Dissertation Defense Enhancing the Reliability of Medium Access Level Wireless Multicast

1. Dissertation DefenseEnhancing the Reliability of Medium Access Level Wireless Multicast By: Vikram Shankar Committee: Dr. Sandeep Gupta Dr. Goran Konjevod Dr. Arunabha Sen Dr. Cihan Tepedelenlioglu

2. Presentation Outline • Motivation • Reliable Multicast Problem • System Model • Fundamental Problems with Wireless Multicast • Multicast Hidden Terminal Problem (MHTP) • Feedback Implosion Problem (FIP) • Problem of increasing error probability • Analytic Results • Multicast Reliability Analysis • Multicast Throughput Analysis • Multicast Delay Analysis • Proposed Protocols • The Improved Leader Based Protocol (LBP-I) • Tone Based Protocol (TBP) • Multi-channel Multicast Feedback Protocol (MMFP) • Simulations Results • Conclusions and future work Preliminaries

3. Motivation • Reliable multicast has applications where the same information must be reliably communicated to a group: • Smart classroom (dissemination of presentation slides, tests etc.) • Impromptu distributed gaming (e.g. ad hoc group waiting at airport) • Multicast Reliability is a well studied problem at • Transport Layer – provides end-to-end reliability. • Network Layer – careful routing reduces packet drops. • We are interested in Multicast Reliability at the MAC: • Takes advantage of wireless broadcast medium • Improves link quality as perceived by higher layers. • Improves end-to-end packet delivery. • Lower cost compared to end-to-end recovery. • Packet Loss – Why we need MAC reliability • Channel noise, interference, collisions • Methods of MAC Reliability • Forward Error Correction Coding • Automatic Repeat Request (ARQ) Preliminaries Smart Classroom

4. Motivation • Several reliable multicast MAC protocols proposed in literature. • LBP, DBP, PBP [KK99] • BMMM [Sun02], BMW [TG01], RMAC [SL04] • Protocols not very effective for various reasons: • Not scalable • Too much packet delay • Poor throughput efficiency • Not reliable! • Unfairness of reliability between members of a group. • Lack of comprehensive research on the effect of MAC protocol design choices on: • Reliability – most work are at network and transport layers. • Tradeoff between reliability, throughput and delay. Preliminaries

5. Components of ARQ-Based MAC Multicast Reliability MAC Multicast Problem Idle Channel Fundamental Problem 1: Multicast Hidden Terminal Problem Wait for Idle channel Reserve Channel Problem Statement New Data Channel Access Failure Wait for Data from higher layers Channel Acquired Failure Abort Success Fundamental Problem 2: Probability of transmission failure increases with group size Exchange Feedback Exchange Data Send Data Fundamental Problem 3: Feedback Implosion Problem

6. Contributions of this Research Presents a comprehensive theoretic, analytic and simulative study of MAC level reliability and its cost in terms of throughput and delay. • Theoretical study investigates the three fundamental problems. • Study assumes packet corruption only due to collisions. • Analytic study examines the tradeoff between reliability, throughput and delay. • Error model relaxed to include packet corruption due to channel noise. • Two new reliable multicast protocols that serve different application requirements are proposed. • Tone Based Protocol: is scalable to large group sizes. • Multi-channel Multicast Feedback Protocol: tracks the receive status of individual members. • Improvements to Leader Based Protocol. • Extensive simulations on ns-2 compares proposed protocols against LBP and BMW. • Error model accounts for distance between stations, capture, interference, channel noise and station mobility. • Results confirm conclusions of theoretic and analytic studies. Preliminaries

7. Interaction of MAC with other components • Internet Group Management Protocol • Manages group membership. • Detects station migration via query broadcast. • Multicast Routing Protocol • Neighbor Discovery • Route Discovery Network System Model Logical Link Control Sub-layer Queuing Address Resolution Protocol Data Packet with MAC Address • MMG Group Management • Membership • Leader Selection • Group Timers • MAC Sub-layer Management Entity • Association/Disassociation • Synchronization • Security • Beaconing Data Link MAC Sub-layer Channel Access Control MHTP Prevention Local Error Recovery FIP Prevention RSSI CCA Bytes Enable/Disable Detect Tone • Additional Requirements for TBP • Tone Generation and Detection • Sub-channel assignment PLCP PHY • PMD • Modulation/Demodulation • Start/Stop Symbol Detection Physical Layer Management Entity RF Signal

