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Network Coding Schemes for Underwater Networks The Benefits of Implicit Acknowledgement

Network Coding Schemes for Underwater Networks The Benefits of Implicit Acknowledgement. Daniel E. Lucani, Muriel Médard, Milica Stojanovic Massachusetts Institute of Technology. Introduction. Acoustic underwater communications [1]: Low propagation speed: High transmission delays

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Network Coding Schemes for Underwater Networks The Benefits of Implicit Acknowledgement

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  1. Network Coding Schemes for Underwater NetworksThe Benefits of Implicit Acknowledgement Daniel E. Lucani, Muriel Médard, Milica Stojanovic Massachusetts Institute of Technology

  2. Introduction • Acoustic underwater communications [1]: • Low propagation speed: High transmission delays • Trade-off: probability of collision and transmission delay • Path loss dependent on transmission distance and signal frequency: Bandwidth determined by distance • Random fading channel with high packet erasures • Application: fixed acoustic sensor networks • Battery-powered devices • Expected to operate for long time • New network coding method: relies on implicit acknowledgements to achieve best performance in terms of power consumption and transmission delay for all loads • Minimize power consumption: Reduce transmissions per packet • Trade-off: Minimize transmission delay [1] Stojanovic, M., ”On the Relationship Between Capacity and Distance in an Underwater Acoustic Communication Channel”, in Proc. WUWnet ’06, pp. 41-47, Los Angeles, Sept. 2006

  3. Coded Code Data Coded Code Data Network Coding • Originally developed for wired networks [2] • Nodes can perform mathematical operations on packets • Routing is particular case of network coding: forwarding and replication • Simple, powerful: Linear combination of packets [3, 4] • Random linear combination of packets [5] • Distributed computation • Good performance in erasure channels Node Packet A Packet B λPacket A μA+ρB λ μ, ρ Packet A Packet B [2] Ahlswede, et al, ”Network Information Flow”, IEEE Trans. Inf. Theory, pp. 1204-1216, Jul. 2000 [3] S.-Y. R. Li, et al, “Linear network coding”, IEEE Trans. Inf. Theory, pp. 371–381, Feb. 2003. [4] R. Koetter and M. Médard, “An algebraic approach to network coding,” IEEE/ACM Trans. Networking, vol. 11, no. 5, pp. 782–795, Oct. 2003. [5] T. Ho, et al,“A random linear network coding approach to multicast,” IEEE Trans. on Info. Theory, vol. 52, no. 10, pp. 4413- 4430, October 2006.

  4. Network Coding • Degree of freedom (dof): Number of independent equations used to generate packet • Good match for underwater acoustic communications: • Good performance in channels with high packet erasures • Outperforms routing in wireless scenario for number of transmissions per packet and delay [6, 7] • Expect reduced power consumption • Expect low transmission delay • Distributed computation of codes at each node: topology independent • Previous work in [8] • End-to-end packet loss and total transmissions (No retransmission) • Simplified channel: link erasure probability [6] Lun, D. S., et al,”Minimum-Cost Multicast Over Coded Packet Networks”, IEEE Trans. on Info. Theory, vol. 52, no. 6, pp. 2608-2623, Jun. 2006 [7] Lun, D. S., et al, ”Network Coding for Efficient Wireless Unicast”, In Proc. IEEE International Zurich Seminar on Communications 2006, pp. 74-77, Zurich, Feb. 2006 [8] Z. Guo, P. Xie, J. H. Cui and B. Wang. "On Applying Network Coding to Underwater Sensor Networks", In Proc. of WUWNet '06, pp. 109-112, Los Angeles, Sept. 2006

  5. Channel Model • Attenuation: With the spreading factor and Thorp’s formula • Noise: is the power sprectral density (p.s.d) decays with frequency at approximately 18dB/dec • SNR[1]: With optimum bandwidth for distance power to achieve SNR level for given . [1] Stojanovic, M., in Proc. WUWnet ’06, pp. 41-47, Los Angeles, Sept. 2006

  6. Channel Model • SNR changing distance but keeping B(l): • Equivalent bit SNR: where • Erasure probability: computed using this with PSK bit error probability, assuming fast channel decorrelation and fixed packet size

  7. R2 R2 High SNR Low SNR R1 R1 MAC Model • Previous work includes various MAC protocols [9], e.g. CSMA, polling, CDMA, TDMA, FDMA • CSMA/TDMA/Polling: latency compromises usefulness • FDMA: reduction in bandwidth with • CDMA: high SNR limits performance. Also, difficult to do effective power control • Most modems (designed for point to point communication) support one-way polling or fixed transmission assignment • Simulations: using last assumption. Transmissions occur every time T [9] Kilfoyle, D. B. and Baggeroer, A. B. ” The State of the Art in Underwater Acoustic Telemetry”, IEEE Journal of Oceanic Engineering,vol. 25, no. 1, Jan. 2000

  8. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Coded Pq. Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code Code MAC Model Routing Node 1 Node 2 Node 3 Data Data Data Data Data Network Coding with Implicit ACK Network Coding Rateless Fashion Node 1 Node 2 Node 3 ACK ACK

  9. D1 D2 D3 Node 3 Node 1 Node 2 Node 4 Simulation Results • Delay (measured in T units) and power consumption to transmit N packets from Tx node to Rx • Packets generated with probability Psource each T time • Fixed number of packets • Network Coding: • Better delay performance • Rateless fashion: poor power consumption performance in low Psource

  10. Simulation Results • Network coding with implicit acknowledgement: • Best power consumption performance over all Psource • Best delay performance over all loads • Approximates opportunistic routing with link-by-link ACK as load decreases • Cannot take advantage of linear combination Delay: High Psource Power: Low Psource

  11. Simulation Results • Increasing network size network coding with implicit acknowledgement: • Improves power consumption per node • Gives lowest delay per added node • Similar results to [7]. However, [7] does not consider implicit acknowledgement method [7] Lun, D. S., et al, In Proc. IEEE Inter. Zurich Seminar on Comms. 2006,Zurich, Feb. 2006

  12. Conclusions • Conventional routing schemes have limitations: power consumption and delay • Why network coding? Good match to underwater acoustic scenario: • Good performance in channels with high packet erasures • Outperforms routing in wireless scenario in terms of number of transmissions per packet and delay • Network coding with implicit acknowledgements: • New proposed method: use transmitted data packets as implicit ACK for upstream nodes • Has best overall performance: transmission delay and power consumption

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