IEEE 802.15.4 to IEEE 802.15.4e30

Limits of model-based algorithms
PerLab

Effectiveness bounds to the model accuracy
Assumptions on packet arrival process typically does
not fit real scenarios
The communication channel is never ideal

Requires to know configuration parameters
They may change over time (e.g., # of sensor nodes)

Unsuitable for multi-hop WSNs

From IEEE 802.15.4 to IEEE 802.15.4e

31

Measurement-based Adaptation
PerLab

ADAPT: Adaptive Access Parameters Tuning
Completely heuristic approach
Provides the desired reliability with minimum energy
consumption (and latency)
Based on local measurements of unreliability
No a-priori knowledge about operating conditions
Suitable for real-life scenarios

M. Di Francesco, G. Anastasi, M. Conti, S. K. Das, V. Neri, Reliability and Energy-efficiency in IEEE 802.15.4/ZigBee Sensor
Networks: An Adaptive and Cross-layer Approach, IEEE Journal on Selected Areas in Communications, 2011.
From IEEE 802.15.4 to IEEE 802.15.4e

32AdAPT: Unreliability Measurements
PerLabSources of message loss
Congestion (Contention)
Noisy channel (fading, interferences, …)Unreliability Measurements
Messages discarded due to exceeded # of backoffs
congestion

Messages discarded due to exceeded # of rtx
Channel errors
Collisions (can be neglected in the considered scenario)

From IEEE 802.15.4 to IEEE 802.15.4e

33

AdAPT: Adaptation Strategy
PerLabFrom IEEE 802.15.4 to IEEE 802.15.4e

34

Limits of ADAPTPerLab

Oscillations
ADAPT tends to oscillate between adjacent sets
Thus consuming more energy than necessary

Memory-less
ADAPT does not take advantage of a recurrent states

From IEEE 802.15.4 to IEEE 802.15.4e

35

LEAP
PerLab

LEAP: LEarning-based Adaptive Parameter Tuning
based on local measurements, like ADAPT
does not require any input parameterit can be used in a real scenario

learns the past history
it can take advantage of recurrent states in thesystembehaves very close to an ideal algorithm

S. Brienza, D. De Guglielmo, C. Alippi, G. Anastasi, M. Roveri, A Learning-based Algorithm for Optimal MAC Parameters
Setting in IEEE 802.15.4 Wireless Sensor Networks, Proceedings of ACM International Symposium on Performance
Evaluation of Wireless Ad Hoc, Sensor, and Ubiquitous Networks (PE-WASUN 2013), Barcelona, Spain, November 3-7, 2013.
From IEEE 802.15.4 to IEEE 802.15.4e36

LEAP Algorithm
PerLabLEAP selects the optimal parameters setting
using the knowledge learned in the past history.

LEAP algorithm
Exploration Phase
An adaptive approach is used to set CSMA-CA parameters
Sensor node builds a Learning Table using the knowledge
learned during the Exploration phase.
Then it enters the Exploitation phase

Exploitation Phase
Sensor node checks the Learning Table to derive the optimal
setting, given the current operating conditions.

From IEEE 802.15.4 to IEEE 802.15.4e37

Performance Comparison
PerLab

PHY layer

2.4 GHz

Bit Rate

250 Kbps

Sensor nodes

from 1 to 50

Distance from Coordinator Node10m

CS range

30m

RX range

15m

Traffic Generation
Message Size

Periodic
100 bytes

Messages per Period

10

Message Loss Rate0-%

Coordinator always ON

From IEEE 802.15.4 to IEEE 802.15.4e

38

Performance Measures
PerLab

Packet Delivery Ratio
ratio between # of packets correctly received by the sink, and the total # ofpackets generated by all sensor nodes.

Miss ratio
fraction of times when the packet delivery probability – calculated over the current
Beacon Interval – drops below the threshold required by the application

Average Energy per Packet
total energy consumed by each sensor node divided by the # of data packets
correctly delivered to the sink.

