1. Voice over the Dins: Improving Wireless Channel Utilization with Collision Tolerance Xiaoyu Ji Xiaoyu Ji, Yuan He, Jiliang Wang, Kaishun Wu, Ke Yi, Yunhao Liu ICNP, 2013, Göttingen Tsinghua UniversityHong Kong University of Science and Technology

2. Motivation 2 Collisions in wireless networks – Broadcast nature and lack of collision detection – Event-driven mode (WSNs) Network performance, e.g., channel utilization is harmed Researchers have designed a lot of medium access control protocols (MAC) – A-MAC, B-MAC, …, Z-MAC

3. The long-held philosophy Collision avoidance – Avoid simultaneous access from multiple users to a shared channel – Conservative strategy against collisions – Side effect: idle slots, as a result of random backoff 3 Collision avoidance limits the wireless channel to be better utilized!!!

4. Why not more aggressive? Collision tolerance – Allowing collisions to happen, i.e., the overlap of signals in time domain – Potential gain in channel utilization 4 AB CD R A B B C D B A C B D C B C C B D C D B A A B D B D C B B C D B A C B 4 senders with a common receiver ABD CBD A Tolerance Avoidance Collision tolerance enables one to approach the upper limit of channel utilization!

5. Formulation Np Suppose N senders choose probability p to transmit to the receiver, then three possible states for any given time slot: – Idle – Successful – Corrupted And the utilization ratio: 5

6. The potential gain For backoff-based approaches, the success probability is ： The simulation results CT- Collision tolerance CA- Collision avoidance η: parameter related to T s, T c, T slot 20% improvement in general case! 6

7. Feasibility of collision tolerance PHY layer implementation – Redundancy – Stronger signal dominates the weak ones – Capture effect 1,2, message in message (MIM) 3 Therefore, even packets collide, the strongest one could still be correctly received under certain conditions! 7 1, The capture effect in FM receivers, KRIJN LEENTVAAR and JAN H.FLINT, IEEE TOC, 1976 2, Sniffing out the correct physical layer capture model in 802.11 b, MLA Kochut, Andrzej, et al., ICNP, 2004 3, Order Matters: Transmission Reordering in Wireless Networks, Justin Manweiler etc, Mobicom, 2009

8. Investigating collision tolerance (1/2) Experiment setup – Indoor, TelosB nodes, Contiki-OS – 2 senders with a common receiver 8 1. Timing requirement 160μs Case 1: Strong first. The receiver receives the strong signal without knowing the weak ones. Case 2: Weak first, strong coming within the preamble window of the weak ones. Case 3: Weak first, strong coming out of the preamble window of the weak ones. Strong Weak P P P P Case 1 P P Case 3 P P Case 2

9. Investigating collision tolerance (2/2) 9 2. Concurrency requirement C(k): concurrency of 2, 3 even 4 are beneficial! Especially, the collision probability with concurrency 2 is very small. All packets should come within 160μs; Concurrency should be proper. All packets should come within 160μs; Concurrency should be proper.

10. Protocol overview (Coco) Timing requirement – ACK triggers the transmission Concurrency requirement p – Transmitting with probability p 9

11. ACK-triggered transmission ACK – Triggering transmissions (synchronization) p – Piggybacking the probability p – Coping with hidden terminals The offset can be controlled within 1 μs as shown in the evaluation. 10

12. p Transmitting with p p Optimum: How to decide whether p is proper or not? Adjustment: If not, how to adjust it? Convergence: How well can the adjustment converge? Errors: The error involved in the adjustment. 12

13. Optimum: Is p proper? The utilization is a function of N and p, and under some N, the best p should be: p Claim: when p is proper, which means Util(p) achieves its best value, P c, P i, P s and Util(p) converge when N  ∞ The criteria 12

14. Adjustment of p The feedback control algorithm Sliding window: recording status of past packets, to calculate P c The dichotomous algorithm 13

15. Convergence speed of p p p is in the range of [0:1] The resolution is 0.001 when N=20 (Tab.1) p It takes at most 10 iterations to regulate p from initial 1 value to 0.001. With 100 data packets in each iteration, a maximum 4.096 ms for each packet, the maximum time is 4.096 s 14

16. Error in the process The error introduced by ε: ΔUtil(p) For different ε, we calculate the errors: 15

17. Evaluation Metrics – The accuracy of timing – The adjustment algorithm – Overall performance Setup – 21 TelosB nodes, Contiki-OS – Single hop, single receiver 16

18. Timing accuracy 11 senders with a common receiver The offset between the tested sender and a reference sender at receiver side 18 The offset among senders can be as small as nano seconds!

19. The feedback control algorithm With N changing dynamically, p can converge to its best value quickly! 18

20. Overall evaluation (1/2) Comparing with B-MAC, default in Tiny-OS Performance evaluation with different network parameters Parameter EnvironmentIndoor: Hall, Testbed Outdoor: ground Packet length20, 60, 100 (bytes) TopologyLine, circle random 19

21. Overall evaluation (2/2) Packet length: longer packets are better. Topology: resistance to hidden terminals. Compared with B-MAC (linear and exponential) Avg. 20% improvement in general case. 20

22. Summary Question the avoidance principle and improve channel utilization with collision tolerance Investigate the sufficient condition in achieving beneficial collision tolerance Design a protocol to exploit collision tolerance for improving channel utilization Real implementation and extensive evaluation 21

23. THANK YOU! Xiaoyu JI xji@cse.ust.hk 22