Reliable bursty convergecast in wireless sensor networks
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Reliable Bursty Convergecast in Wireless Sensor Networks. Hongwei Zhang, Anish Arora Young-ri Choi, Mohamed Gouda. Thanks: Lites & ExScal teams. Application context. A Line in the Sand (Lites)

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Reliable bursty convergecast in wireless sensor networks

Reliable Bursty Convergecast in Wireless Sensor Networks

Hongwei Zhang, Anish Arora Young-ri Choi, Mohamed Gouda

Thanks: Lites & ExScal teams


Application context
Application context

  • A Line in the Sand (Lites)

    • field sensor network experiment for real-time target detection, classification, and tracking

  • A target can be detected by tens of nodes

    • Traffic burst

  • Bursty convergecast

    • Deliver traffic bursts to a base station nearby


Problem statement
Problem statement

  • Only 33.7% packets are delivered with the default TinyOS messaging stack

    • Unable to support precise event classification

  • Our objectives

    • Close to 100% reliability

    • Close to optimal event goodput (real-time)

  • Experimental study for high fidelity


Outline
Outline

  • Testbed

  • Limitations of two commonly used mechanisms

  • Protocol RBC

  • Experimental results

  • Concluding remarks


Network setup

base station

Network setup

  • Network

    • 49 MICA2s in a 7 X 7 grid

    • 5 feet separation

    • Power level: 9 (for 2-hop reliable communication range)

  • Logical Grid Routing (LGR)

    • It uses reliable links

    • It spreads traffic uniformly


Traffic trace from lites
Traffic trace from Lites

  • Packets generated in a 7 X 7 subgrid, when a vehicle passes across the middle of the Lites network

  • Optimal event goodput:

    6.66 packets/second


Outline1
Outline

  • Testbed

  • Limitations of two commonly used mechanisms

  • Protocol RBC

  • Experimental results

  • Concluding remarks


Retransmission based packet recovery
Retransmission based packet recovery

  • At each hop, retransmit a packet if the corresponding ACK is not received after a constant time

    • Synchronous explicit ack (SEA)

      • Explicit ACK immediately after packet reception

      • Shorter retransmission timer

    • Stop-and-wait implicit ack (SWIA)

      • Forwarded packet as an ACK

      • Longer retransmission timer


SEA

  • Retransmission does not help much, and may even decrease reliability and goodput

  • Similar observations when adjusting contention window of B-MAC and using S-MAC

  • Retransmission-incurred contention


SWIA

  • Again, retransmission does not help

  • Compared with SEA, longer delay and lower goodput/reliability

    • longer retransmission timer & blocking flow control

    • More ACK losses, and thus more unnecessary retransmissions


Outline2
Outline

  • Testbed

  • Limitations of two commonly used mechanisms

  • Protocol RBC

  • Experimental results

  • Concluding remarks


Protocol rbc
Protocol RBC

  • Differentiated contention control

    • Reduce channel contention caused by packet retransmissions

  • Window-less block ACK

    • Non-blocking flow control

    • Reduce ack loss

  • Fine-grained tuning of retransmission timers


Window less block ack
Window-less block ACK

Non-blocking window-less queue management

  • Unlike sliding-window based black ACK, in order packet delivery is not considered

    • Packets have been timestamped

  • For block ACK, sender and receiver maintain the “order” in which packets have been transmitted

    • “order” is identified without using sliding-window, thus there is no upper bound on the number of un-ACKed packet transmissions


Sender queue management

VQ0

1

2

high

VQ1

3

4

5

occupied

ID of buffer/packet

VQM

low

VQM+1

empty

6

static

physical queue

ranked

virtual queues (VQ)

Sender: queue management

M: max. # of retransmissions


Sender gets a packet from an upper layer

empty queue buffer?

Sender: gets a packet from an upper layer

VQ0

1

2

VQ1

3

4

5

VQM

VQM+1

6


Sender transmits a packet

2

Sender: transmits a packet

1,

2

VQ0

fresher

1

VQ1

3

4

5

earlier

later

order of

transmission

VQM

older

VQM+1

6


Receiver loss detection

no loss

=

if no packet loss, expecting packet j

i’= j

some loss

Receiver: loss detection

i, j

i

i’


Receiver block ack

 

k’

Receiver: block ACK

i

j

k

  

i, k’ 

i, j 

i, i 

i, k

ACK replication !


Sender processes a block ack

3

4

5

Sender: processes a block ACK

3, 5

VQ0

1

2

VQ1

VQM

VQM+1

6


Differentiated contention control
Differentiated contention control

  • Schedule channel access across nodes

  • Higher priority in channel access is given to

    • nodes having fresher packets

    • nodes having more queued packets


Implementation of contention control
Implementation of contention control

  • The rank of a node j = M - k, |VQk|, ID(j) , where

    • M: maximum number retransmissions per-hop

    • VQk: the highest-ranked non-empty virtual queue at j

    • ID(j): the ID of node j

  • A node with a larger rank value has higher priority

  • Neighboring nodes exchange their ranks

    • Lower ranked nodes leave the floor to higher ranked ones


Fine tuning retransmission timer
Fine tuning retransmission timer

  • Timeout value: tradeoff between

    • delay in necessary retransmissions

    • probability of unnecessary retransmissions

  • In RBC

    • Dynamically estimate ACK delay

    • Conservatively choose timeout value; also

      reset timers upon packet and ACK loss


Outline3
Outline

  • Testbed

  • Limitations of two commonly used mechanisms

  • Protocol RBC

  • Experimental results

  • Concluding remarks


Event wise
Event-wise

  • Retransmission helps improve reliability and goodput

    • close to optimal goodput (6.37 vs. 6.66)

  • Compared with SWIA, delay is significantly reduced

    • 1.72 vs. 18.77 seconds


Distribution of packet generation and reception
Distribution of packet generation and reception

  • RBC

    • Packet reception smoothes out and almost matches packet generation

  • SEA

    • Many packets are lost despite quick packet reception

  • SWIA

    • Significant delay and packet loss


Breakdown of rbc
Breakdown of RBC

RBC-NoDiffCtrl: RBC without Differentiated Contention Control

  • Contention control plays an increasingly important role as RT (thus channel contention) increases


Field deployment
Field deployment

  • A Line in the Sand (Lites)

    • ~ 100 MICA2’s

    • 10 X 20 meter2 field

    • Sensors: magnetometer, micro impulse radar (MIR)

  • ExScal

    • ~ 1,000 XSM’s, ~ 200 Stargates

    • 288 X 1260 meter2 field

    • Sensors: passive infrared radar (PIR), acoustic sensor, magnetometer


Outline4
Outline

  • Testbed

  • Limitations of two commonly used mechanisms

  • Protocol RBC

  • Experimental results

  • Concluding remarks


Concluding remarks
Concluding remarks

  • With its unique traffic pattern and performance requirements, bursty convergecast

    • poses new challenges to error control

      • Non-blocking packet delivery

      • Retransmission scheduling

    • also offers opportunities

      • E.g., reorder-tolerance

  • Other applications

    • Continuous event convergecast

    • Data aggregation

      • to use explicit ack


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