Speaker: Po-Hung Chen Advisor: Dr. Ho-Ting Wu 2016/10/12 A Model-based Beacon Scheduling algorithm for IEEE 802.15.4e TSCH networks Speaker: Po-Hung Chen Advisor: Dr. Ho-Ting Wu 2016/10/12

Outline Introduction IEEE 802.15.4e TSCH Network Formation Process Model-Based Beacon Scheduling Algorithm Algorithm for Comparison Simulation Setting Simulation Result Conclusion Reference

Introduction Time Slotted Channel Hopping (TSCH) is an emerging MAC protocol defined in the IEEE 802.15.4e standard, combining time slotted access with multi-channel and channel hopping capabilities. In this paper we focus on the network formation process of TSCH networks. This relies on periodic advertisement of Enhanced Beacons (EBs), however, the standard does not specify any advertising strategy. Finally, we propose a new Model-based Beacon Scheduling (MBS) algorithm that approximates the optimal strategy in real networks.

IEEE 802.15.4e Application Presentation IEEE 802.15.4 is a standard which specifies the physical layer and mac layer for low-rate wireless personal area networks (LR-WPANs) and it has defined it in 2003. IEEE 802.15.4 MAC is used CSMA/CA. IEEE 802.15.4 Frequency is 2405+5(k-11)MHz, k=11~26 IEEE 802.15.4e is the first MAC amendment which has defined in 2012. Application Presentation Session Transport Network Data Link Physical (0~15) IEEE 802.15.4e MAC IEEE 802.15.4 PHY

IEEE 802.15.4e(Cont.)

IEEE 802.15.4e(Cont.) Tree Topology

TSCH The Time Slotted Channel Hopping (TSCH) protocol is one of the new MAC Behavior modes introduced by the IEEE 802.15.4e standard. TSCH combines Time slotted access Predictable and bounded latency, Guaranteed bandwidth Multi-channel communication increased network capacity (More node can communication at the same time using different channels) Channel hopping mitigates the effects of interference and multipath fading, improves reliability

TSCH(Cont.) Time slotted access

TSCH(Cont.) TSCH provides two types link: Dedicated Links: Tx, Rx Shared Links Channel hopping 𝑓: physical channel ASN: absolute slot number N c : number of channels 𝑓= 𝐴𝑆𝑁+𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑂𝑓𝑓𝑠𝑒𝑡 % 𝑁 𝑐

TSCH(Cont.) The special Enhanced Beacons(EBs) for TSCH Synchronization information Channel hopping information Timeslot information Initial link and slotframe information The special EBs for TSCH 1. Synchronization information: allows new devices to synchronize to the network 2. Channel hopping information: allows new devices to learn the channel hopping sequence 3. Timeslot information: describes when to expect a frame transmission and when to send an acknowledgment 4. Initial link and slotframe information: allows new devices to know: a. when to listen for transmissions from the advertising device b. when to transmit to the advertising device

Network Formation Process A node wishing to join a TSCH network turns its radio on and starts scanning for possible EBs. As soon as it receives a valid EB, its switches the device into TSCH mode. When radio-on for a long time, the joining node will waste a lot of energy and long time to join the network. Therefore, have a good algorithm for how to advertise and when to advertise an EB is a very important thing. But IEEE 802.15.4e standard does not specify any EB advertising strategy.

Network Formation Process(Cont.) In TSCH networks nodes use a link to exchange messages, each link is represented by a couple [timeslot, channelOffset]. 𝑁 𝑠 : the number of timeslots in a slotframe 𝑁 𝑐 : the number of available frequencies 𝑁 𝑠 and 𝑁 𝑐 are coprime. 𝑓=(𝐴𝑆𝑁+ 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑂𝑓𝑓𝑠𝑒𝑡 ) 𝑚𝑜𝑑 𝑁 𝑐

Network Formation Process(Cont.) An example, 𝑁 𝑠 =3, 𝑁 𝑐 =4 The sequence of channels used for communication in a certain link repeats every 𝑇 = 𝑁 𝑐 ∗ 𝑁 𝑠 . At successive slotframes, all links change frequency. All links follow the same channel hopping sequence.

