Study on Window-Based Reliable Multicast Protocols for Wireless LANs

1. Study on Window-Based Reliable Multicast Protocols for Wireless LANs Huei-Wen Ferng, Ph.D. Assistant Professor Department of Computer Science and Information Engineering (CSIE) Nation Taiwan University of Science and Technology (NTUST) Wireless Communications and Networking Engineering (WCANE) Lab E-mail: hwferng@mail.ntust.edu.tw

2. Introduction Description of the proposed protocols Performance study and numerical examples Conclusions Outline NTUST/WCANE Lab

3. Unicast vs. Multicast Wired vs. Wireless Unreliable vs. Reliable We deal with the issue of incorporating the reliability into the multicast of wireless LANs. Two major problems: ACK/NAK implosion and media access Introduction (1/2) NTUST/WCANE Lab

4. Introduction (2/2) • Existing approaches (Kuri and Kasera [6]): • Delay feedback-based protocol (DBP) • Probabilistic feedback-based protocol (PBP) • Leader-based protocol (LBP) • Based on LBP, we further propose • LBP with a sliding window (LBPW) • LBP with a sliding window and n-fold acknowledgement reduction (LBPR(n)) • To achieve reliability, automatic repeat request (ARQ) is applied in this paper. NTUST/WCANE Lab

5. Introduction Description of the proposed protocols Performance study and numerical examples Conclusions Outline NTUST/WCANE Lab

6. Scenario • Basic network architecture: AP and several mobile hosts • We split the communication link into • Sender to APs • An AP to group members (GMs) • Merits of such an arrangement • Scalability • Local error recovery • LBPW and LBPR(n) are designed for the basic network architecture. NTUST/WCANE Lab

7. LBPWPhase of RTS/CTS exchange • Event PCW1 - AP to GMs (starting in slot k): • Send an RTS to all GMs • Event PCW2 - Leader/GMs to AP (in slot k+1): • Leader Send a CTS if it is ready to receive data frames; otherwise, do nothing. • Other GMs Send an NCTS if it is not ready to receive data frames; otherwise, do nothing. NTUST/WCANE Lab

8. LBPWPhase of data frames transfer • Event PTW1 - AP to GMs (in slot k+2): • If a CTS was received by the AP in slot k + 1, start to transmit contiguously available na(<= WS) data frames with labels, say, 1, 2, . . . , na; otherwise, go back to event PCW1. • Event PTW2 - Leader/GMs to AP (during slot k + 2 + ⌈(fl* ttr* na+ tpc+ tpp)/tst⌉and slot k + 1 + ⌈(fl* ttr* na+ tpc+ tpp)/tst ⌉+ na) : • Leader If the leader received the ith frame correctly, it sends an ACK in slot k+1+ ⌈(fl* ttr* na + tpc + tpp)/tst ⌉+i; otherwise, it sends a NAK. NTUST/WCANE Lab

9. Other GMs If the ith frame was received with error bits by any GM, it sends a NAK in slot k + 1 + ⌈(fl* ttr* na+ tpc+ tpp)/tst ⌉+ i; otherwise, it does nothing. NTUST/WCANE Lab

10. LBPW • Based on feedbacksfrom GMs, AP should make a decision. • Three cases AP faces: • An ACK is received • Nothing is received • A collision occurs • Case I: frame is correctly received. • Other cases: retransmission is required. NTUST/WCANE Lab

11. LBPR(n) Phase of RTS/CTS exchange • Event PCW1 - AP to GMs (starting in slot k): • Send an RTS to all GMs • Event PCW2 - Leader/GMs to AP (in slot k+1): • Leader Send a CTS if it is ready to receive data frames; otherwise, do nothing. • Other GMs Send an NCTS if it is not ready to receive data frames; otherwise, do nothing. NTUST/WCANE Lab

