1. David Culler Fall 2003 University of California, Berkeley
CS294-1 Deeply Embedded Networks Broadcast / Dissemination Sept 18, 2003
David Culler
Fall 2003
University of California, Berkeley
Thank you
Tag team: software / tinyos part, Kris hardware/smart dust part
Hopefully let you play with the toys so you don’t get too anxious before lunch

2. BCAST: Fundamental building block
Command
Wake-up
Form routing tree
Discover route
Source-destination discovery (DSR, AODV)
Exploration in directed diffusion
Time-synch
Constructed from underlying local broadcast (cell)
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3. Flooding Simple Epidemic Algorithm Schema Command, dissemination
if (new bcast msg) then
take local action
retransmit modified request
Command, dissemination
Build spanning tree
record parent
Naturally adapts to available connectivity
Minimal state and protocol overhead
=> surprising complexity in this simple mechanism
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4. Network Discovery: Radio Cells
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5. Network Discovery
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6. Controlled Empirical Study
Experimental Setup
13x13 grid of nodes
separation 2ft
flat open surface
Identical length antennas, pointing vertically upwards.
Fresh batteries on all nodes
Identical orientation of all nodes
The region was clean of external noise sources.
Range of signal strength settings
Log many runs
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7. Example “epidemic” tree formation
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8. Final Tree
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9. Open Territory => Many Children
Example: Level 1
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10. Open Territory => Many Children
Example: Level 2 – variation in distance
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11. Open Territory => Many Children
Example: Level 3 – long links
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12. Power Laws ? Most nodes have very small degree (ave = .92)
Some have degree = 15% of the population
Few large clusters account for most of the edges
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13. Understanding Connectivity
16 transmit power settings
For each transmit power setting, each node transmits 20 packets.
Receivers log successfully received packets.
Nodes transmit one after the other in a token-ring fashion
No collisions.
Define “range”: radius where 75% of enclosed nodes receive 75% of packets
Often good nodes at a distance
probability of reception from center node vs xmit strength
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14. Sensitivity
Hardware platform?
MAC?
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15. Broadcast Storm Redundant broadcasts Contention Collisions
Transmission when all nbrs have the message
Contention
After a transmission, many neighbors likely to rebroadcast at nearly same time
Highly correlated behavior
Collisions
Node is likely to hear multiple conflicting transmissions
Storm paper cites lack of RTS/CTS, but is more fundamental
=> everyone need not transmit
Reduce redundancy.
Will also reduce contention and collision
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16. Collisions Nodes out of range may have overlapping cells
hidden terminal effect
Collisions => these nodes hear neither ‘parent’
become stragglers
As the tree propagates
folds back on itself
rebounds from the edge
picking up these stragglers.
Seen in many experiments
Mathematically complex because behavior is not independent beyond singe cell
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17. SPIN version of problems
Implosion
Node hears same msg from many neighbors
Overlap
Two transmitting nodes may have large intersection of neighbors
Resource blindness
Some nodes may have to do more than others
Favor energy-rich nodes when winowing redundancy
Coming from IP multicast perspective
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18. Estimating Redundancy
Geometric overlap
At best an additional 41% area
Decreasing yield with number of neighbors msgs
Estimation of contention
Potential contention
Delay can always take prob. Of contention to zero
Collisions
No RTS/CTS to help
Observes that MACs don’t pay enough attention one-to-many case
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19. Selective Retransmission Schemes
Probabilistic Retransmission
Fixed prob.
What would be the right choice?
Counter
When hear msg, start random delay
If hear C msgs during wait, don’t retransmit
Distance
If nearest node from which msg is heard is less than some threshold, don’t retransmit
Location
If portion of cell not covered by transmitting neighbors is less than some threshold, don’t retransmit
Cluster-based
Partition graph into cluster heads, gateways, and members
Members don’t transmit
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20. Ni findings: Probabilistic Squelch
What is the expected degree of each configuration?
100 in nxn => ~40/n (except n=1)
Is “latency” meaningful when reachability is low
How many neighbors transmit?
Savings versus nodes per cell
Reducing contention trims latency tail
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21. Are random-placement graphs reasonable?
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22. P = Prob(exists unbounded connected component)
Percolation of Random Graphs λ P λ2 1 λc λ1
P = Prob(exists unbounded connected component)
Ed Gilbert (1961)
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23. Example
l=0.3
l=0.4
= …[Quintanilla, Torquato, Ziff, J. Physics A, 2000] lc r2
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24. A different connectivity model
Presence of long links dramatically change overlap estimate
Changes the percolation point too.
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25. CNP for the standard connection model (disc) Shifting and squeezing
Squishing and squashing
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MASSIMO FRANCESCHETTI

26. Asymmetric Links Asymmetric Link: 10%-25% of links are asymmetric
>65% successful reception in one direction
<25% successful reception in the other direction
10%-25% of links are asymmetric
Many long links are asymmetric
in large field it is likely that someone far away can hear you
what does this mean for protocol design?
OK for broadcast, not for tree build
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27. Counter Scheme
At reasonable degree, (16 in 5x5) need C=4 and only save ~10%
Delay improves latency
Delay may be more fundamental than squelch
But Ni uses single delay, independent of nbhd
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28. Distance and location Modest Savings at reasonable degree. 11/12/2018
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29. Flooding vs Gossip
In gossip protocols, at each step pick a random neighbor
Assumes an underlying connectivity graph
Typically used when graph is full connected
E.g., ip
Much slower propagation
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30. SPIN Assume data much larger than protocol message 3-phase handshake
600 bytes vs 16
3-phase handshake
ADV – advertise new data
Send to indicate have data to xmit
REQ – request for data
Response saying go ahead
Data
If all nbrs have data, no req comes back
If one req’s, others snoop data and squelch own request
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31. Spin Example
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32. Delay Upon receiving ADV, If receive REQ What if links not symmetric?
If not already received data or ADV
set random timer
When expires, if still no data, send REQ
If receive REQ
Cancel own REQ
What if links not symmetric?
If not enough energy to finish, don’t REQ
Spin-RL
Set timeout on req and retry till get
Limit resend rate
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33. Study
Lossless
No contention!
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34. With Loss (and contention)
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35. Traffic ADV(i) is fundamentally no different from data(i)
SPIN does nothing to eliminate redundancy in ADV
Does introduce delay to reduce collision and contention
Loss is fundamental since hidden nodes are prevasive
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36. Generalizing from one graph?
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37. Adaptive BCAST rate Upon first msg of epoch Upon expiration of delay
Start random delay
If new msg arrives during delay
Filter message (eg., discard if signal strength below threshold)
If passes filter, Utilize message
Start new delay
Upon expiration of delay
Complete local processing
E.g., pick lowest depth node with strongest signal as parent
Retransmit
Delay is proportion to cell density
Wait till ngbrs go quiet before transmit
Approx uniform transmissions per unit area, regardless of node density
Exploit long links when appropriate
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38. Example Tree
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39. Is this a good broadcast tree?
Only internal nodes retransmit?
How much will you eliminate?
Graph analysis of spatially constrained trees
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40. What more to ensure reliability
Retransmit till neighbors get it
Must maintain underlying connectivity graph
Ambient repair
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41. questions Methodology for evaluating bcast
Data Size, one-time, repetitive
Net Scale, density variation
Underlying mac, radio
Reliability, convergence
What is the right way to do it?
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