RedFixHop: Efficient Ultra-Low-Latency Network Flooding - IEEE Xplore

RedFixHop: Efficient Ultra-Low-Latency Network Flooding Antonio Escobar∗ , Cristian Gonzalez∗ , Francisco J. Cruz† , Javier Garcia-Jimenez‡ , Jirka Klaue§ and Angel Corona§ ∗ Infineon

Technologies AG, [email protected] GmbH, [email protected] ‡ Kinexon, [email protected] § Airbus Group Innovations, [email protected]

† eesy-innovation

Abstract—A Constructive Interference-based flooding mechanism - using hardware-generated ACKs - is proposed to achieve highly reliable source-to-sink communication of very short critical packets in a wireless sensor network working in harsh environments. Constructive interference is only achieved if the senders synchronize their packet transmissions with sub-microsecond accuracy. In order to achieve the required synchronization accuracy, the packet retransmissions at the relay nodes are triggered by using hardware acknowledgements. Keywords—Wireless Sensor Networks, Low-Power, LowLatency, Flooding, Constructive Interference, Hardware Acknowledgements.

I.

I NTRODUCTION

The exploitation of constructive interference in WSNs has already been demonstrated, for instance in [1], [2], [3] and [4]. It allows the simultaneous transmission of identical packets by multiple senders. However, constructive interference is only achieved if the senders synchronize their packet transmissions with sub-microsecond accuracy. For example, with an IEEE 802.15.4 radio working at 2.4 GHz, the maximum tolerable temporal displacement of distributed packet transmissions is 0.5 µs [1]. In order to improve the synchronization, we trigger the distributed packet transmissions at the relay nodes by using the hardware acknowledgements of the transceiver; the microcontroller is thus bypassed. Using a flooding mechanism, instead of dynamic routing, allows the transmission of data packets with optimal latency and minimum complexity, since no previous knowledge of the network state is used. By combining flooding with a synchronous radio duty cycling mechanism, as shown in Figure 2, energy consumption is also optimized. In order to minimize the delivery delay, packet retransmissions based on acknowledgements (ACKs) are abandoned as well. The reason behind is that in a harsh environment packets, as well as ACKs, will get lost with a certain probability, dramatically affecting retransmissions. That would only increase the delay, while not greatly improving the packet delivery chances. To increase the reliability, time redundancy will be exploited. Since the wireless channel properties are timedependant, retransmissions increase the reception probability. However, time diversity is also problematic, because energy consumption is affected and packet reception can only be modelled using probabilistic channel models.

Fig. 1.

Source-to-Sink communication with intermediate Relays

Fig. 2.

Synchronous Radio Duty Cycling Mechanism

II.

R ED F IX H OP

The protocol exploits flooding with simultaneous transmissions triggered by hardware-ACKs, exploiting constructive interference, and a fixed number of repetitions. From this spatial redundancy and fixed repetition scheme, the name RedFixHop is derived. The flooding mechanism uses synchronous radio duty cycling based on periodic active bursts (Figure 2). Between the flooding bursts, the radio of the nodes can be kept off to save power. Every node in the network get synchronized after its first packet reception, since every transmitted packet contains enough information to be located inside the burst period. How RedFixHop works (Figure 3): 1) 2) 3)

Every node is synchronized to turn its radio on at the beginning of the flooding period. The transmission source sends the packet. All forwarding nodes (relays) receiving the original packet from the source transmit it immediately using hardware acknowledgements; potentially interfering constructively or exploiting the capture effect.

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R EFERENCES [1]

[2]

[3]

Fig. 3.

RedFixHop Protocol [4]

4)

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Back to Step 2 until a predefined number of burst repetitions to increase reliability and ensure that all the potential receivers in the range of the source, are synchronized and cooperating to generate the constructive interference. When the repetitions are over, the radios switch off until the beginning of the next flooding cycle.

The accurate time synchronization needed for the constructive interference to work is achieved using the automatically generated hardware-ACKs from the radio transceivers. This way, the retransmissions are triggered simultaneously, independently of the tasks running in the nodes and avoiding errors due to different clock sources. For example, IEEE 802.15.4 2.4 GHz radio transceivers usually implement the automatic sending of an ACK after 12 symbol periods following the end of the received frame. This mechanism limits the maximum payload of the transmitted packets to 1 byte -including the bits allocated for the counter field-, since the information must be encoded in the Data Sequence Number field of the MAC Header. III.

D EMO E XPLANATION

The demo shows the deterministic low-latency and high reliability achieved using RedFixHop. The source node has a button attached, while the sink node controls a light, quickly following the state of the button. Relays are necessary, as the sink is not within communication range of the source. Relays can be freely turned on and off, without affecting the smooth operation of the protocol. They join and leave the network seamlessly, with minimum complexity, since no previous knowledge of the network state is used. At the beginning of every flooding period, the state of the button is propagated through the network, reaching the sink with optimal latency, independently of the number of connected relays. RedFixHop effectiveness was proved at the Dependability Competition of the International Conference on Embedded Wireless Systems and Networks (EWSN 2016), achieving the first place [5].

[5]

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