Jianfeng Wang, Hongqiang Zhai and Yuguang Fang Department of Electrical & Computer Engineering

1. Opportunistic Packet Scheduling and Media Access Control for Wireless LANs and Multi-hop Ad Hoc Networks Jianfeng Wang, Hongqiang Zhai and Yuguang Fang Department of Electrical & Computer Engineering University of Florida

2. Overview • Motivation • Contributions • OSMA Protocol • Performance Evaluation • Conclusions • Future work

3. Motivation • Head-of-Line (HOL) blocking problem • Outgoing packets are buffered in a FIFO queue waiting for transmission. • If the first in line packet is blocked, all subsequent packets are denied service, even if their corresponding destinations are not blocked. • This phenomenon contributes to an increase in the average queue length, packet latency and packet loss probability • Ultimately, it causes a reduction of the useful system throughput and an unfairness problem.

4. 1 2 1 4 3 1 2 3 4 Motivation • Head-of-Line (HOL) blocking problem

5. Motivation • The HOL blocking problem worsens in the wireless LANs or mobile ad hoc networks for two reasons. • The HOL packet may fail in retransmission of RTS or DATA frames many times due to the fading, the interference and the collision. • Random nature of the contention-based MAC protocols, say Binomial Exponential Backoff scheme.

6. Contributions • Introduce a new protocol - Opportunistic Packet Scheduling and Media Access Control (OSMA). • This protocol exploits high quality channel condition under certain fairness constraints. • The first paper to exploit the multi-user diversity in the CDMA/CA based wireless networks.

7. OSMA Protocol - Overview • Multicast RTS • A channel probing message which includes a list of candidate receivers. • Guarantees “fairness” • Priority-based CTS • The candidate receiver with the highest priority would be granted to access the channel by replying CTS in this first place. • Optimizes “throughput”

8. OSMA Protocol - Multicast RTS • At the sender node, one separate queue is maintained for each next hop 1 1 1 1 1 2 2 2 3 3 4 4 4 4 4 4

9. OSMA Protocol - Multicast RTS • The sender node determines a set of candidate receivers which have their packets queued. A candidate receiver list = {1,2,4} 1 1 1 1 1 1 2 2 2 2 3 3 4 4 4 4 4 4 4

10. OSMA Protocol - Multicast RTS • Based on the weight of the HOL packet in each queue, the scheduler assigns media access priority to each candidate receiver. • Possible scheduling schemes: Round Robin, Earliest Timestamp First etc.

11. 1 8 2 RTS Src 3 7 RTS RTS RTS 6 4 5 OSMA Protocol - Multicast RTS • The sender multicasts a RTS frame with a media access priority list to those chosen candidate receivers. Selected group of candidate receivers: 1,3,5,7

12. OSMA Protocol - Multicast RTS • To ensure long term fairness among links, the weight adjustor is used to update the weight of each link after each transmission.

13. 1 8 2 RTS Src 3 7 RTS RTS RTS 6 4 5 OSMA Protocol - Priority-based CTS • Each candidate receiver evaluates the instantaneous channel condition based on the physical-layer analysis of the RTS frame.

14. OSMA Protocol - Priority-based CTS • If the channel condition is better than a certain level and its NAV is zero, the receiver is allowed to transmit a CTS. Eligible candidate receivers: 1 1 8 2 CTS Src 3 7 6 4 5

15. OSMA Protocol - Priority-based CTS • It is possible that more than one candidate receiver is qualified to receive data. Eligible candidate receivers: 1,5 1 8 2 CTS Src 3 7 CTS 6 4 5

16. Format of Multicast RTS frame OSMA Protocol - Priority-based CTS • To avoid collisions, the media access priority list in the multicast RTS frame announces the order of media access among qualified candidate receivers.

17. Prioritized CTS frames SIFS CTS1 1st receiver SIFS CTS2 2nd receiver SIFS CTS3 3rd receiver Time_slot … SIFS CTSn Nth receiver OSMA Protocol - Priority-based CTS • To prioritize the receivers, different Inter-Frame Spacings (IFSs) are employed. • the IFS of the nth receiver = SIFS + (n-1) * Time_slot Only ONE of these CTS frames will be received by the sender

18. RTS DIFS Sender SIFS CTS1 1st receiver SIFS CTS2 2nd receiver SIFS CTS3 3rd receiver … SIFS CTSM Mth receiver OSMA Protocol - Priority-based CTS • If the sender can’t receive any CTS frames after DIFS, there is no qualified receiver. • DIFS = SIFS + M * Time_slot where M is the maximal number of receivers which can be included into the multicast RTS.

19. Performance Evaluation - Objective • Ns-2 is used as simulation tool • Evaluate the performance of OSMA • Compare it with the base rate IEEE 802.11 scheme.

20. Performance Evaluation - Setup • Physical Propagation model is Ricean fading. • Background noise = 100dbm • Data packet size = 1000 bytes • Introduce Average Fade Probability to characterize the channel condition. • The probability that the received power is less than the received power threshold defined by 802.11 MAC

21. Performance Evaluation - Setup • Scenario 1 - WLAN • Number of flows vs throughput • Channel quality vs TCP throughput • Channel quality vs TCP fairness 1 8 2 7 AP 3 4 6 5

22. Performance Evaluation - Setup • Scenario 2 - Multihop network • One-hop and multi-hop flow • Total throughput vs Offered load Grid topology with 100 nodes 3 6 9 10 1 2 4 5 7 8 12 13 19 20 11 14 15 16 17 18 ... 99 100 91 92 93 94 95 96 97 98

23. Performance Evaluation - Results • WLAN - Number of users vs throughput 44% throughput gain

24. Performance Evaluation - Results • WLAN - Channel quality vs TCP throughput 12% throughput gain 87% throughput gain

25. Jain’s Fairness Index = f where xi is the flow rate for the flow i Performance Evaluation - Results • WLAN - Channel quality vs TCP fairness

26. Performance Evaluation - Results • Multihop network with One-hop flow • Total throughput vs Offered load

27. Performance Evaluation - Results • Multihop network with Multi-hop flow • Total throughput vs Offered load

28. Conclusions • OSMA, an Opportunistic scheduling and channel aware media access protocol for WLANs and multihop ad hoc networks. • By using multicast RTS and prioritized CTS, OSMA • explores the multi-user diversity • alleviates HOL blocking problem significantly.

29. Conclusions • Simulation results show that compared with 802.11 MAC, OSMA normally obtains throughput gains of: • 50% or above in WLANs and • several times in multi-hop networks • This is the first paper to address multi-user diversity by opportunistic scheduling in the CSMA/CA based wireless networks.

30. Future work • The scheduling among unicast data packets, control packets and broadcast packets. • Design details of packet scheduling algorithms. • Studies on incorporating power control, rate adaptation and directional antenna into this general framework OSMA.

31. Q & A