Packet Dispersion inPacket Dispersion inIEEE 802.11IEEE 802.11

Wireless NetworksWireless Networks

Mingzhe Li, Mark Claypool andBob Kinicki

WPI Computer Science DepartmentWorcester, MA 01609

rek@cs.wpi.edu

Second International Workshop on Performance and Management of Wireless Local Area Networks (P2MNet)

Tampa, Florida, November 14, 2006

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OutlineOutline• Introduction• Packet Dispersion in WLANs

• WLAN Packet Dispersion Model

• Model Validation

• Model Analysis

• Conclusions

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IntroductionIntroduction

• Bandwidth estimation techniques focus on network capacity or available bandwidth.

• Most bandwidth estimation has involved only wired networks.

• This paper focuses on packet dispersion in wireless LANs (WLANs).

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OutlineOutline• Introduction

• Packet Dispersion in WLANs• WLAN Packet Dispersion Model

• Model Validation

• Model Analysis

• Conclusions

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Packet Dispersion Packet Dispersion TechniquesTechniques

• Packet pair dispersion– two equal-sized packets are sent back-

to-back through the network.

• Packet train dispersion– multiple back-to-back probe packets are

sent through the network.

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Packet Pair DispersionPacket Pair Dispersion

narrow linknarrow link

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NS-2 AdditionsNS-2 Additionsto Model WLANsto Model WLANs

• Receiver Based Auto Rate (RBAR) module was re-implemented in NS 2.27 to model 802.11 dynamic rate adaptation [ Sadeghi et al.]

• An NS-2 extension to model Rayleigh fading was implemented [Ratish et al.]

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Wireless Rate AdaptationWireless Rate Adaptationunder Rayleigh Fadingunder Rayleigh Fading

NS-2 simulation results

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• Probing traffic– Packet pairs or trains sent downstream

through the AP to the wireless client.

• Crossing traffic– Downstream traffic going through the AP to

other wireless clients.

• Contending traffic– Upstream traffic that accesses the shard

wireless channel and competes with probe packets on the estimated path.

Wireless Traffic Wireless Traffic ClassificationClassification

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Wireless Traffic Wireless Traffic ClassificationClassification

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WLAN Packet WLAN Packet Dispersion IssuesDispersion Issues

• Wireless frame retries– increase the packet delay variance that

produce packet dispersion inconsistencies.

• Dynamic Rate Adaptation• Fading channel• Noisy wireless channel - BER• Contending Traffic

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Simulated Wireless Simulated Wireless Effects on Bandwidth Effects on Bandwidth

Estimation Estimation

• CDF represents 1000 packet-pair estimates• 1000-byte probe packets and CBR contending packets• Contending traffic = 1Mbps upstream flow

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OutlineOutline• Introduction

• Packet Dispersion in WLANs

• WLAN Packet Dispersion Model• Model Validation

• Model Analysis

• Conclusions

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WLAN Packet Dispersion WLAN Packet Dispersion ModelModelAssumptions

Bottleneck link is the WLAN last hop.No crossing traffic.

• Dispersion, T, between two packets in a packet pair is characterized in terms of E[T] and V[T] for a WLAN that includes: packet size, link data rate, BER and access methods (base or RTS/CTS).

• While wireless channel conditions can be characterized by received signal strength indicator (RSSI), SNR and BER, our simplified model assumes these other factors impact BER.– The impact of channel conditions on bandwidth estimation

is evaluated via V[T], the packet dispersion variance.

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WLAN Packet Dispersion WLAN Packet Dispersion ModelModel

• From previous 802.11 models, we build a new model for wireless packet dispersion where dispersion is defined as the delay between the arrival times of the first and second packets in the packet pair.