12. LBPR(n)Phase of data frames transfer • Event PTR1 - AP to GMs (in slot k+2): • If a CTS was received by the AP in slot k + 1, start to transmit contiguously available na(<= WS) data frames with labels, say, 1, 2, . . . , na; otherwise, go back to event PCW1. • Event PTR2 - Leader/GMs to AP (during slot k + 2 + ⌈(fl* ttr* na+ tpc+ tpp)/tst⌉and slot k + 1 + ⌈(fl* ttr* na+ tpc+ tpp)/tst ⌉+ ⌈na/n ⌉) : • Leader Send an acknowledgement in a bit map, including the receiving status for at most n frames at a time. Hence ⌈na/n ⌉ times of ACKs are NTUST/WCANE Lab

13. required to send during slot k+2+ ⌈(fl* ttr* na + tpc + tpp)/tst ⌉and slot k+1+ ⌈(fl* ttr* na + tpc + tpp)/tst ⌉ + ⌈na/n ⌉. • Other GMs Break the na frames into ⌈na/n ⌉ subsegments (each including exactly n frames except the last one). If one of frames for subsegment i was received with error bits by any GM, it sends a NAK directly in slot k + 1 + ⌈(fl* ttr* na+ tpc+ tpp)/tst ⌉+ i; otherwise, it does nothing. NTUST/WCANE Lab

14. LBPR(n) • Based on feedbacksfrom GMs, AP should make a decision. • Two cases AP faces: • An ACK in a bit map is received • A collision occurs • Case I: erroneous frames are retransmitted. • Case II: all frames are retransmitted. NTUST/WCANE Lab

15. GSM/GPRS system Description of the proposed protocols Performance study and numerical examples Conclusions Outline NTUST/WCANE Lab

16. Assumptions • A minimal wireless LAN is considered. • MAC is neglected. • Perfect time synchronization is assumed. • One multicast group is considered. • Each GM is assumed to be always ready to receive data frames. • Data frames may be corrupted but not lost. • Control frames are always correctly received. • Frames are generated according to Batch Poisson. NTUST/WCANE Lab

17. Performance Metrics • Cost the average time lasting since the AP contends the channel until the AP ascertains that all group members correctly receive the frame. • Exposure ratio of the number of mobile hosts actually receiving the frame and the number of mobile hosts who do need the frame. • Average queueing delay/queue length, Number of ACKs or NAKs. NTUST/WCANE Lab

18. LBPW vs. LBP (1/2) • These results evidently show that LBPW with a large window size, say 10, performs much better than LBP. • The reductions when WS = 2 and WS = 10 compared to LBP are 4.3% and 7.0%, when fl = 20 and nGM = 10 • 7.3% and 13.3%, when fl = 10 and nGM = 10 • The increase of the window size WS causes a lower cost, i.e., higher throughput. NTUST/WCANE Lab

19. We see that the queueing delay goes down as the window size increases or the number of group members decreases. LBPW vs. LBP (2/2) • Exposure is not affected by the increase of the window size but it increases as the group grows up. NTUST/WCANE Lab

20. LBPR(n) vs. LBPW (LBP) (1/3) LBPR(n) achieves ACKs/NAKs reduction approximately by a factor of n compared to LBP or LBPW. NTUST/WCANE Lab

21. LBPR(n) vs. LBPW (LBP) (2/3) NTUST/WCANE Lab

22. LBPR(n) vs. LBPW (LBP) (3/3) • LBPR(n) performs better than LBPW due to the saving of ACKs/NAKs. • The cost reduction is more obviously when FEP is high. NTUST/WCANE Lab

23. GSM/GPRS system Description of the proposed protocols Performance study and numerical examples Conclusions Outline NTUST/WCANE Lab

24. Conclusions • The cost of LBPR(n) is lower than that of LBPW which is subsequently lower than LBP. • The attainable cost reduction of LBPWcompared to LBP can be over 10%. • Both LBPW and LBPR(n) perform better than LBP in terms of queueing delay. • LBPW mostly performs better than LBPR(n) for n ≥3, while LBPR(2) performs better than LBPW when the frame loss probability is low. • As for the exposure metric, LBPW is the same as LBP and smaller than LBPR(n). For larger n, the exposure of LBPR(n) becomes higher. • So, we suggest LBPW and LBPR(2) to be used. NTUST/WCANE Lab

25. Thank You! NTUST/WCANE Lab