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Improving Wireless LAN Performance via Adaptive Local Error Control

Presented by

Yuanfang Cai

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Outline

Local error control introduction Evaluations

Simple local error control• MAC & LLC design and implementation• Experimental approach• Results

Adaptive local error control• MAC & LLC design and implementation• Experimental approach• Results

Summary

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Local versus End-to-end Error Control Attractions:

Understand local characteristicsMore efficientEasier to deploy

Problems:Confusing higher layer protocolsUndesirable interactionWasted Effort

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Design Tradeoffs for Local Error Control Hardware error control

Simple Can not differentiate flows

“Pure” link-layer approaches Per-packet basis Flow-aware

“Protocol-aware” link-layer protocols Requires gateways to understand a wide variety of

protocols. “Gateway-style”/”indirect” error control

Might have to understand multiple protocols Routing changes

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Simple local error control

MAC designMaster/slave transactionsINVITE and JOINPOLL-DATA and DATA-ACK

LLC designEntirely lost, partially lost, corruptedStop-and-wait retransmission

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Simple local error control—MAC and LLC design

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Simple local error control—Implementation and Performance

Intel 80486 and Pentium laptops using 915 MHz PCMCIA card WaveLAN units

NetBSD Unix 43% throughput loss

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Simple local error control--Experimental Approach

Single Hop

Ethernet + wireless

WAN extension

Basestation25 MHz 80486

DEC pc-4255SL

Client: 75 MHz Pentium Toshiba Satellite pro 400CDT

Wireless Host

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Evaluation—Pure local error control Pattern-based evaluation

Packet killer Basic robust evaluation

TCP without local error control TCP with local error control

Broader scenarios Ethernet + wireless WAN extension Competing TCP streams

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TCP without local error control

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TCP without local error control

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TCP with local error control

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TCP with local error control

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Ethernet + wireless

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WAN extension

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Competing TCP streams

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Simple local error control--Analysis Steady state conditions (Assume that

TCP is stable)Lost packets always indicate

congestion.Avoid packet reorderingDon’t have long delay

Dynamic error environmentUpgradeDegrade

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Simple local error control--Analysis

Persistence of local error controlPerpetual retransmissionGive up after a few transmissionsThe higher error environment, the

more persistent the retransmission need to be.

Packet Delay by persistent local retransmission

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Simple local error control--Analysis

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Simple local error control--Analysis

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Simple local error control--Analysis

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Simple local error control--Analysis 3% overlap End-to-end retransmission timeouts should

be substantially longer than the single-hop round-trip time

TCP features that allow persistent retransmission with a small efficiency loss Delay variation Cautious minimum timeout Slow-start probing

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Adaptive local error control

LLC Design Add FEC and packet shrinking

• Packet truncation• Rare for short packets

• Bit corruption• Have only a few bit errors

• Packet Shrinking• Forward Error Correction (FEC)

• Reed-Solomon codes Observe the quality of the link Tell slaves using POLL-DATA Employ adaptive policies

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Adaptive local error control

LLC Implementation Implement packet shrinking through packet

segmentation and reassembly • Data transmission:

• Add to the packet sequence number: • starting byte offset,• a byte count• a packet complete bit

• Acknowledgement: • A package sequence number• A cumulative length indicating correctly received bytes

• Rare for short packets Emulates the effects of Forward Error Correction

(FEC)

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Adaptive local error control

Static Policies BOLD—Without coding or shrinking LIGHT—5% coding overhead Robust—Sends minimally-sized packets with nearly

1/3 of each devoted to coding overhead. Adaptive policies

BIMODAL• BOLD in good conditions• ROBUST in poor conditions

BI-CODE—BIMODAL that only adjust coding overhead

BI-SIZE—BIMODAL that only adjust coding overhead FLEX—adapts the packet size and degree of FEC

redundancy independently

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Adaptive local error control

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Adaptive local error control

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Adaptive local error control

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Summary

“Pure” link-layer local error control mechanism can greatly increase the efficiency of data transfer in wireless LAN’s.

Flow-aware instead of Protocol-aware Simple adaptive policies outperformed

static policies across a range of error environments.