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.

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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