Link Estimation, CTP and MultiHopLQI

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

Prerequisites Basic concepts of wireless communications Basic concepts of physical layer, data link layer, and network layer

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 –Alternating between high (100% packet reception ratio PRR) and low (0% PRR) quality Link asymmetries Hardware variations Ref. [LinkEstimation_1 ]: Section 1

Detour: Time-varying nature of a wireless channel

802.11b and Spectrum Utilization Ref: [Implication_1] Section 3 and 4

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

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

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

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

Reception Probability [Other_1]: Figure 1

Reception Probability [Other_1]: Figure 1

Link Estimation – ETX (Expected Transmission Count)

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

Results on Experimental Testbed Figure 2 of [ETX_1]

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

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

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

Link Estimation Metric - ETX Forward delivery ratio: d f –The probability that a data packet successfully arrives at the recipient Reverse delivery ratio: d r –The probability that the ACK packet is successfully received The expected probability that a transmission is successfully received and acknowledged is d f X d r ETX = 1 / (d f X d r ) Ref. [ETX_1]: Section 1, 3

How to Measure d f and d r Each node broadcasts link probes of a fixed size, at an average period of τ –Receive a probe every τ seconds Each node remembers the probes it receives during the last w seconds The ETX of a route is the sum of the link metrics

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

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

Link Estimation – Four-bit Wireless Link Estimation

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

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

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

[LinkEstimation_1]: Section 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

Four Bit Interface Details WHITE Packets on this channel experience few errors ACK A packet transmission on this link was acknowledged PIN Keep this link in the table COMPARE Is this a useful link?

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

Physical Layer Information is NOT Sufficient Unacked PRR LQI

Physical Layer Information is NOT Sufficient Unacked PRR LQI