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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 - PowerPoint PPT Presentation
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Opportunistic Packet Scheduling and Media Access Control for Wireless LANs and Multi-hop Ad Hoc Networks
Jianfeng Wang, Hongqiang Zhai and Yuguang FangDepartment of Electrical & Computer EngineeringUniversity of Florida
Overview
MotivationContributionsOSMA ProtocolPerformance EvaluationConclusionsFuture 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
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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 R
TS 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
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OSMA Protocol - Multicast RTS
The sender node determines a set of candidate receivers which have their packets queued.
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A candidate receiverlist = {1,2,4}
OSMA Protocol - Multicast RTS
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
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RTS
RTSRTS
RTS6
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Selected groupof 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.
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RTS
RTSRTS
RTS6
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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.
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CTS
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Eligible candidate receivers: 1
OSMA Protocol - Priority-based CTS
It is possible that more than one candidate receiver is qualified to receive data.
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CTS
CTS6
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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
SIFS CTS1
Prioritized CTS frames
Time_slot
SIFS CTS2
SIFS CTS3
SIFS CTSn
1st receiver
2nd receiver
3rd receiver
Nth 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 D
IFS, 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.
CTS2
SIFS CTS3
SIFS CTSM
SIFS CTS1 1st receiver
2nd receiver
3rd receiver
Mth receiver
RTS DIFS Sender
…
SIFS
Performance Evaluation - Objective
Ns-2 is used as simulation toolEvaluate the performance of OSMACompare it with the base rate IEEE 802.11
scheme.
Performance Evaluation - Setup
Physical Propagation model is Ricean fading.
Background noise = 100dbmData packet size = 1000 bytesIntroduce Average Fade Probability to ch
aracterize the channel condition. The probability that the received power is less t
han the received power threshold defined by 802.11 MAC
Performance Evaluation - SetupScenario 1 - WLAN
Number of flows vs throughput Channel quality vs TCP throughput Channel quality vs TCP fairness
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Performance Evaluation - Setup
Scenario 2 - Multihop network One-hop and multi-hop flow Total throughput vs Offered load
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91 92 93 94 9796 9895 99 100
...
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 xi 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 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.
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