1 Improving Wireless LAN Performance via Adaptive Local Error Control Presented by Yuanfang Cai

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 characteristics More efficient Easier to deploy Problems: Confusing higher layer protocols Undesirable interaction Wasted 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 design Master/slave transactions INVITE and JOIN POLL-DATA and DATA-ACK LLC design Entirely lost, partially lost, corrupted Stop-and-wait retransmission

6 Simple local error control— MAC and LLC design

7 Simple local error control— Implementation and Performance Intel 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 Basestation 25 MHz 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

10 TCP without local error control

11 TCP without local error control

12 TCP with local error control

13 TCP with local error control

14 Ethernet + wireless

15 WAN extension

16 Competing TCP streams

17 Simple local error control-- Analysis Steady state conditions (Assume that TCP is stable) Lost packets always indicate congestion. Avoid packet reordering Don’t have long delay Dynamic error environment Upgrade Degrade

18 Simple local error control-- Analysis Persistence of local error control Perpetual retransmission Give up after a few transmissions The higher error environment, the more persistent the retransmission need to be. Packet Delay by persistent local retransmission

19 Simple local error control-- Analysis

20 Simple local error control-- Analysis

21 Simple local error control-- Analysis

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

26 Adaptive local error control

27 Adaptive local error control

28 Adaptive local error control

29 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.