1 Penn State, 4-29-08 Wireless Mesh with Mobility 3Q Update Thomas F. La Porta ([email protected]) & Guohong Cao ( [email protected]) The Pennsylvania State University Students: Hosam Rowaihy, Mike Lin, Tim Bolbrock, Qinghua Li Wireless Mesh with Mobility 1. Executive Summary 2. Schedule 3. Centralized 4. Distributed 5. Status
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Readers1. Receive queries2. Determine who serves3. Move to read data4. Upload results to cache
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Area of Responsibility
Areas of responsibility
– Change dynamically according to queries served (weighted moving average)
– If no readers covers a crate, closest serves it
Resting circle
– Mobile reader can reach any location within area of responsibility in < tseconds
– other basic scheme; return to center
Heavy load,Small area
Resting circle
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Area of Responsibility
Scenario: query arrives for tag located outside all areas of responsibility
a) Mobile RFID reader 1 calculates that it should move
b) Mobile RFID reader 1 moves
c) New AR is calculated
1
1 1
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Rest point
Readers must reside on or within circumference of rest circle
– Center will reposition based on movement
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Flexible Grid
Area of responsibility center remains constant
– Circumference changes based on movement
– Leads to stable data distribution
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Centralized Architecture Evaluation
• Consider both skewed and uniform queries
• Skewed queries are distributed using a burstiness algorithm to model temporal locality of queries and the Zipf distribution to model popular items
• 1,000,000 sq. ft. warehouse with 10,000 uniformly distributed RFID tags
• 1000 queries to 4 and 16 mobile readers
• Skewed and uniform results are similar
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Centralized Algorithm: Delay Results
Naïve solution is the best
16 robots
4 robots
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Centralized Algorithm: Distance Results
Naïve results are the best
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Distributed Architecture
Multi-hop network: may become disconnected due to mobility
– Algorithm updates required
–“Connected readers” run algorithm; search for others while moving
– Results returned to query point (similar process)
Implications
– Pre-positioning may help maintain connectivity
– Limiting movement may help maintain connectivity
reader
Crate
Crate Crate
Crate
Crate
Crate
Crate
CrateCrate
Crate
Crate
Crate
Crate
Crate
Crate
Crate
reader
reader
readerReaders1. Receive queries2. Locate “server”3. Return answer4. Local cache
Crate
Crate Crate
Crate
Crate
Crate
Crate
Cratereader
reader
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Distributed Architecture Example and Analysis
Comm
Q
R1
R2
R3
Gets Query
Moves
Comm
Moves
d1
d2
lg
,
A
typenetdDefinitions:
Alg – RP (rest point) or Naïve (N)
Net – multi-hop (MH) or centralized (C)
Type – non-reader (nr), or reader (r)
Total delay, T:
For centralized:
For fully connected network:
lg,
lg,
lg Arnet
nr
Anrnet
Anet ddT
0lg, Anrcd
0lg, Anrnetd
lg,ArcdT
lg,
ArnetdT
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Distributed Architecture vs. Centralized
More realistic case: network has some partitions
Naïve algorithm
AR algorithms
Nrmh
nr
Nnrmh
Nmh ddT ,,
Nrc
Nrmh dd ,, (we may or may not pick the optimal reader)
ARrmh
nr
ARnrmh
ARmh ddT ,,
ARrc
ARrmh dd ,, (we may or may not pick the optimal reader)
Nrmh
ARrmh dd ,, (based on empirical data)
nr
Nnrmh
nr
ARnrmh dd ,,
(based on empirical data)
ARmh
Nmh TT
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Multihop Evaluation
• 1,000,000 sq. ft. warehouse with 10,000 RFID tags
• Skewed and uniform queries (results are similar)
• Queries now originate from query sources on the edge of the warehouse
• Wireless transmission range of 300 ft.
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Multi-hop Results
Flexible grid performs the best, naive is one of the worst
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Multi-hop Results
Flexible grid outperforms by a significant margin
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Analysis
Flexible grid outperforms the other algorithms by a wide margin
Performance can be characterized by looking at the secondary distance travelled
– Secondary distance is the total distance travelled to respond to a query by readers that were not the first reader to receive the query:
Comm
Q
R1
R2
R3
Gets Query
Moves
Comm
Moves
d1
d2
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Secondary Distance
Flexible grid has a very small secondary distance compared to other algorithms
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Analysis
The forced structure of the flexible grid algorithm reduces the secondary distance
df - distance saved by forwarding query
dfFlex Grid is much higher relative to the overall distance travelled
Naive Flex Grid
dt 138745.5 16099.7
ds 118432.8 10072.8
ds/dt 85% 62%
df 391874.6 113249.8
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Analysis
Although all algorithms begin on a grid, only the flexible grid algorithm retains the structure, which increases the efficiency of forwarding queries and reduces the average distance the reader must travel
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Discussion
Centralized scheme will always be the best
– Always choose optimal reader
– No extra movement
– BUT: not always feasible
Flexible Grid scheme is best in a disconnected network
– Network is more “connected”
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Caching
Cache Path: keep record of how to reach data
– This is done in all mobile robots
– Used to determine nearest robot
– Results included in mobile reader results presented in previous slides
Cache Data: keep copies of data that have been gathered or forwarded