Fast Join and Synchronization
Schema in the IEEE 802.15.4e
MAC
Speaker: Liang-Lin Yan
Advisor: Dr. Ho-Ting Wu
2015/10/29
Outline
• Introduction
• Overview on the IEEE 802.15.4e TSCH
• Fast synchronization
algorithms
• Analytical models
• Experimental evaluation
• Conclusion
• Reference
2
Introduction
• The
Internet of Things (IoT) represents, in the context of
networking, one of the most relevant innovations of the third
millennium .
• In
enabling IoT the low power and short range wireless
communication technologies play a key role .
3
Introduction(cont.)
• One
of the leading standard in this field is the IEEE 802.15.4
MAC, and the new IEEE 802.15.4e amendment extends its
features making it more suitable for the industrial market.
• It introduces the Time Slotted Channel Hopping (TSCH) mode to
increase the reliability and the energy efficiency of short range
wireless communications in noisy environments.
4
Introduction(cont.)
• In
a IEEE 802.15.4e network, all nodes share a common time
slotted baseline, organized as a periodic sequence of slotframes,
and they can wake up only in those timeslots according to the
needs of higher layer protocols.
• In this manner, the duty cycle could be minimized, extending the
network lifetime due to a smaller energy consumption.
5
Introduction(cont.)
•A
critical aspect of TSCH, which is worth to be investigated, is
related to the set up of the global synchronization at the
network bootstrap.
• Electing
at least one node to broadcast the Absolute Slot
Number (ASN). To this aim, specific Enhanced Beacon (EB)
frames are used.
6
Introduction(cont.)
• The
adoption of TSCH for the transmission of EBs may lead to a
longer time needed for the global synchronization of all the
nodes, because it is necessary that the synchronizer and the new
joining nodes are aligned on the same frequency.
• TSCH
may impairs the overall energy efficiency of the network
because nodes are forced to remain awake for a long time till
they are able to gain the synchronization.
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IEEE 802.15.4e TSCH
• The IEEE802.15.4e TSCH MAC adds channel hopping to time
slotted access. Channel hopping mitigates the effects of
interference and multipath fading
• Moreover,
the use of several frequencies increases the network
capacity, because more nodes can transmit their frames at the
same time, using different channel offsets.
•
8
IEEE 802.15.4e TSCH(cont.)
• In
TSCH, as stated before, motes synchronize on a slotframe
structure. Each mote follows a schedule which tells it what to do
in each slot: transmit, receive, or sleep.
•A
TSCH PAN is formed when a device, usually the PAN
coordinator, advertises network presence by sending EBs. The
device wishing to join the network begins “passively” (using a
preferred channel) or “actively” (scanning for the network).
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IEEE 802.15.4e TSCH(cont.)
• Once
new node received a valid EB, it joins the network by
sending a Join Request command frame to the advertising
device.
• When the new mote is accepted into the network, the advertiser
activates it by setting up slotframes and links between it and
other existing motes.
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IEEE 802.15.4e TSCH(cont.)
• It
is important to highlight that in TSCH, device-to-device
synchronization is necessary to maintain connectivity with
neighbors.
•
To this aim, Acknowledgement-Based and Frame- Based
methods have been defined; they allow the receiver to calculate
the difference between expected and actual (ACK or frame)
arrival times and to tune its clock accordingly to stay
synchronized with the sender.
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IEEE 802.15.4e TSCH(cont.)
• To
maintain synchronization in a network with a very low dutycycle, there is the need for keep-alive packets. An adaptive
synchronization technique where the nodes measure the clock
drifts of the neighbors and adjust the period of keep-alive
packets accordingly.
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Fast synchronization algorithms
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Fast synchronization algorithms
(cont.)
• Random
Vertical filling (RV) -The coordinator transmits EBs in
the first advertisement slot of the multi-superframe structure
using chof = 0.
• Any
new synchronized node, instead, has to transmit in the
same advertisement slot with a randomly chosen channel offset.
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Fast synchronization algorithms
(cont.)
• Random
Horizontal filling (RH) - The coordinator transmits EBs
in the first advertisement slot of the multislotframe structure
using a chof = 0
•
Other synchronized node will chose randomly one of the
available advertisement slots of the multi -slotframe structure
using again a chof = 0.
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Analytical models
Based on the following assumptions (unless otherwise specified):
• There are N nodes, already synchronized, in radio visibility that send
EBs. They will be referred to as “synchronizer” nodes.
• The
EBs are sent with a frequency 1/TM where TM is the multislotframe duration.
• The
probability that each synchronizer node transmits at a certain
frequency in a given timeslot is uniformly distributed and it is equal to
1/C, where C is the number of channels in use.
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Analytical models(cont.)
• The
joining node is tuned on one of the available channels
listening for an EB.
• The
nodes willing to join the network have initially a duty-cycle
equal to 100 %, that is, their radios are always on, till they gain
the synchronization.
17
Analytical models(cont.)
18
Analytical models-RV(cont.)
• In
the RV scheme, the EB transmission is allowed in only one
timeslot every multi-slotframe and the corresponding channel is
chosen randomly.
• The
joining node A acquires the synchronization need to
satisfied two conditions:
1)
2)
the channel, fB, used by a node sending an EB is the same
channel, fA,used by the node A to listen EBs (i.e., fB = fA);
no collisions nor errors happen.
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Analytical models-RV(cont.)
Probability of a channel is selected by only one synchronizer:
Mean number of channel where only one EB is transmitted:
20
Analytical models-RV(cont.)
Mean number of TM periods need for synchronization:
When packet delivery ratio < 1:
21
Analytical models-RV(cont.)
When packet delivery ratio < 1 and N >1:
Average synchronization time:
22
Analytical models-RV(cont.)
The optimal number of node(N):
The optimal synchronization:
23
Analytical models-RH (cont.)
• As
for the RV model, there are N nodes (including the PAN
coordinator) already synchronized that send periodically EB frames
with a period equal to TM = Sf *Tf .
• Each
of the N nodes chooses randomly an advertisement slot in the
first Sf slotframes (after it joins the network) and it uses always this
advertisement slot for transmitting EBs.
• According
to TSCH operations, the physical channel used in the
selected slot will be different at every consecutive sloframe.
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Analytical models-RH(cont.)
Probability of a slotframe is selected by only one synchronizer :
Mean number of ad. slot where only one EB is transmitted:
25
Analytical models-RH(cont.)
Average period for EB transmission:
26
Analytical models-RH(cont.)
Average synchronization time:
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Analytical models(cont.)
28
Experimental evaluation
• Implemented in the OpenWSN stack.
• Using TelosB motes.
• Multi-superframe
of 15 slotframes & each slotframe 101
timeslots.
• Topology:
29
Experimental evaluation (cont.)
30
Experimental evaluation (cont.)
31
Experimental evaluation (cont.)
32
Conclusion
• They
provide the closed form expression. The theoretical and
experimental result are similarly the same. So we can use these
expression to calculate the optimal situation.
• The
joining operations required by a node to become part of a TSCH
network can be effectively boosted by increasing the node density or
reduce the average inter arrival time between consecutive EBs.
• They don’t consider collision scenario , when N
channel.
> number of available
33
Reference
• Vogli,
E.; Ribezzo, G.; Grieco, L.A.; Boggia, G, “Fast join and
synchronization schema in the IEEE 802.15.4e MAC,” IEEE
Wireless Communications and Networking Conference Workshops
(WCNCW), 9-12 March, 2015, pp.85-90
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