Challenges to Reliable Data Transport Over Heterogeneous Wireless Networks

1
Challenges to Reliable Data Transport Over
Heterogeneous Wireless Networks
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Motivation (Ch 12)

• 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

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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

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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

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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

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Wireless Testbed (ch 3)
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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

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NS-2

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

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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

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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 lt 1, closer to 1 - more efficient
• Utilization
• How often contended resource is idle
• Fairness
• How evenly shared, Jan's fairness index

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Jan's Fairness Index

• n connections
• xi throughput for node I
• f (åxi)2/(nåxi2)

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Berkeley Snoop Protocol (Ch 4)

• Significantly improves TCP performance in
  error-prone cellular networks
• Uses cross-layer protocol optimisations

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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)

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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

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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

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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

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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

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Mobility

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

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Performance
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Asymmetry

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

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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 lt3 Duplicate ACKs