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

3. 2 of 19 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

4. 3 of 19 MAC (Multiple Access Control) Controlling access to the channel Key roles Determining channel capacity utilization network delays network delays power consumption power consumption Congestion, fairness in channel usage Requirements High energy efficiency High channel utilization Low latency Reliability Scalability Robustness and adaptability to changes

5. 4 of 19 Hybrid-MAC Hybrid (CSMA+TDMA) Hybrid (CSMA+TDMA) High channel utilization and low latency, low cost High channel utilization and low latency, low cost Channel Utilization TDMA CSMAIDEAL # of Contenders

6. 5 of 19 CSMA vs TDMA CSMA (Carrier Sense Multiple Access, contention based) CSMA (Carrier Sense Multiple Access, contention based) Carrier-sensing before transmission Carrier-sensing before transmission Simple, no time synch, and robust to network changes Simple, no time synch, and robust to network changes High control overhead (for two-hop collision avoidance) High control overhead (for two-hop collision avoidance) High idle listening and overhearing overheads High idle listening and overhearing overheads TDMA (Time Division Multiple Access, scheduled based) TDMA (Time Division Multiple Access, scheduled based) Nodes within interference range transmit during different times, so Nodes within interference range transmit during different times, so collision free collision free Requires time synch and not robust to changes Requires time synch and not robust to changes Low throughput and high latency even during low contention Low throughput and high latency even during low contention Low idle listening and overhearing overheads Low idle listening and overhearing overheads

7. 6 of 19 Related works : S-MAC, T-MAC S-MAC S-MAC listen sleep listen sleep Listen Period Listen Period Sleep / Wake schedule synchronization with neighbors Sleep / Wake schedule synchronization with neighbors Receive packets from neighbors Receive packets from neighbors Sleep Period Sleep Period Turn OFF radio Turn OFF radio Set timer to wake up later Set timer to wake up later Transmission Transmission Send packets only during listen period of intended receiver(s) Send packets only during listen period of intended receiver(s) Collision Handling Collision Handling RTS / CTS RTS / CTS T-MAC T-MAC Improve the energy efficiency of S-MAC Improve the energy efficiency of S-MAC

8. 7 of 19 Related works : B-MAC B-MAC (adaptive preamble sampling) B-MAC (adaptive preamble sampling) Lightweight MAC protocol Lightweight MAC protocol LPL (Low Power Listening) LPL (Low Power Listening) CCA (Clear Channel Accessment) CCA (Clear Channel Accessment) Higher throughput and better energy efficiency than S-MAC and Higher throughput and better energy efficiency than S-MAC and T-MAC T-MAC

9. 8 of 19 Z-MAC : setup phase Neighbor discovery Neighbor discovery Periodically broadcasts a ping to its one-hop neighbors to gather its Periodically broadcasts a ping to its one-hop neighbors to gather its one-hop neighbor list one-hop neighbor list Slot assignment Slot assignment The two-hop neighbor list is used as input to a time slot assignment The two-hop neighbor list is used as input to a time slot assignment algorithm (using DRAND) algorithm (using DRAND) Local framing Local framing Each node needs to decide on the period in which it can use the time Each node needs to decide on the period in which it can use the time slot for transmission ( Period : time frame) slot for transmission ( Period : time frame)

10. 9 of 19 Z-MAC : Slot assignment using DRAND algorithm Not exceed the size of its local two-hop neighborhood(δ) 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) The running time and message complexity of DRAND is also bounded by O(δ) C D A F B C D A E B E F Radio Interference Map Input Graph C D A E B F DRAND slot assignment 0 0 1 3 2 1

11. 10 of 19 Z-MAC : Local framing The period in which can use the time slot for transmission ☞ time frame The period in which can use the time slot for transmission ☞ time frame Time frame rule Time frame rule 2 a-1 ≤ F i < 2 a -1, 2 a-1 ≤ 2 < 2 a -1 (a=2), 2 a-1 ≤ F i < 2 a -1, 2 a-1 ≤ 2 < 2 a -1 (a=2), A’s time frame : 4 (=2 a ) A’s time frame : 4 (=2 a )

