Upload
hiram-williams
View
33
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
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.