Network Formation Process(Cont.) We assume that the joining node activates in a random timeslot and starts listening for EBs on a frequency 𝑓𝑗. Hence, it has a probability 1⁄𝑇 to be initially in any state 𝑆 𝑖 (𝑖=0,1,…,𝑇−1). State 𝑆𝑖: EBs are sent on frequency 𝑓𝑗 during this slot. 𝑝 𝑙𝑜𝑠𝑠 : the probability a non-valid EB is received, due to collisions and/or channel errors. (1− 𝑝 𝑙𝑜𝑠𝑠 ): the probability a valid EB is received.

Network Formation Process(Cont.) Discrete Time Markov Chain

Network Formation Process(Cont.) And this paper propose a average joining time 𝜏 𝑗𝑜𝑖𝑛 equation by calculating the average absorption time 𝜂 in the DTMC.

Network Formation Process(Cont.) We implemented an ad-hoc simulator in C++. It’s performed 100000 replications, and 95% confidence intervals.

Model-Based Beacon Scheduling Algorithm An example of optimal schedule. 𝑁 𝑠 =23, 𝑁 𝑐 =16 𝑎𝑛𝑑 𝑁 𝑏 =5 𝑇= 𝑁 𝑠 ∗ 𝑁 𝑐 =368 𝑑 𝑗 =𝑑( 𝑏 𝑗 , 𝑏 𝑗+1 ) We used the AMPL modeling language, in conjunction with the CPLEX solver. 𝑑 0 =73, 𝑑 1 =74, 𝑑 2 =74, 𝑑 3 =74, 𝑑 4 =73

Model-Based Beacon Scheduling Algorithm (Cont.)

Model-Based Beacon Scheduling Algorithm (Cont.) In fact, it may not be possible for a node to transmit all the Nb EBs specified in the schedule, for two main reasons. Beacon collisions Transmission unfeasibility Therefore, in a real network it is necessary to determine which nodes will be in charge of transmitting EBs, and assign links for EB transmission to these nodes, according to the optimal schedule.

Algorithm for Comparison Random (RD) The 𝑁 𝑏 link devoted to EB advertisement are randomly distributed within the slotframe. Channel Offset 3 2 1 0 1 2 timeslot

Algorithm for Comparison(Cont.) Random Vertical (RV) EBs are sent during the first timeslot of the slotframe using all the available channel offsets. 𝑁 𝑏 ≤ 𝑁 𝑐 Channel Offset 3 2 1 0 1 2 timeslot

Algorithm for Comparison(Cont.) Random Horizontal (RH) EBs are transmitted during all the timeslots of the slotframe, always using the same channel offset (i.e., 0). Channel Offset 3 2 1 0 1 2 timeslot

Simulation Setting Two scenarios: Single Joining Node Network Setup 𝑁 𝑠 =101 𝑁 𝑐 =16 It’s performed 100000 independent replications, and 95% confidence intervals.

Simulation Result Single Joining Node, 𝑝 𝑙𝑜𝑠𝑠 =0%

Simulation Result(Cont.) Single Joining Node, 𝑝 𝑙𝑜𝑠𝑠 =10%

Simulation Result(Cont.) Single Joining Node, 𝑝 𝑙𝑜𝑠𝑠 =30%

Simulation Result(Cont.) Network Setup, 20 nodes

Simulation Result(Cont.) Network Setup, 40 nodes

Simulation Result(Cont.) Network Setup, 60 nodes

Conclusion In this paper we have investigated the network formation process in 802.15.4e TSCH networks. Finally, we have defined a Model-based Beacon Scheduling (MBS) algorithm that allows network nodes to autonomously select the links to use for advertising EBs, starting from the optimal EB schedule provided by the model. Through simulations, that MBS outperforms all the other solutions present in the literature, providing a significant reduction in the average joining time of single nodes and the average network formation time.

Reference Domenico De Guglielmo; Simone Brienza; Giuseppe Anastasi, “A Model-based Beacon Scheduling algorithm for IEEE 802.15.4e TSCH networks” in 2016 IEEE 17th International Symposium on A World of Wireless, Mobile and Multimedia Networks (WoWMoM), June. 2016 , pp1-9 Domenico De Guglielmo; Alessio Seghetti; Giuseppe Anastasi; Marco Conti, “A performance analysis of the network formation process in IEEE 802.15.4e TSCH wireless sensor/actuator networks” in 2014 IEEE Symposium on Computers and Communication (ISCC), June 2014, pp1-6 IETF RFC 7554

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