Fast Join and Synchronization Schema in the IEEE 802.15.4e MAC Speaker: Liang-Lin Yan Advisor: Dr. Ho-Ting Wu 2015/10/29.

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

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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 .

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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.

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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.

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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.

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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.

•

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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 duty-cycle, 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 modelsBased 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 multi-slotframe 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.

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Analytical models(cont.)

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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) 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);

2) no collisions nor errors happen.19

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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:

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Analytical models-RV(cont.)Mean number of TM periods need for synchronization:

When packet delivery ratio < 1:

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Analytical models-RV(cont.)When packet delivery ratio < 1 and N >1:

Average synchronization time:

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Analytical models-RV(cont.)

The optimal number of node(N):

The optimal synchronization:

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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:

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Analytical models-RH(cont.)Average period for EB transmission:

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Analytical models-RH(cont.)Average synchronization time:

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Analytical models(cont.)

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Experimental evaluation•Implemented in the OpenWSN stack.

•Using TelosB motes.

•Multi-superframe of 15 slotframes & each slotframe 101 timeslots.

•Topology:

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Experimental evaluation (cont.)

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Experimental evaluation (cont.)

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Experimental evaluation (cont.)

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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 > number of available channel.

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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