8. Assumptions and Definitions Radio Model Network Multicast Group • Two distinct level of Multicast groups • MAC Multicast Group (MMG) • Network Multicast Group (NMG) • Group membership is constant during a session. Definitions • Gs – Set of stations in the multicast group of interest. Gs = {1,2,3,4}. • Cs– Set of stations in the Coverage Area of the Source S. Cs = {1,2,3,4,6}. • NGS– Set of stations that don’t belong to Gs but are in the Coverage Area of at least one member of {Gs U S}. NGS = {5,6,7,8,9}. • BX– Set of stations blocked from transmitting during a multicast exchange by a method X. • Half duplex radios with bidirectional links. • Signal power fades as a function of distance. • Capture effect: Stronger signal can completely mask a weaker signal if the difference in power is greater than a threshold. MMG A System Model Multicast Group Model MMG B Coverage Area Source A Out of Range A B 5 1 2 9 S 12 6 10 11 3 8 7 4

9. Multicast Hidden Terminal Problem (MHTP) Condition 1 (MHTP Prevention): MHTP is averted by method X if and only if every member of NGS is a member of BX. That is NGS is a subset of BX. Condition 2 (Optimality of MHTP Solution): We say X, a solution to MHTP, is optimal w.r.t. blocking if and only if the set of blocked stations BX does not include stations other than those in NGS. That is NGS = BX. Theoretic Study 1 2 5 9 Member S 12 10 6 11 Interfering Station Non-Interfering Station 4 3 8 7

10. Illustration of Bandwidth Wastage by 2-hop Blocking MHTP Solutions B’s Interference Region A’s Interference Region C5 Theoretic Study C3 C1 C2 A B A’s Transmission Region C4 B’s Transmission Region Communication C1 between stations A and B prevents communications C2, C3, C4 and C5.

11. MHTP Solutions • Strategy I: One-Hop Blocking by Source (e.g. IEEE 802.11 multicast) • Strategy II: One-Hop Blocking by Source and Representative Member (e.g. BMW, LBP, DBP, PBP, [Tang], [Sheu]) • Strategy III: Two-Hop Blocking by Source • Strategy IV: One-Hop Blocking by Source and All Members (e.g. RMAC, TBP) • Strategy V: Two-Hop Blocking by Source and one or more Members (e.g. LBP-I) Theoretic Study 5 1 2 9 12 S 6 11 10 4 8 3 7 Interfering Station Non-Interfering Station Member

12. Strategy IV: One-Hop Blocking by Source and All Members Theorem 4A: Strategy IV prevents MHTP. Theoretic Study Theorem 4B: Strategy IV is blocking optimal. 5 1 9 2 Member 6 12 S 10 11 Interfering Station Non-Interfering Station 8 7 4 3

13. Feedback Implosion Problem • Must obtain feedback from multiple members. • Scalability problem if members reply one after the other. • Feedback collision problem if members reply together. • Incomplete feedback information if only a subset of members provide feedback. Theoretic Study Round robin feedback is not scalable. E.g. BMMM, RMAC Representative 1 2 Concurrent feedback results in collisions. E.g. Probability Based Protocol Feedback from only a representative says nothing about status of other members. E.g. Leader Based Protocol ACK 1 ACK 2 ACK ACK 4 ACK 3 4 3

14. Feedback Implosion Problem • Solutions studied • Positive Group Feedback (e.g. LBP, DBP, PBP [KK99]) • Positive Individual Feedback (e.g. BMMM [Sun02], RMAC [SL04] , BMW [TG01], [Tang], [Shue]) • Negative Group Feedback. • Negative Individual Feedback. • Concurrent (e.g. TBP) • Polling (BMW [TG01]) • Choice of solution depends on our application requirements • Scalability? • Feedback from individual members? Theoretic Study NAK from here will destroy ACK NAK from here will be masked by ACK 2 1 3 Multicast Source Leader sends ACK Distance Problem with Group Feedback Problem with Individual Feedback

15. Frame Control Duration Receiver Address Transmitter Address Data Sequence Number CRC New Field (2 bytes) Increasing Error Probability • Let, n = number of members in group Ldata = length of a control packet (in bits) Pb = Probability of bit error. • Probability that at least one member receives packet in error = • A fraction of members would have received data correctly for each transmission attempt. • Reduce group size by excluding members that have correct data. Theoretic Study RTS Frame Format:

16. Multicast Protocols • Most protocols use control packets for channel reservation • Request-To-Send (RTS) from source • Clear-To-Send (CTS) from members • If the channel is busy or collision occurs, back off • Each sender starts a timer that expirees at a random time • Transmit if channel is free when timer expires Clear to Send (CTS) Request to Send (RTS) Clear to Send (CTS)

17. Random Back-off Random Back-off RTS Data Source Data RTS RTS Leader CTS CTS ACK CTS Other Members NCTS Leader Based Protocol (LBP) Improved Leader Based Protocol (LBP-I) • Better group management: • Maintain a (partial) list of group members. • If Leader does not respond, select new leader from list without dissolving group. • Protocol optimizations: • Leader information carried in RTS. • Data Sequence Number in RTS. • Result: • Better average PDR. • Feedback still susceptible to capture effect. • PDR of individual members varies drastically (unfair). Multicast Protocols

18. Tone-Based Protocol (TBP) • Salient Features: • Purely NAK-based. Feedback indicated by channel state. • MHTP solved using Busy Tones. • Result: • Protocol scalable with group size. • No indication whether members received data correctly. • Can be compensated by end-to-end signaling by higher layers. Multicast Protocols NAK Response Time (NRT) Random Back-off NRT Random Back-off Source Data Data RTS RTS RTS Member 1 NCTS Member 2 Busy Tone Busy Tone Receive Timeout NAK Member 1 receives RTS in error Member 1 receives Data in error Data exchanged successfully

19. Multi-channel Multicast Feedback Protocol (MMFP) • ACK-based protocol. • Takes advantage of technologies such as OFDM and MIMO. • Clear indication of how many and which members received data correctly. • Not scalable with group size. Multicast Protocols Random Back-off Random Back-off RTS RTS Data Source Data Member 1 CTS CTS ACK ACK CTS Member 2

20. Performance Metrics • Average Packet Delivery Ratio (PDR): is the ratio of the average number of data packets delivered successfully to each MMG member to the total number of data packets generated in the multicast session. • Fairness of Reliability (σ): is measured by the standard deviation of the PDR. σ = • Throughput Efficiency (η): is the ratio of the actual saturation throughput to the channel capacity. η = S/C • Average Packet Delay: is the expected time that elapses between the moment a data packet is received at the MAC from higher layers to the time the data is successfully transmitted, or dropped. Analytic Study

21. Observation Window RTS DATA DATA Error RTS Error Channel Idle Feedback Feedback Error Throughput Analysis • Saturation Throughput = (Throughput efficiency)*(Channel Capacity) i.e. S = ηC • Throughput Efficiency(η) = • Useful Time = PSUCCTDATA • Total Time = PSUCCTSUCC+ PIDLETIDLE + PCOLLTCOLL + PRERRTRERR + PDERRTDERR Analytic Study

22. Throughput Analysis Analytic Study

23. Throughput Analysis • Throughput reduces with an increase N, the number of members. • Throughput reduces with an increase in error probability. • Threshold k is the number of members that must acknowledge the data for a transmission to be considered successful. Analytic Study

24. Delay Analysis • Three possible outcomes for any transmission: • RTS is in error • RTS is correct but data in error • Transmission is successful Markov Model Analytic Study Delay at the end of the ith transmission attempt is given by: where, Δi-1 – Cumulative delay until (i-1)th retransmission attempt, δre(i) – Delay due to RTS error in ith attempt, δde(i)– Delay due to Data error in ith attempt, δs(i)– Delay due to success in ith attempt P[RTS Exchange Failure] Number of members involved in the ith retransmission attempt is given by: P[Data Exchange Failure] P[Success]

25. Delay Analysis

26. Delay Analysis • Members that receive data correctly must not participate in subsequent retransmissions for that packet. • Delay is a function of group size. • Delay is a function of received SNR. Analytic Study

27. Reliability Analysis Channel Bit Error Rate vs. Required Retransmissions (For Packet Size = 512 Bytes) Data Packet Size vs. Required Retransmissions (For Bit Error Rate = 10-4) Analytic Study

28. Recommendations for Multicast Protocol Design • MHTP prevented by using 1-hop blocking mechanism by source and ALL members. • Choice of feedback depends on application requirement: • Positive Individual not scalable. • Negative Individual does not provide fine-grained feedback information to higher layers. • Prohibit members that already received data correctly from successive retransmissions. • Carry data sequence number in RTS. • Keep data packet size small. • Reduce the number of control packets exchanged.

