Scheduling Enhanced Beacons in the IEEE 802.15.4e MAC for fast network formation

Scheduling Enhanced Beacons in the IEEE 802.15.4e MAC for fast network formation Elvis Vogli, Giuseppe Ribezzo, Luigi Alfredo Grieco, Gennaro Boggia Department of Electrical and Information Engineering (DEI) Politecnico di Bari, Italy Email: name.surname@poliba.it Elvis Vogli INW-Cavalese 13-17 January 2015

OVERVIEW IoT: a connected world A standardized stack for the Industrial IoT TSCH mode in the IEEE 802.15.4 e The problem of network formation Scheduling Enhanced Beacons Mathematical models Performance Evaluation Conclusions

Ericsson s Vision The vision of more than 50 billion connected devices seem ambitious today, but with the right approach, it is within reach

CISCO s Vision Size doubling every 5,32 years. The IoT was born between 2008 and 2009.

Industrial IoT

Standardized Stack for Industrial IoT

TSCH mode in the IEEE 802.15.4 e IEEE802.15.4 e Amendment to enhance and add functionality to the 802.15.42006 MAC to better support the industrial markets Time Synchronized Channel Hopping Nodes are synchronized on a common sense of time Nodes send successive packets on different frequencies using a pseudo-random hopping pattern Resistent to interferences and Multipath fading Needs Syncronization

TSCH mode in the IEEE 802.15.4 e Timeslots long enough to transmit a frame and receive the acknowledgment SlotFrame is formed from a number of timeslots which are repated in time. Absolute Slot Number (ASN) is the number of timeslots passed since the bootstrap. It is shared amon all the motes in the network

TSCH mode in the IEEE 802.15.4 e A slot-frame repeats over time Number of slots in a slotframe is tunable Each cell can be assigned to a pair of motes, in a given direction by assigninga Channel Offset (Choff) and a Slot Offset within the slotframe. There are 16 available physical channels Physical channel f= (ASN + choff ) mod 16 1 3 2 E 4 4 5

TSCH mode in the IEEE 802.15.4 e Cells are assigned according to application requirements Tunable trade-off between packets/second latency robustness and energy consumption 1 3 2 E 4 4 5

Network formation Enhanced Beacons (EB) EB are transmited in broadcast from the Personal Area Network (PAN) Coordinator to advertise the network. EB are transmitted from the Full Function Devices (FFD) once they are syncronized in the network in order to further extend the network. EB should contain the ASN and the minimal schedule requirements. A joining mote should wait to receive an EB. When an EB is received the mote extracts the ASN and syncronizes to the slotframe structure of the network.

EB Frequency Mismatch Frequency Mismatch TX EB the transmitter can send EB only in the first timeslot of each slotframe. Each time the EB will be transmitted in a different physical channel RX EB The Joining mote will be listening in one of the 16 available channels. An elevated Joining time besides meking difficult the deployment of networks with a big number of nodes can be the cause of an important energy consumption by the joining motes which before syncronisation have a Duty Cycle 100%. Solution An efficient beacon scheduling that avoids collisions among EB of neighbouring motes and advertises the network regularly in all the frequencies Hereby are propose four schemas for EB scheuling that improve the syncronisaton time with the increase of the network density.

Scheduling Enhanced Beacons Multi-slotframe It is a group of slotframes repeated in time. A mote advertising EB will be scheduled to transmit in only one cell per multislot frame Why? - transmitting in each advertisement slot will cost in terms of energy and it is not guaranteed that it will improve the joining time because there will be many collisions too. Fill the schedule Elvis Vogli Each time a new mote will join the network it will schedule the transmission of EB in one of the advertisement slots of the multi-slotframe If more than one mote schedules the transmission of EB in the same slot there will be a collision.

Lightweight Random filling Schemas Random Vertical (RV) filling A node joins the network and will transmit EBs in the first slotframe choosing a randomly one channel offset. The firs column in the figure will be filled from the scheduled slots. Random Horizontal (RH) filling - A node joins the network and will chose randomly a Slot Offset in the Multi superframe to transmit EBs using channel offset zero. The first raw of advertisement slots in the figure will be filled in the schedule.

Coordinating the scheduling schema Sensing: A joining node before scheduling the EB transmission listens to the channel. It will schedule the EB transmission only if the channel is free. Enhanced filling: In most of the realistic scenarios coordinators are more powerful and have less energy restrictions, therefore they can be scheduled to transmit in each slotframe. RV Enhanced Coordinated Vertical RH Enhanced Coordinated Horizontal

Mathematical models RV ECV RH ECV Definitions: Ts Syncronisation Time TM Multi-Slotframe length C Number of available channels N Number of syncronized motes ΠD Packet Delievery Ratio Sf Number of Slotframes within an EB period

Experimental Setup Firmware implemented in OpenWSN, an open source implementation of 6TiSCH Each joining node implements the 4 joining algorithms with a few bytes aditional code The coordinato advertises the joining algorithm to use Network scenario for the expriments

Experimental Results RV RH ECV ECH

Conclusions From these results, we can conclude that analytical models can be effectively used to correctly estimate the joining time and help designer in adequately sizing real networks. Albeit equivalent for what concerns the average joining time, RV and RH (ECV and ECH, respectively) can accommodate different architectural choices/constraints on the usage of timeslots and channels in real deployments E. Vogli, G. Ribezzo, L. A. Grieco, and G. Boggia," Fast Join and Synchronization Schema in the IEEE 802.15.4e MAC", Proc. of IEEE Wireless Commun. and Networking Conf.,WCNC, Workshop on Energy Efficiency in the Internet of Things, and Internet of Things for Energy Efficiency, E2IoT, New Orleans, LA, USA, Mar., 2015.

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