cs 410/510 sensor networks portland state university lecture 3 wireless communication
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04/18/23 Nirupama Bulusu 2
Source Acknowledgements
• Alberto Cerpa and Deborah Estrin
• Alec Woo and David Culler
• Jerry Zhao and Ramesh Govindan
04/18/23 Nirupama Bulusu 3
Outline
• IEEE 802.15.4 Wireless Communication Standard
• Single Hop packet loss characteristics– Axes
• Environment, distance, transmit power, temporal correlation, data rate, packet size
IEEE 802.15.4: Why the need?
• Sensor and Personal Area Networks require– Low Power Consumption– Minimal Installation Cost– Low Overall Cost
• Existing Technologies– Wired– 802.11 (WiFi) and Bluetooth
History
• Combination of Two Standards Groups– ZigBee Alliance: “an association of companies
working together to enable reliable, cost-effective, low-power, wirelessly networked, monitoring and control products based on an open global standard.”
– IEEE 802 Working Group 15
• Task Group 4 formed in December 2000– Low-rate Wireless Personal Area Network
Network Layer Guidelines
• 802.15.4 Specification does not address Network Layer
• Expected to be self-organizing and self-maintaining to minimize cost to user
• Two Network Topologies Supported:– Star Topologies– Peer-to-Peer Topologies
Data Link Layer
• Two Parts– Logical Link Control (LLC)
• Standard among many 802.x standards• Communicates with MAC through SSCS• Proprietary LLC’s can communicate directly
– MAC Sublayer• Data Service - Common Part Sublayer• Management Service – Management Entity
Superframe Beacons
• Time between beacons divided in 16 time slots• Can be used to provide bandwidth guarantees• Contention-free period and duration of
superframe announced in beacon
Additional MAC Features
• Channel Access Mediums– Slotted CSMA-CA– Unslotted CSMA-CA
• Acknowledgements
• Security– No security– Access Control Lists– Symmetric Key Security
Physical Layer
• Two Potential Physical Layers– 868/915Mhz– 2.4Ghz– Direct Sequence Spread Spectrum– Same Packet Structure
• 27 Frequency Channels Total
• Dynamic Channel Selection left to network layer
Other Physical Layer Features
• Modulation– 868/915 – Binary Phase Shift Keying– 2.4 – Offset Quadrature Phase Shift Keying
• Sensitivity and Range– 868/915 -92 dBm– 2.4 -85 dBm– 10-20m typical range
04/18/23 Nirupama Bulusu 18
Outline
• IEEE 802.15.4 Wireless Communication Standard
• Single Hop packet loss characteristics– Axes
• Environment, distance, transmit power, temporal correlation, data rate, packet size
04/18/23 Nirupama Bulusu 19
Zhao’s Study of Packet Loss
• Hardware– Mica, RFM 433MHz
• MAC– TinyOS Mac (CSMA)
• Encoding– Manchester (1:2)– 4b/6b (1:1.5)– SECDED (1:3)
• Environment– Indoor, Open Structure, Habitat Environment
04/18/23 Nirupama Bulusu 21
Indoor is the Harshest
• Linear topology over a hallway (0.5/0.25m spacing)• 40% of the links have quality < 70%• Lower transmit power
– yields smaller tail distribution
• SECDEC – significantly helps to lower the heavy tail
04/18/23 Nirupama Bulusu 22
Packet Loss and Distance
• Gray/Transitional Area– ranges from 20% to 50% of the communication range– Habitat has smaller communication range?– Other evidence (Cerpa et al., Woo et al.)– RFM: BAD RADIO??
04/18/23 Nirupama Bulusu 23
ChipCon Radio (Cerpa et al.)
• Higher transmit power doesn’t eliminate transitional region– Range in (a) and (b) are the same?
• Indoor RFM result is worst than that in Zhao’s work– cannot even see the effective region
Mica On Ceiling
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Can better coding help?
• SECDED is effective if start symbol is detected but does not increase “communication range”– Bit error rate (BER) is higher in transitional region
• Missing start symbol is fatal– Better coding for start symbol?
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Loss Variation (Cerpa et al.)
• Variation over distance and over time– binomial approximation for variation over time?
• Zhao shows that SECDED helps decrease the variation over distance (but very large SD here)
04/18/23 Nirupama Bulusu 26
Packet Loss vs. Workload
• Packet loss increases as network load increases– But what is the network load?– How many nodes are in range?
• Not sure! • Is 0.5 packets/s already in saturation?• Difficult to observe is it hidden node terminal
04/18/23 Nirupama Bulusu 27
Packet Loss vs. RSSI
• Low packet loss => good RSSI– But not vice versa– Too high a threshold limits number of links
• Network partition??
04/18/23 Nirupama Bulusu 28
Other Findings• Correlation of Packet Loss
– correlation at the gray (transitional) region for indoor– Habitat: much less
• Independent losses are reasonable
• 50%-80% of the retransmissions are wasted– Neighbor = hear a node once
• Asymmetric links are common– > 10% of link pairs have link quality difference > 50%– Cerpa et al.
• Moving a little bit doesn’t help• Swap the two nodes, asymmetrical link swaps too
– i.e. not due to the environment
04/18/23 Nirupama Bulusu 29
Packet Size (Cerpa et al.)
• Loss over distance is relatively the same for different packet size (25 bytes and 150 bytes) at different transmit power
04/18/23 Nirupama Bulusu 30
Lessons to Take Away• Who to blame?
– Radio?• Similar results found over RFM and ChipCon radio• Hardware calibration! Yeah!
– Base-band radio• Multi-path will remain unless spread-spectrum radio is used
– But 802.11 is also not ideal (Decouto et al. Mobicom 03)
• What is the effective communication range?– What does it mean when you deploy a network
• What defines a neighbor?• Why study high density sensor network?