29. Simulator Set-up • Implemented packet capture • Ns-2 default implementation captures only first of concurrently arriving packets. • New implementation captures the strongest packet that arrives within the PLCP header reception time of the current packet. • Uses the most commonly adopted capture model. • Bit Error probability consistent with BPSK modulation. • Noise includes: channel noise, interference. Both assumed to be white. • Signal attenuates according to: • Frii’s propagation model in near-region of transmitter; • Two-Ray Ground Model in the far region. • Constant Bit Rate (CBR) data traffic. Simulation Results

30. Protocols Compared • Leader Based Protocol (LBP) • One-hop blocking by source and representative. • Positive Group Feedback (ACK only from representative). • Improved Leader Based Protocol (LBP-I) • Two-hop blocking by source and representative. • Same feedback mechanism as LBP. • Improved group management. • Tone Based Protocol (TBP) • One-hop blocking by source and all members. • Negative Individual - channel state indicates feedback. • Multi-channel Multicast Feedback Protocol (MMFP) • One-hop blocking by source and all members. • Positive Individual Feedback. • Broadcast Medium Window (BMW) • One-hop blocking by source and representative. • Feedback similar to multiple Go-back-N unicasts.

31. Main Results • MHTP prevention necessary • LBP has lower Packet Delivery Ratio (PDR) compared TBP and LBP-I. • Positive Individual feedback not scalable. • BMW and MMFP face excessive delay. • Packet loss due to queue overflow. • Positive Group feedback not effective • LBP and LBP-I have lower PDR because of capture effect. • Negative Group feedback is scalable • TBP better handles large group sizes. • Negative feedback does not provide information on individual members. (e.g. TBP, LBP, LBP-I) • Reliable protocols provide lower PDR than less reliable protocols when channel conditions deteriorate due to queue overflows. Simulation Results

32. Reliability in WLANs Received SNR 7 dB • TBP provides 100% PDR for SNR 7 and higher. • PDR of BMW affected due to queue overflows at SNR 7 and below. • PDR of BMW and TBP affected due to queue overflow at SNR 5 and below. Simulation Results Received SNR 5 dB Received SNR 9 dB

33. Reliability in Ad Hoc Networks • PDR of LBP suffers due to MHTP. • PDR of LBP and LBP-I reduced due to capture effect. Simulation Results • Two points to note about fairness: • Lower value of σ indicates better fairness. • Amount of σ scatter indicates the dependence of fairness performance on network topology. • TBP, MMFP and BMW are consistently fair. • LBP and LBP-I are not fair. Their fairness depends on network topology.

34. S1 S2 Throughput Performance • Two-hop blocking reduces throughput of LBP-I. • LBP source transmits much more packets than LBP-I and TBP. However, a large fraction of packets are lost. • LBP has lower throughput in ad hoc mode due to collisions (MHTP) • BMW does not always harness multicast advantage in ad hoc environments. • Throughput of MMFP is reduced due to its longer feedback time. S4 S5 Simulation Results S1 S2 S3

35. Reliability-Throughput Tradeoff • Reliability comes at the cost of throughput. • Throughput depends on: • Reliability, in terms of k – the minimum number of members that must receive data correctly. • Number of members in the group – larger groups provide better diversity. • Improving throughput by reducing k will result in higher unfairness of reliability.

36. Delay Performance • Delay depends on: • Reliability, in terms of minimum number of members that must receive data correctly. • Number of members in the group – larger groups provide better diversity. • Lower delay comes at the cost of fairness of reliability. Simulation Results • For SNR 7 and above, all group feedback protocols have similar performance. • BMW always has a significantly higher delay compared to other protocols. • For SNR 5, TBP and BMW suffer far more delay than LBP.

