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

Overview Motivation Contributions OSMA Protocol Performance Evaluation Conclusions Future work

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.

Motivation Head-of-Line (HOL) blocking problem

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.

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.

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” OSMA Protocol - Overview

At the sender node, one separate queue is maintained for each next hop 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} 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.

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

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.

OSMA Protocol - Priority-based CTS Each candidate receiver evaluates the instantaneous channel condition based on the physical-layer analysis of the RTS frame. Src RTS

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. Src CTS Eligible candidate receivers: 1

OSMA Protocol - Priority-based CTS It is possible that more than one candidate receiver is qualified to receive data. Src CTS Eligible candidate receivers: 1,5

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. Format of Multicast RTS frame

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 SIFSCTS 1 Prioritized CTS frames Time_slot SIFSCTS 2 SIFSCTS 3 SIFSCTS n 1 st receiver 2 nd receiver 3 rd receiver N th receiver … Only ONE of these CTS frames will be received by the sender

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. CTS 2 SIFSCTS 3 SIFSCTS M SIFSCTS 1 1 st receiver 2 nd receiver 3 rd receiver M th receiver RTSDIFS Sender … SIFS

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

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 MAC

Performance Evaluation - Setup Scenario 1 - WLAN  Number of flows vs throughput  Channel quality vs TCP throughput  Channel quality vs TCP fairness AP

Performance Evaluation - Setup Scenario 2 - Multihop network  One-hop and multi-hop flow  Total throughput vs Offered load Grid topology with 100 nodes

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

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

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

Performance Evaluation - Results Multihop network with One-hop flow  Total throughput vs Offered load

Performance Evaluation - Results Multihop network with Multi-hop flow  Total throughput vs Offered load

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.

Conclusions Simulation results show that compared with 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.

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.

Q & A