• Thus, the model must include the delay before the transmission of the second packet, E[D], and the time to transmit it, Ts, and dispersion is:

E[T] = E[D] + Ts

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WLAN Packet Dispersion WLAN Packet Dispersion ModelModel

• Since E[D] depends on the wireless link rate, Cl , the average packet size, L and the number of nodes in the contention domain, n, we have

E[D] = d (Cl, L, n)

• Similarly, for the transmission time,

Ts = ts (Cl, L)

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Bandwidth EstimateBandwidth Estimate

• Defining, Cest, as the bandwidth estimate based on the wireless packet pair dispersion model, our model is defined by:

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OutlineOutline• Introduction

• Packet Dispersion in WLANs

• WLAN Packet Dispersion Model

• Model Validation• Model Analysis

• Conclusions

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Model ValidationModel Validation

• Validation results include an ideal WLAN and a scenario with contention and BER.

• Ideal validation includes NS-2 simulations, analytic model and actual measurements.

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Ideal WLAN ScenarioIdeal WLAN Scenario

The ideal scenario consistsof an AP with a singlewireless client for bothbasic (BAS) and RTS/CTSaccess methods.

Simulations: 500 pairsMeasurements: 100 pairs

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Model ValidationModel Validation

Random ModelSimulation Parameters Topology

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Models with Contention and Models with Contention and Wireless Transmission Wireless Transmission

ErrorsErrors

RTS/CTS Basic RTS/CTS Basic

Mean

Error8.05% 4.90% 9.40% 7.67%

Standard

Deviation6.72% 4.28% 5.30% 3.82%

Errors in the Bandwidth Estimation ModelCompared with Simulations

Error Free BER = 10-5

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OutlineOutline• Introduction

• Packet Dispersion in WLANs

• WLAN Packet Dispersion Model

• Model Validation

• Model Analysis• Conclusions

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Model AnalysisModel Analysis• In a non-saturated WLAN with low BER,

low contention, packet-pair dispersion estimates the maximal channel capacity, effective capacity, Ce .

• With considerable contending traffic and/or BER, the dispersion metric is referred to as the achievable throughput, At , for the current level of contending traffic.

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Model AnalysisModel Analysis• Effective Capacity

• Achievable Bandwidth

• Relationship with Available Bandwidth

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Achievable ThroughputAchievable Throughput

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Impact of Channel RateImpact of Channel Rateon Effective Capacityon Effective Capacity

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Impact of BER Impact of BER on Achievable Throughputon Achievable Throughput

Five node, BAS wirelessnetwork with 1500-bytepackets

no achievable throughput

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Impact of RTS/CTSImpact of RTS/CTSon Achievable Throughputon Achievable Throughput

Model Results for Wireless Networks with 1500 Byte Packets

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Standard Deviation of Standard Deviation of Achievable ThroughputAchievable Throughput

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Packet Size Effect onPacket Size Effect onStandard Deviation ofStandard Deviation ofBandwidth EstimationsBandwidth Estimations

Five node, BAS wirelessnetwork with 1500-bytepackets

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Impact of Channel RateImpact of Channel Rateon Standard Deviation of on Standard Deviation of Bandwidth EstimationsBandwidth Estimations

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Impact of BER on Impact of BER on Standard Deviation of Standard Deviation of Bandwidth EstimationsBandwidth Estimations

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OutlineOutline• Introduction

• Packet Dispersion in WLANs

• WLAN Packet Dispersion Model

• Model Validation

• Model Analysis

• Conclusions

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ConclusionsConclusions• Packet dispersion measures the

effective capacity and the achievable throughput of a wireless network instead of the capacity as in a wired network.

• Wireless channel conditions, such as BER and RTS/CTS access method impact the bandwidth estimation results and the variance of the results.

Packet Dispersion inPacket Dispersion inIEEE 802.11IEEE 802.11

Wireless NetworksWireless NetworksMingzhe Li, Mark Claypool and Bob Kinicki

WPI Computer Science DepartmentWorcester, MA 01609

rek@cs.wpi.eduhttp://web.cs.wpi.edu/~rek/

Second International Workshop on Performance and Management of Wireless Local Area Networks (P2MNet)

Tampa, Florida, November 14, 2006

Thank You !!Thank You !!

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