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• Understand the impact of link estimation on

representative TinyOS network protocols

Prerequisites

• Module 2• Basic concepts of wireless communications• Basic concepts of computer networks

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 802.15.4 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: 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

How to Measure df and dr

• 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 3.1 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

Four Bit Interface Details

WHITEPackets on this channel experience few errors

ACKA packet transmission on this link was acknowledged

PINKeep this link in the table

COMPAREIs 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

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.