2008. 11. 11. Systems Modeling & Simulation Lab. Kim Jeong Hoon

1. Z-MAC : a Hybrid MAC for Wireless Sensor NetworksInjong Rhee, Ajit Warrier, Mahesh Aia and Jeongki Min ACM SenSys 2005 2008. 11. 11.Systems Modeling & Simulation Lab.Kim Jeong Hoon

2. Outline 1. MAC(Multiple Access Control) 2. Hybrid-MAC 3. CSMA vs TDMA 4. Related works : S-MAC, T-MAC, B-MAC 5. Z-MAC 6. Performance evaluation 7. Conclusion

3. MAC (Multiple Access Control) Controlling access to the channel Key roles Determining channel capacity utilization network delays power consumption Congestion, fairness in channel usage Requirements High energy efficiency High channel utilization Low latency Reliability Scalability Robustness and adaptability to changes

4. IDEAL Hybrid-MAC • Hybrid (CSMA+TDMA) • High channel utilization and low latency, low cost CSMA Channel Utilization TDMA # of Contenders

5. CSMA vs TDMA • CSMA (Carrier Sense Multiple Access, contention based) • Carrier-sensing before transmission • Simple, no time synch, and robust to network changes • High control overhead (for two-hop collision avoidance) • High idle listening and overhearing overheads • TDMA (Time Division Multiple Access, scheduled based) • Nodes within interference range transmit during different times, so • collision free • Requires time synch and not robust to changes • Low throughput and high latency even during low contention • Low idle listening and overhearing overheads

6. listen listen listen listen listen listen sleep sleep Related works : S-MAC, T-MAC • S-MAC • Listen Period • Sleep / Wake schedule synchronization with neighbors • Receive packets from neighbors • Sleep Period • Turn OFF radio • Set timer to wake up later • Transmission • Send packets only during listen period of intended receiver(s) • Collision Handling • RTS / CTS • T-MAC • Improve the energy efficiency of S-MAC

7. Related works : B-MAC • B-MAC (adaptive preamble sampling) • Lightweight MAC protocol • LPL (Low Power Listening) • CCA (Clear Channel Accessment) • Higher throughput and better energy efficiency than S-MAC and • T-MAC

8. Z-MAC : setup phase • Neighbor discovery • Periodically broadcasts a ping to its one-hop neighbors to gather its • one-hop neighbor list • Slot assignment • The two-hop neighbor list is used as input to a time slot assignment • algorithm (using DRAND) • Local framing • Each node needs to decide on the period in which it can use the time • slot for transmission ( Period : time frame)

9. E A C D B F E E A A D C D C F B F B Z-MAC : Slot assignment using DRAND algorithm Radio Interference Map 1 0 2 3 DRAND slot assignment 0 Input Graph 1 • Not exceed the size of its local two-hop neighborhood(δ) • The running time and message complexity of DRAND is also bounded by O(δ)

10. Z-MAC : Local framing • The period in which can use the time slot for transmission ☞ time frame • Time frame rule • 2a-1≤ Fi< 2a-1, 2a-1≤ 2 < 2a-1 (a=2), • A’s time frame : 4 (=2a)

11. Z-MAC : Transmission control of Z-MAC • Slot Ownership • If current timeslot is the node's assigned time-slot, then it is the Owner, and all other • neighbouring nodes are Non-Owners. • Low Contention Level : Nodes compete in all slots, albeit with different priorities. • High Contention Level • A node sends an ECN when it experiences high contention • Compete in the current slot only if it is the owner of the slot or a one-hop neighbor to • the owner of that slot • A node receives an ECN message • Before transmitting • If I am the Owner – take backoff = Random(T0) • Else if I am Non-Owner – take backoff = T0+Random(Tn0) • After backoff, sense channel, if busy repeat above, else send. • Switches between CSMA and TDMA automatically depending on Contention level • Performance depends on specific values of T0 and Tn0 • From analysis, we use T0 = 8 and Tn0 = 32 for best performance

12. F D C E A B Z-MAC : ECN(Explicit Contention Notification) Thick Line – Routing Path Dotted Line – ECN Messages • C experience high contention • C broadcasts one-top ECN message to A, B, D • A, B not on routing path(C→D→F), so discard • ECN • D on routing path, so it forwards ECN as two-hop • ECN message to E, F. • Now, E and F will not complete during C’s slot as • Non-Owners. • A, B and D are eligible to compete during C’s slot, • albeit with lesser priority as Non-Owners. forward forward discard discard

13. Performance Evaluation • Z-MAC vs B-MAC • Setup • Single-hop, Two-hop and Multi-hop topology experiments on Mica2 • Comparisons with B-MAC, default MAC of Mica2, with different backoff • window sizes. • Metrics : Throughput, Energy, Fairness

14. Performance Evaluation : Throughput • Single-hop Throughput Z-MAC B-MAC

15. Performance Evaluation : Throughput • Two-hop Throughput Z-MAC B-MAC

16. Z-MAC B-MAC Performance Evaluation : Throughput • Multi-hop Throughput

17. Performance Evaluation : Fairness • Two-hop

18. Z-MAC HCL B-MAC Performance Evaluation : Energy Efficiency • Multi-hop

19. Conclusion • Z-MAC combines the strength of TDMA and CSMA • High throughput independent of contention • Robustness to timing and synchronization failures and radio • interference from non-reachable neighbors • Always falls back to CSMA • Compared to existing MAC • It outperforms B-MAC under medium to high contention • Achieves high data rate with high energy efficiency