Average Latency
average time from when the packet transmission starts at the source node towhen the packet is correctly received by the sink

From IEEE 802.15.4 to IEEE 802.15.4e

39Performance in Stationary Conditions
PerLabRequirements
Delivery Ratio: > 80%
Miss Ratio:< 15%
Delivery ratio (%)

From IEEE 802.15.4 to IEEE 802.15.4e

Miss Ratio (%)

40Performance in Stationary Conditions
PerLabEnergy per packet (mJ)

From IEEE 802.15.4 to IEEE 802.15.4e

Latency (ms)41Summary
PerLabThe desired reliability can be achieved
Through an appropriate CSMA/CA parameter setting

At the cost of
Increased energy consumption
Increased latency

From IEEE 802.15.4 to IEEE 802.15.4e

42Other limits of 802.15.4
PerLab

Unbounded latency
Due to contention-based CSMA/CA algorithm

No guaranteed bandwidth
Unless GTS is used
GTS only provides a limited service (7 slots)

No built-in frequency hopping technique
Prone to failure due to interferences and multi-pathfadingFrom IEEE 802.15.4 to IEEE 802.15.4e43

Limits of 802.15.4/ZigBee
PerLab

Unbounded latency
Due to contention-based CSMA/CA algorithm
No guaranteed bandwidth
Unless GTS is used
GTS only provides a limited service (7 slots)
No built-in frequency hopping technique
Prone to failure due to interferences and multi-pathfadingFrom IEEE 802.15.4 to IEEE 802.15.4e

44

Link Reliability vs. Channel
PerLabConnectivity traces were collected by J. Ortiz and D. Culler in a UC Berkeley office space (traces are made available
at http://wsn.eecs.berkeley.edu/connectivity/). 46 IEEE802.15.4-compliant TelosB motes are deployed in a 50m by 50m
indoor environment, and are constantly listening for packets. One after the other, each mote transmits a burst of 100
packets, with a 20ms inter-packet time and a transmission power of 0dBm, on each of the 16 frequency channels whichspan the 2.4-2.485GHz band.
From IEEE 802.15.4 to IEEE 802.15.4e

45

Channel Hopping
PerLabSubsequent packets are sent at a different frequency
following a pseudo-random hopping pattern.

If a transmission fails, retransmission will happen ona different frequency
Greater chance of successful transmissionthat if the retransmission happened on the same channel
Since a different frequency means different effects of multi-pathfading and interference

Increased security
Against Selective Jamming attack

Channel Hopping used inBluetoothWirelessHart
From IEEE 802.15.4 to IEEE 802.15.4e

46

Key Question
What to do in criticalscenarios?

IEEE 802.15.4e
PerLabIEEE 802.15 Task Group 4e
chartered to define a MAC amendment to the existing
standard 802.15.4-2006.
The intent of this amendment was to enhance andadd functionality to the 802.15.4-2006 MAC
better support the industrial markets
permit compatibility with modifications being proposed
within the Chinese WPAN.

On February 6, 2012 the IEEE Standards Association
Board approved the IEEE 802.15.4e MAC
Enhancement Standard document for publication.
http://www.ieee802.org/15/pub/TG4e.html
From IEEE 802.15.4 to IEEE 802.15.4e

48

Major Changes
PerLab

MAC Behavior Modes
support of specific application domains

General functional improvements
not tied to any specific application domain.

Comment:
Many ideas taken from previous industrial standards
WirelessHART and ISA 100.11.a
slotted access, shared and dedicated slots, multichannel communication, and frequency hopping.

From IEEE 802.15.4 to IEEE 802.15.4e

49

General functional improvements
PerLabLow Energy (LE)
Information Elements (IE)
Enhanced Beacons (EB)Multipurpose Frame
MAC Performance Metrics
Fast Association (FastA)

From IEEE 802.15.4 to IEEE 802.15.4e

50

MAC Behavior Modes
PerLabRadio Frequency Identification Blink (BLINK)
item and people identification, location, and tracking

Asynchronous multi-channel adaptation (AMCA):
application domains where large deployments are required (e.g., process
automation/control, infrastructure monitoring, etc.);

Deterministic & Synchronous Multi-channel Extension (DSME)
industrial and commercial applications with stringent timeliness andreliability requirements;

Low Latency Deterministic Network (LLDN)
applications requiring very low latency requirement (e.g., factory
automation, robot control)

Time Slotted Channel Hopping (TSCH)
application domains such as process automation

From IEEE 802.15.4 to IEEE 802.15.4e51

Deterministic & Synchronous Multi-channel Extension (DSME)PerLabDSME networks run on Beacon-enabled PAN
All devices synchronize to multi-superframe via
beacon framesMulti-superframe enhances GTS in 802.15.4
By grouping multiple superframes
and extending single channel operation to multi-channel
operationeither by adapting (switching) channels or by hopping channels.

