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Link Estimation, CTP and MultiHopLQI. Motivation. Data Collection needs to estimate the link quality To select a good link. Challenges of Link Quality Estimation. Prevalence of intermediate-quality links Time-varying nature of a wireless channel

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Link Estimation, CTP and MultiHopLQI


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    Presentation Transcript
    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 dfanddr • Each node broadcasts link probes of a fixed size, at an average period of τ • 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 [LinkEstimation_1]: Section 3.1

    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