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Link Estimation, CTP and MultiHopLQI

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Link Estimation, CTP and MultiHopLQI – PowerPoint PPT presentation

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Title: Link Estimation, CTP and MultiHopLQI


1
Link Estimation, CTP and MultiHopLQI
2
Motivation
  • Data Collection needs to estimate the link
    quality
  • To select a good link

3
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
4
Detour Time-varying nature of a wireless channel
5
802.11b and 802.15.4 Spectrum Utilization
  • Ref Implication_1 Section 3 and 4

6
Packet Reception Ratio (PRR)
  • Ref Implication_1 Section 3 and 4

7
PRR vs. RSSI
  • Ref Implication_1 Section 3 and 4

8
Distribution of the Mode of Noise Readings
  • Ref Implication_1 Section 3 and 4

9
Behavior of a Single Node
  • Ref Implication_1 Section 3 and 4

10
Reception Probability
  • Other_1 Figure 1

11
Reception Probability
  • Other_1 Figure 1

12
Link Estimation ETX (Expected Transmission
Count)
13
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
14
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
15
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
16
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
17
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
18
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

19
Link Estimation Four-bit Wireless Link
Estimation
20
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

21
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

22
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

23
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

24
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
25
Data Collection in TinyOS 2.x
  • CTP
  • Bi-directional probe-based link estimation
  • MultiHopLQI
  • Only uses physical layer information to estimate
    link

26
Physical Layer Information is NOT Sufficient
PRR
LQI
Unacked
27
Physical Layer Information is NOT Sufficient
PRR
LQI
Unacked
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