Link Estimation, CTP and MultiHopLQI

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Link Estimation, CTP and MultiHopLQI
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Motivation

• Data Collection needs to estimate the link
  quality
• To select a good link

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Challenges of Link Quality Estimation

• Prevalence of intermediate-quality links
• Time-varying nature of a wireless channel
• Alternating between high (100 packet reception
  ratio PRR) and low (0 PRR) quality
• Link asymmetries
• Hardware variations

Ref. LinkEstimation_1 Section 1
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Detour Time-varying nature of a wireless channel
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802.11b and 802.15.4 Spectrum Utilization

• Ref Implication_1 Section 3 and 4

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Packet Reception Ratio (PRR)

• Ref Implication_1 Section 3 and 4

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PRR vs. RSSI

• Ref Implication_1 Section 3 and 4

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Distribution of the Mode of Noise Readings

• Ref Implication_1 Section 3 and 4

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Behavior of a Single Node

• Ref Implication_1 Section 3 and 4

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Reception Probability

• Other_1 Figure 1

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Reception Probability

• Other_1 Figure 1

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Link Estimation ETX (Expected Transmission
Count)
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Link Estimation Metric - ETX

• Minimum Hop Count is not a good metric
• Assume that links either work well or do not work
  at all
• Many wireless links have intermediate loss ratios
• ETX Expected Transmission Count
• Choose routes with high end-to-end throughout
• Finds paths with the fewest expected number of
  transmissions (including retransmissions)
  required to deliver a packet to the destination

Ref. ETX_1 Section 1, 3
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Why some superficially attractive metrics are not
suitable?

• Hop-count
• Ignoring links with loss ratios above a certain
  threshold
• Product of the per-link delivery ratios
• Fail to account for inter-hop interference

Ref. ETX_1 Sectiion 3
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Link Estimation Metric - ETX

• ETX
• Consider the wide range of link loss ratios
• The existence of links with asymmetric loss
  ratios
• The interference between successive hops of
  multi-hop paths

Ref. ETX_1 Section 1, 3
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Link Estimation Metric - ETX

• ETX of a link
• The predicted number of data transmissions
  required to send a packet over a link, including
  retransmissions
• Calculated using the forward and reverse delivery
  ratios of a link
• How to measure Broadcasting of probe packets and
  derives link quality information from each
  direction
• ETX of a route
• The sum of the ETX for each link in the route

Ref. ETX_1 Section 1, 3
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Link Estimation Metric - ETX

• Forward delivery ratio df
• The probability that a data packet successfully
  arrives at the recipient
• Reverse delivery ratio dr
• The probability that the ACK packet is
  successfully received
• The expected probability that a transmission is
  successfully received and acknowledged is df X dr
• ETX 1 / (df X dr)

Ref. ETX_1 Section 1, 3
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How to Measure df and dr

• Each node broadcasts link probes of a fixed size,
  at an average period of t
• Each node remembers the probes it receives during
  the last w seconds

• The ETX of a route is the sum of the link metrics

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Link Estimation Four-bit Wireless Link
Estimation
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Four-bit Wireless Link Estimation

• Physical layer
• Measure channel quality during a packet
• Measured for single received packet
• Fast and Inexpensive
• Sometimes can be misleading because the
  time-varying nature
• Decoding error
• The physical layer can provide immediate
  information on the quality of the decoding of a
  packet
• Example
• MultiHopLQI uses Link Quality Indication (LQI)
  a feature of the CC2420 radio
• RSSI, SNR

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Four-bit Wireless Link Estimation

• Link layer
• Measure whether packets are delivered and
  acknowledged
• Such as ETX use periodic broadcast probes to
  measure incoming packet reception rates
• Slow to adapt

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Four-bit Wireless Link Estimation

• Network layer
• Knows which links are most useful for routing
• Is a link useful?
• Keep useful links in the table

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Four Bit Interface

• Physical Layer Packet decoding quality
• If set, the white bit denotes that each symbol in
  the received packet has a very low probability of
  decoding error
• Link Layer Packet acknowledgements
• A link layer sets the ack bit on a transmit
  buffer when it receives a layer 2 ack for that
  buffer
• Network Layer Relative important links
• Pin bit when the network layer sets the pin bit
  on one link table entry, the link estimator
  cannot remote it from the table until the bit is
  cleared
• Compare bit indicate whether the route provided
  by the sender of the packer is better than the
  route provided by one or more of the entries in
  the link table

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Four Bit Interface Details
PINKeep this link in the table
COMPAREIs this a useful link?
ACKA packet transmission on this link was
acknowledged
WHITEPackets on this channel experience few
errors
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Data Collection in TinyOS 2.x

• CTP
• Bi-directional probe-based link estimation
• MultiHopLQI
• Only uses physical layer information to estimate
  link

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Physical Layer Information is NOT Sufficient
PRR
LQI
Unacked
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Physical Layer Information is NOT Sufficient
PRR
LQI
Unacked