37. Conclusions • MAC-level reliability can significantly improve packet delivery ratio for a MMG. • MHTP must be prevented for: • Higher reliability • Better fairness of reliability • Complete MHTP prevention possible using 1-hop blocking by source and all members. • Probability of retransmission may be reduced by prohibiting members that receive data correctly from successive retransmissions of that data. • No protocol suitable for all applications: • scalability versus feedback certainty • reliability versus throughput • reliability versus delay • Reliability must be traded off for throughput and delay.

38. Future Work • Cross-layer optimization of error-recovery. • Application dependent dynamic adaptation of reliability, throughput and packet delay. • let application decide what it wants • adapt parameters to achieve target • Starvation prevention in centrally located multicast groups. Thank You!

39. References

40. 5 1 2 9 12 S 6 11 10 4 8 3 7 Strategy I: One-Hop Blocking by Source Theorem 1: Strategy I does not completely prevent MHTP MHTP Prevention Member Interfering Station Non-Interfering Station Blocked Station Unnecessary Block

41. Representative 5 1 2 9 12 10 S 6 11 4 8 3 7 Strategy II: One-Hop Blocking by Source and Representative Member Theorem 2: Strategy II does not completely prevent MHTP MHTP Prevention Member Interfering Station Non-Interfering Station Blocked Station Unnecessary Block

42. 5 9 1 2 S 10 12 11 6 4 8 7 3 Strategy III: Two-Hop Blocking by Source Theorem 3A: Strategy III prevents MHTP MHTP Prevention Theorem 3B: Strategy III is not blocking optimal Member Interfering Station Non-Interfering Station Blocked Station Unnecessary Block

43. 5 1 2 9 6 12 10 S 11 7 8 3 4 Strategy V: Two-Hop Blocking by Source and one or more Members Theorem 5A: Strategy V prevents MHTP. MHTP Prevention Theorem 5B: Strategy V is not blocking optimal. Member Interfering Station Non-Interfering Station Blocked Station Unnecessary Block

44. Positive Individual (PI) Feedback • Polling • Control packet overhead increases linearly with group size. • Source must maintain state information for each member. • E.g. BMMM, RMAC Feedback Mechanisms Let n = number of members in group; LCP = length of a control packet (in bits); Pb = Probability of bit error. Control packet overhead is nLCP. Probability of at least one control packet being in error =

45. NAK from here will destroy ACK NAK from here will be masked by ACK 2 3 1 Multicast Source Leader sends ACK Distance Positive Group (PG) Feedback • Positive Feedback by Representative Member • Only one member sends an ACK. • Other members send NAK if packet in error. • ACK can mask NAK due to capture effect. • E.g. LBP • Feedback through random delays • All members start a random timer • Member transmit on timer expiry; other members cancel their timers on receiving feedback. • All members must be within range severely restricting cell size. • Feedback collision possible. • E.g. DBP • Probabilistic Feedback • Each member transmits an ACK only with a certain probability. • Feedback collision likely. • E.g. PBP Feedback Mechanisms

46. Negative Individual (NI) Feedback • Polling • Control packet overhead increases linearly with group size. • Concurrent • Stations that receive packet in error will transmit NAK with probability 1. • Busy channel during feedback period considered as a NAK. • NAK collisions likely to occur. Feedback Mechanisms

47. Negative Group Feedback • Feedback by Representative • Difficult to select Representative – we don’t know a priori which members will receive packet in error. • Impractical. Feedback Mechanisms

48. Summary of Results • TBP: • Provides best PDR for SNR 7 and over. • Does not provide information on individual members. • Reliability is fair to all members; fairness is consistent. • LBP-I: • Provides best PDR for SNR 5 and lower. • Does not provide information on individual members. • Reliability of feedback affected by capture. • Is not throughput efficient in ad hoc networks. • LBP: • Provides best PDR for SNR 5 and lower. • Does not provide information on individual members. • Reliability of feedback affected by capture. • Reliability affected by MHTP. • BMW: • Suffers from high packet delay. • Reliability affected by queue overflow. • Obtains feedback information from individual members.

49. Station 1 Station 1 APP APP Stable Connection Source Destination NET NET Good Link MAC MAC Local Error Recovery PHY PHY Poor Channel Conditions Source Destination Motivation • Multicast Reliability at the MAC: • Improves link quality as perceived by higher layers. • Improves end-to-end packet delivery? • Lower cost compared to end-to-end recovery. • Takes advantage of wireless broadcast medium Preliminaries