A pair of peer devices wakes up at a reserved GTS
slot to exchange a data frame and an ACK frame.

From IEEE 802.15.4 to IEEE 802.15.4e52

Low Latency Deterministic Network (LLDN)
PerLabTarget: factory and process automation
automotive robots, machine tools, milling/turning/ robot
Very low and deterministic latency (e.g., 5-50 ms)

Designed for small networks and small frames
Operates in star topology

LLDN MAC uses a superframe, based on time slots,
to provide determinism
beacon timeslot
management timeslots (if present)
base transmission timeslots (of equal length)
The base timeslots include uplink timeslots andbidirectional timeslots.
From IEEE 802.15.4 to IEEE 802.15.4e

53

Low Latency Deterministic Network (LLDN)
PerLabGroup Acknowledgement (GACK) slots
Configured to promote the retransmission of failed tx
in uplink timeslots.

Two categories of base timeslots
Dedicated timeslots
The slot owner has access privileges in dedicatedtimeslots

Shared timeslots
devices assigned to a shared timeslot use the contentionbased access method.

From IEEE 802.15.4 to IEEE 802.15.4e54Time Slotted Channel Hopping (TSCH)
PerLab

Combines time slotted access, multi-channel
communication and channel hopping
Particularly suitable for multi-hop networks

Time-slotted accessPredictable and bounded latency
Guaranteed bandwidth

Multi-channel communication
More nodes can communicate at the same time (i.e., same slot) using
different channels (identified by different channel offsets)
increased network capacity

Channel hopping
mitigates the effects of interference and multipath fading
improves reliability
From IEEE 802.15.4 to IEEE 802.15.4e55

Periodic Slotframe
PerLabDuring a timeslot, one node typically sends a frame, and another sends back an acknowledgement if it
successfully receives that frame. If an acknowledgement is not received within the timeout period,
retransmission of the frame waits until the next assigned transmit timeslot (in any active slotframe) to that
address occurs.

From IEEE 802.15.4 to IEEE 802.15.4e

56

Frequency Translation
PerLab

The channel offset is translated in an operating
frequency f

ASN: total # of slots that elapsed since the
network was deployed
ASN=(k·S+t) where S is the slotframe size, k the slotframe
cycle
nch : number of used channels

F is implemented as a look-up-table containing
the sets of available channels

From IEEE 802.15.4 to IEEE 802.15.4e57

Frequency Translation
PerLab

2

3 4 5

6 7

slotOffset

0 1

2 3

4 5 6

Slotframe cycle0channelOffset

ASN 1

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

15

1
0

Channel

t
2

1
ch15

From IEEE 802.15.4 to IEEE 802.15.4e

ch22

3
ch13

ch20

58

TSCH Link
PerLab

Link=Pairwise assignment of a directed communication
between devices in a specific slot, with a given channel
offsett
chOf

From IEEE 802.15.4 to IEEE 802.15.4e

slot in the slotframe
channel offset

59

TSCH Scheduling
PerLabThe standard only explains how the MAC layer
executes a schedule
it does not specify how such a schedule is built

Centralized Schedulinga manager node is responsible for building and maintaining the
network schedule

Distributed Scheduling
No central entity
Each node decides autonomously

From IEEE 802.15.4 to IEEE 802.15.4e60Centralized Scheduling
PerLabEvery node regularly updates the manager withthe list of other nodes it can hear
the amount of data it is generating

The manager draws the connectivity graph,assigns slots to different links in the graphBased on data generation demands

and informs each node about links in the schedule itis participating in.
If the connectivity graph changes (i.e., a node lost a neighbor), themanager updates its schedule and informs affected nodes

