1 improving wireless lan performance via adaptive local error control presented by yuanfang cai
TRANSCRIPT
2
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
3
Local versus End-to-end Error Control Attractions:
Understand local characteristicsMore efficientEasier to deploy
Problems:Confusing higher layer protocolsUndesirable interactionWasted Effort
4
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
5
Simple local error control
MAC designMaster/slave transactionsINVITE and JOINPOLL-DATA and DATA-ACK
LLC designEntirely lost, partially lost, corruptedStop-and-wait retransmission
7
Simple local error control—Implementation and Performance
Intel 80486 and Pentium laptops using 915 MHz PCMCIA card WaveLAN units
NetBSD Unix 43% throughput loss
8
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
9
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
17
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
18
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
22
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
23
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
24
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)
25
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