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Challenges to Reliable Data Transport Over Heterogeneous Wireless Networks

This article discusses the challenges faced in achieving reliable data transport over heterogeneous wireless networks, including poor performance, asymmetric effects and latency variation, and low channel bandwidths. It also explores split-connection protocols, wireless testbeds, measurement techniques, performance metrics, and the Berkeley Snoop Protocol.

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Challenges to Reliable Data Transport Over Heterogeneous Wireless Networks

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  1. Challenges to Reliable Data Transport Over Heterogeneous Wireless Networks

  2. Motivation (Ch 1+2) • Everybody went nuts about wireless (cell phones, etc) and the data networks (the internet) in the 90's • Then, why are wireless networks not more popular? • Is there no demand? • No

  3. Then, why are wireless networks not more popular? • Poor performance • Too large a difference from wired technology

  4. Heterogeneity • Makes it difficult to identify performance bottlenecks

  5. Three Fundamental Challenges • Wireless bit errors • TCP assumes losses are due to congestion • Asymmetric effects and latency variation • TCP relies on consistent rtt's for good performance • Low channel bandwidths • Long range channels are often orders of magnitude slower than the wired alternative

  6. Split-Connection Protocols • Put a layer under tcp that is error free • Now losses are due to congestion • Asymmetric rtt's lead to poor performance

  7. Wireless Testbed (ch 3)

  8. Simulation Environment • Initially used REAL • Realistic TCP modules • Inflexible • Written in C with parts in assembler • Hard to extend • Simulation written in propriety script language • Now use NS-2

  9. NS-2 • Added LAN object • Formerly only point-to-point link • Error Models • Tested on real wireless network to determine error behaviour

  10. BARWAN • WaveLan • 2Mb/s DS • Throughput between 50k and 1.5M • Usually closer to the low end • Ricochet • Half-duplex FH • Cable • 10Mb/s shared up, dialup down

  11. Measurement Techniques • Wrote netperf • Measures TPC workloads • Tcpdump • Detailed packet traces

  12. Performance Metrics • Throughput • Received bytes /unit time • Goodput • Ratio of useful bytes to number transmitted • Always < 1, closer to 1 - more efficient • Utilization • How often contended resource is idle • Fairness • How evenly shared, Jan's fairness index

  13. Jan's Fairness Index • n connections • xi = throughput for node I • f = (åxi)2/(nåxi2)

  14. Berkeley Snoop Protocol (Ch 4) • Significantly improves TCP performance in error-prone cellular networks • Uses cross-layer protocol optimisations

  15. Topology • End node(s) connected to Base station via wireless link • Rest of hops over wired network • Using TCP Reno a bit error rate of 5% makes a transfer take 4.5 times longer than ideal TCP(2MB transfer)

  16. Extra layer • Transfer to • Agent at base station • Uses info in ACKs • Soft state • Transport aware link protocol • Transfer from • Explicit loss notification • Retransmits lost packets • No congestion control • Link aware transport protocol

  17. Design Goals • Local solution • Transparent to fixed internet host • Eliminate adverse interaction between layers • Enable incremental deployment • Preserve end-to-end semantics • Use soft state

  18. Transfer From a Fixed Host • Caches data to be forwarded to MH • ACKs are forwarded to fixed host if not due to loss • Duplicate ACKs can mean loss • Packet is resent with high priority • DupACKs after first not forwarded

  19. Transfer From Mobile Host • Negative ACKs • Built on SACKs • Dependant on SACK implementation • Not used • ELN • BS keeps list of “holes” • Hole is set only when BS is not receiving close to max # of packets • If DupACK corresponds to hole ELN bit is set

  20. Mobility • Handoffs can lead to packet loss • Multi-cast based buffering • Intermediate “home” agent does snoop and sends to each base-station

  21. Performance

  22. Asymmetry • ACK speed on slow link limits throughput on fast link • Compress ACKs • Reduce ACK frequency

  23. Small Windows • Fast retransmissions are infrequent • Most due to timeouts • Results in idle channel • Usually fix with SACKs and ELN • ER (Enhanced Recovery) • Probe network after <3 Duplicate ACKs

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