Palattella, M. R., Accettura, N., Dohler, M., Grieco, L. A., & Boggia, G., Traffic Aware Scheduling
Algorithm for reliable low-power multi-hop IEEE 802.15. 4e networks. IEEE International Symposium
on Personal Indoor and Mobile Radio Communications (PIMRC 2012), pp. 327-332, 2012
From IEEE 802.15.4 to IEEE 802.15.4e61Distributed Scheduling
PerLabEach node decides locally on which links toschedule with which neighbors.
Suitable for highly dynamic networks
e.g., networks with mobile nodes
or when the network has many gateway nodes
centralized schedules are known to be superior to distributed ones forfairly static networks
P. Zand, A. Dilo, and P. Havinga, D-MSR: A distributed network management scheme for real-timemonitoring and process control applications in wireless industrial automation, Sensors, vol. 13, no. 7,
pp. 8239–8284, 2013.
Accettura, N., Palattella, M. R., Boggia, G., Grieco, L. A., & Dohler, M., Decentralized Traffic Aware
Scheduling for multi-hop Low power Lossy Networks in the Internet of Things”. In IEEE International
Symposium and Workshops on a World of Wireless, Mobile and Multimedia Networks (WoWMoM 2013),
2013.
Tinka, A., Watteyne, T., & Pister, K., A decentralized scheduling algorithm for time synchronizedchannel hopping. Ad Hoc Networks, pp. 201-216, Springer Berlin Heidelberg (2010)
From IEEE 802.15.4 to IEEE 802.15.4e

62

Network Formation Protocol
PerLabAdvertiser nodes emits Enhanced Beacons(EBs) at regular times
EBs are special TSCH frame containing
Synchronization information
allows new devices to synchronize to the network

Channel hopping information
allows new devices to learn the channel hopping sequence

Timeslot information
describes when to expect a frame transmission and when to send an
acknowledgment

Initial link and slotframe information
allows new devices to know:– when to listen for transmissions from the advertising device– when to transmit to the advertising device

Network Formation Protocol
PerLab

A joining device starts scanning for possible EBs.
Upon receiving an EB from an advertiser node
the MAC layer notifies the higher layer
The higher layer initializes the slotframe and links
Using information in the received EB message

and switches the device into TSCH mode.
At this point the device is connected to the network.

Then, the device allocates communication resources(i.e., slotframes and links)

And can start advertising, on its turn
The EB advertising policy is not part of the 802.15.4e
standard

Advertising Policy
PerLabRandom Advertisement Protocol
Each node is assigned a link,for
EB advertisement
Timeslots devoted to EB advertisement repeat periodicallywith period(
andare assumed to be relatively prime)
All nodes use the same slot, possibly with different channel offset

Each advertiser node transmits its EB, using the scheduled
link, with a probability
is derived autonomously by each nodeso as to minimize the collision probability
= 1⁄
)
!(
D. De Guglielmo, A. Seghetti, G. Anastasi, M. Conti, A Performance Analysis of the Network Formation
Process in IEEE 802.15.4e TSCH Wireless Sensor/Actuator Networks, Proc. IEEE International
Symposium on Computers and Communications (ISCC 2014), Madeira, Portugal, June 23-26, 2014.

Some results
PerLab

Average Joining Time vs. Number of Links used for EBs

Conclusions
PerLabIEEE 802.15.4 suffers many performance
limitations …
in terms of both delay and packet dropping probability
due to the CSMA-CA access method and default parametervalues… and is vulnerable to fading and interferences
IEEE 802.15.4e overcomes the previous limitations
To meet the requirements of industrial and time-constrained
applications
TSCH combines
Time-slotted accessMulti-channel communication
Frequency hopping

Open Research Issues
PerLabLink Scheduling Algorithm
Centralized vs. distributed

EB Advertisement protocol
for network formation

Interaction between TSCH and IoT protocols
Interactions between TSCH and RPL
IETF 6TiSCH standardizes mechanisms for running an
IPv6-enabled protocol stack on top of TSCH.

Additional Info on IEEE 802.15.4e and 6TiSCHPerLabIEEE 802.15 WPAN™ Task Group 4e (TG4e), [Online],
http://www.ieee802.org/15/pub/TG4e.html
http://datatracker.ietf.org/wg/6tisch/charter/
G. Anastasi, D. De Guglielmo, A Seghetti, “From IEEE 802.15.4 to IEEE
802.15.4e: a Step towards the Internet of Things”, Chapter 10 in Advances
onto the Internet of Things (S. Gaglio, G. Lo Re, Editors), pp. Pages 135152. January 2014. Springer. Also available at:
http://info.iet.unipi.it/~anastasi/papers/book-lo-re-10.pdf

From IEEE 802.15.4 to IEEE 802.15.4e69

Questions