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Wireless Link Estimation

Wireless Link Estimation. Learning Objectives. Understand the motivation of link estimation protocols – the time varying nature of a wireless channel Understand the metric of ETX Understand the four-bit link estimation

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Wireless Link Estimation

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  1. Wireless Link Estimation

  2. Learning Objectives • Understand the motivation of link estimation protocols – the time varying nature of a wireless channel • Understand the metric of ETX • Understand the four-bit link estimation • Understand the impact of link estimation on representative TinyOS network protocols

  3. Prerequisites • Module 2 • Basic concepts of wireless communications • Basic concepts of computer networks

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

  5. 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

  6. Detour: Time-varying nature of a wireless channel

  7. 802.11b and 802.15.4 Spectrum Utilization • Ref: [Implication_1] Section 3 and 4

  8. Packet Reception Ratio (PRR) • Ref: [Implication_1] Section 3 and 4

  9. PRR vs. RSSI • Ref: [Implication_1] Section 3 and 4

  10. Distribution of the Mode of Noise Readings • Ref: [Implication_1] Section 3 and 4

  11. Behavior of a Single Node • Ref: [Implication_1] Section 3 and 4

  12. Reception Probability • [Other_1]: Figure 1

  13. Reception Probability • [Other_1]: Figure 1

  14. Link Estimation – ETX (Expected Transmission Count)

  15. 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

  16. Results on Experimental Testbed • Figure 2 of [ETX_1]

  17. 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 • End-to-end delay • Change with network load • Load adaptive routing metrics Ref. [ETX_1]: Sectiion 3

  18. 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

  19. 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

  20. 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

  21. How to Measure dfanddr • Each node broadcasts link probes of a fixed size, at an average period of τ • Receive a probe every τseconds • Each node remembers the probes it receives during the last w seconds • The ETX of a route is the sum of the link metrics

  22. ETX Example • Ref: Figure 4 of “ExOR: Opportunistic MultiHop Routing for Wireless Networks”

  23. ETX Example • Ref: Figure 5 of “ExOR: Opportunistic MultiHop Routing for Wireless Networks” • Each node’s ETX value is the sum of the link ETX value along the lowest-ETX path to the destination node E

  24. Link Estimation – Four-bit Wireless Link Estimation

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

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

  31. Physical Layer Information is NOT Sufficient PRR LQI Unacked

  32. Physical Layer Information is NOT Sufficient PRR LQI Unacked

  33. Assignment • 1. What is the basic idea of the four-bit link estimation in wireless sensor networks? • 2. Why do we need wireless link estimation? • 3. What are the disadvantages if we only use physical layer estimation, data link layer estimation, and network layer estimation, respectively? • 4. Please give one example to illustrate why sometimes hop-count is not a good metric in WSN data forwarding.

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