1. CS 15-849E: Wireless Networks (Spring 2006) MAC Layer Discussion Leads: Abhijit Deshmukh Sai Vinayak Instructor: Srinivasan Seshan

2. Papers “An Energy-Efficient MAC Protocol for Wireless Sensor Networks” Wei Ye, John Heidemann, Deborah Estrin “The Case for Heterogenous Wireless MACs” Chung-cheng Chen, Haiyun Luo “Design and Evaluation of a new MAC Protocol for Long-Distance 802.11 Mesh Networks” Wei Ye, John Heidemann, Deborah Estrin

3. Outline Motivation MAC – Wireless Sensor Networks Heterogenous Wireless MACs MAC for Mesh Networks Take Aways Similarities and Differences Q & A

4. Motivation Last Lecture MACAW, Carrier Sense, Idle Sense Basic Terms, Algorithms Major Focus on Fairness Very Generic Special Requirements for Sensor Networks Heterogeneous Mesh Networks

5. MAC for Sensor Networks Sensor Networks Sensors, Embedded processor, Radio, Battery Ad hoc fashion Proximity, short-range multi-hop communication Committed to One or few applications MAC Protocol Energy Efficiency Scalability Accommodate network changes Fairness, Latency, Throughput and Bandwidth

6. Sensor Networks Sources of Energy Waste ? Collision Overhearing Control packet overhead Idle Listening Tradeoff of fixing these Reduction in per-hop fairness and latency. How? Message Passing, Fragment long message Why not a big concern in Sensor Networks? Application-level performance

7. Energy Saving … turn off radio. Issues? Latency In-network processing. Power consumption?

8. Related Work PAMAS Avoid overhearing among neighbors Two independent radio channels Suffers from idle listening TDMA Natural Savings Scheduling Static Piconet Periodic Sleep

9. Sensor-MAC Protocol Design Periodic Listen and Sleep Message Passing Collision and Overhearing Avoidance

10. Periodic Listen and Sleep Basic Scheme Turn off Radio, set timer to wake up, sleep Clock Drift Sync using relative timestamps Long listen period Reduce Control Overhead Sync with neighbors, exchange schedules Advantage over TDMA ? Looser Synchronization Disadvantage? Latency due to switching, RTS/CTS

11. Periodic Listen and Sleep Choosing and Maintaining Schedules Schedule Table Synchronizer Follower Listen Wait (random) SYNC Wait (random) Rebroadcast

12. Periodic Listen and Sleep Maintaining Synchronization SYNC packet Listen Interval SYNC + RTS

13. Collision & Overhearing Avoidance Collision Avoidance NAV Virtual vs. Physical Carrier Sense Overhearing Avoidance Listening to all transmissions Who all should sleep? All neighbors of sender and receiver ECABDF xx

14. Message Passing Long vs. Short Message Length Stream of Fragments, single RTS-CTS Problem? No Fairness 802.11 Methodology? Why send ACK after each fragment? Prevent hidden terminal problem

15. Implementation Rene Motes + Tiny OS Simplified IEEE 802.11 Message Passing (overhearing avoidance) S-MAC (Message Passing + Periodic Sleep) Topology used

16. Results Low performance for high loads? Synchronization overhead (SYNC packets) Latency

17. Heterogeneous Wireless MACs Basic Service Set (BSS) Careful Channel Assignment Wireless interference Limited orthogonal channels

18. Motivation Exposed Receiver – Hidden Sender data ACK S1  R1 ? data Blocked x CTS / RTS ?

19. 4-way Handshake? Hidden Receiver Exposed Sender

20. Incomplete vs. Inconsistent Channel status at sender Incomplete estimate of receiver Inconsistent at multiple competing senders Incomplete channel status == high packet loss Inconsistent channel status == unfair channel sharing

21. Intra-BSS Interference Mitigation When to use 4-way handshake? Client detecting data transmission vs. Client’s data transmission being detected Access point to initiate channel access? BSS in center Less chance of interference from other BSS

22. Inter-BSS Interference Mitigation RTR (Request to receive) RTR-DATA vs. RTS-CTS-DATA ACK in form of next RTR Stateless Approach Alternating between MAC protocols Simple Design and Implementation Low Channel Utilization

23. Fairness Why is flow 2  3 getting unfair treatment? Client 3 is exposed receiver Receiver 1 is not interfered by 2  3 How to solve it ? Switch to receiver initiated protocol Increase power levels of CTS/RTS

24. MAC for Long Dist. 802.11 Mesh Motivation Extend 802.11 for long haul Challenges Use off-the shelf hardware Low cost

25. Overview Basic Principle SynRx & SynTx

26. Design and Implementation Design decisions driven by Low cost considerations Usage of off-the-shelf 802.11 hardware Achieving SynOp Get rid of immediate ACKs Get rid of carrier sense backoffs

27. Design and Implementation (contd.) Immediate Acks Use IBSS mode of operation Convert IP unicast to MAC broadcast No ACKs for broadcast packets in IBSS mode Broadcast = Unicast since link is 1-1 ACKs can be implemented at the driver level Carrier Sensed Backoffs Make use of feature provided by Intersil Prism chipsets

28. 2P Operation on Single Link Marker acts as a token Loose Synchrony

29. 2P Operation on Single Link (contd.) Need to handle 2 scenarios Temporary loss of synchrony (loss of marker) Link recovery after failure 2P handles both using timeouts Advantages Link-resync process is quick CRC errors do not cause timeout (other than marker) …. Why ?

30. 2P Operation on Single Link (contd.) Two ends of a link get out of synchrony at the same time and timeout together …. So? They would not hear each others marker packets since both SynTx coincides … So? Repeated Timeouts … !!! Solution …? Staggered timeouts  Bumping

31. Topology Formation What are the topologies in which 2P? Bipartite ? A tree is trivially bipartite Bad in terms of fault tolerance Add redundancy but turn on only one tree at a time (Morphing) 3 Heuristics Reduce length of links used Avoid short angles between links Reduce hop-count

32. Evaluation Goal is threefold Measure impact of step by step link establishment Study effect of 2P in a large topology Study performance of TCP over 2P Link Establishment 12.9 ms for first case (delay due to bumping) 4.9 afterwards

33. Throughput

34. 2P vs TCP

35. Similarities and Differences Similarities MAC protocol implementations Extend 802.11 for a specific environment Others? Differences Deployment scenarios Energy Saving, Long haul, Heterogeneity Writing Style Others?

36. Q & A