12. 11 of 19 Z-MAC : Transmission control of Z-MAC Slot Ownership Slot Ownership If current timeslot is the node's assigned time-slot, then it is the Owner, and all other If current timeslot is the node's assigned time-slot, then it is the Owner, and all other neighbouring nodes are Non-Owners. neighbouring nodes are Non-Owners. Low Contention Level : Nodes compete in all slots, albeit with different priorities. Low Contention Level : Nodes compete in all slots, albeit with different priorities. High Contention Level High Contention Level If I am the Owner – take backoff = Random(T 0 ) If I am the Owner – take backoff = Random(T 0 ) Else if I am Non-Owner – take backoff = T 0 +Random(T n0 ) Else if I am Non-Owner – take backoff = T 0 +Random(T n0 ) After backoff, sense channel, if busy repeat above, else send. After backoff, sense channel, if busy repeat above, else send. Switches between CSMA and TDMA automatically depending on Contention level Switches between CSMA and TDMA automatically depending on Contention level Performance depends on specific values of T 0 and T n0 Performance depends on specific values of T 0 and T n0 From analysis, we use T 0 = 8 and T n0 = 32 for best performance From analysis, we use T 0 = 8 and T n0 = 32 for best performance Before transmitting Before transmitting Compete in the current slot only if it is the owner of the slot or a one-hop neighbor to 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 the owner of that slot A node receives an ECN message A node receives an ECN message A node sends an ECN when it experiences high contention A node sends an ECN when it experiences high contention

13. 12 of 19 Z-MAC : ECN(Explicit Contention Notification) A F E D B C discard forward Thick Line – Routing Path Dotted Line – ECN Messages C experience high contention C experience high contention C broadcasts one-top ECN message to A, B, D C broadcasts one-top ECN message to A, B, D A, B not on routing path(C→D→F), so discard A, B not on routing path(C→D→F), so discard ECN ECN D on routing path, so it forwards ECN as two-hop D on routing path, so it forwards ECN as two-hop ECN message to E, F. 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.

14. 13 of 19 Performance Evaluation Z-MAC vs B-MAC Z-MAC vs B-MAC Setup Setup Single-hop, Two-hop and Multi-hop topology experiments on Mica2 Single-hop, Two-hop and Multi-hop topology experiments on Mica2 Comparisons with B-MAC, default MAC of Mica2, with different backoff Comparisons with B-MAC, default MAC of Mica2, with different backoff window sizes. window sizes. Metrics : Throughput, Energy, Fairness Metrics : Throughput, Energy, Fairness

15. 14 of 19 Performance Evaluation : Throughput Single-hop Throughput Single-hop Throughput Z-MAC B-MAC

16. 15 of 19 Performance Evaluation : Throughput Two-hop Throughput Two-hop Throughput B-MAC Z-MAC

17. 16 of 19 Performance Evaluation : Throughput Multi-hop Throughput Multi-hop Throughput Z-MAC B-MAC

18. 17 of 19 Performance Evaluation : Fairness Two-hop Two-hop

19. 18 of 19 Performance Evaluation : Energy Efficiency Multi-hop Multi-hop Z-MAC HCL B-MAC

20. 19 of 19 Conclusion Z-MAC combines the strength of TDMA and CSMA Z-MAC combines the strength of TDMA and CSMA High throughput independent of contention High throughput independent of contention Robustness to timing and synchronization failures and radio Robustness to timing and synchronization failures and radio interference from non-reachable neighbors interference from non-reachable neighbors Always falls back to CSMA Always falls back to CSMA Compared to existing MAC Compared to existing MAC It outperforms B-MAC under medium to high contention It outperforms B-MAC under medium to high contention Achieves high data rate with high energy efficiency Achieves high data rate with high energy efficiency