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Outline
- Motivation
- Existing compression techniques
– Standard compression (LZW, Adaptive Huffman) – Compression with packet retransmissions (RT)
- Proposed fault-tolerant compression (FT)
- Evaluation and conclusions
Low-delay compression for sensor networks Alexandre Guitton - - PowerPoint PPT Presentation
Low-delay compression for sensor networks Alexandre Guitton - - PowerPoint PPT Presentation
Low-delay compression for sensor networks Alexandre Guitton University of Oxford, Computing Laboratory Joint work with Niki Trigoni and Sven Helmer 1 MSN 2007, 12 th -13 th of July 2007 Outline Motivation Existing compression
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Motivation
- Sensor nodes are battery powered
– To save energy: compressing data before
transmitting it
- Challenge: lossy communication channels
- Performance metrics
– Energy-efficiency: – Delay between first transmission and decoding
Bytes of uncompressed data at the receiver Bytes transmitted by the sender
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Our focus
Application Application Compression Error correction encoding Error correction decoding Decompression raw data compressed packets compressed packets uncompressed data sent packets Sender Receiver Link
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Existing approaches Standard compression
Zebra sensing Zebra sensing LZW Hamming encoding Hamming decoding LZW “Zebra seen at 8:00 in (20,30)” 0010 1011 1100 1010 0010 X 1100 1010 “Zebra ” 0010(01) 1100(00) 1011(10) 1010(11) 0010(01) 1100(01) 0010(01) 1011(11)
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Existing approaches Standard compression
Compressing data with dynamic dictionaries is less energy-efficient than not compressing it when the packet drop rate exceeds 10%
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Existing approaches Compression with retransmissions
- Retransmission (RT) mechanism [Sadler and
Martonosi, 2006] to cope with packet losses
– Packets are grouped in blocks – Receiver sends block ACKs – Sender retransmits dropped packets – Compression is restarted at each block
- RT is applied to LZW (RT-LZW)
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Existing approaches Compression with retransmissions
Sender Receiver ghi abc def 10111 def 11111 jkl mno 11111
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Existing approaches Compression with retransmissions
Problem 1 Packets cannot be decoded as they arrive, they have to be decoded in order
Sender Receiver ghi abc def 10111 def 11111 jkl mno 11111
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Existing approaches Compression with retransmissions
Problem 1 Packets cannot be decoded as they arrive, they have to be decoded in order Problem 2 In the event of a disconnection, the effort of the sender is wasted
Sender Receiver ghi abc def 10111 10111 jkl mno 10111 10111 10111
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Existing approaches Compression with retransmissions
- To address these two problems
– Small blocks are used (the delay is not too large,
and energy is not wasted in case of a disconnection)
– The dictionary is restarted at the beginning of each
block (blocks are independent of each other)
- But
– Small blocks reduce the potential for compression
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Proposed fault-tolerant mechanism
- Fault-Tolerant (FT) mechanism
– Packets are grouped in blocks (as in RT) – Block ACKs (as in RT) – Dictionary is updated after each block (NOT after
each symbol)
– Each packet of a block can be decoded
independently of the other packets of the block
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno Dictionary updated with abcghijklmno
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno 1 def 1 pqr Dictionary updated with abcghijklmno
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno 1 stu 1 def 1 pqr 1 vwx 1 yza
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno 1 stu 1 def 1 pqr 1 vwx 1 yza 01111
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno 1 stu 1 def 1 pqr 1 vwx 1 yza 01111 0 efg 0 def 0 bcd 0 hij 0 klm
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno 1 stu 1 def 1 pqr 1 vwx 1 yza 01111 0 efg 0 def 0 bcd 0 hij 0 klm
Problem 1? Packets can be decoded as soon as they arrive
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Proposed fault-tolerant mechanism
Sender Receiver 0 ghi 0 abc 0 def 10111 0 jkl 0 mno 1 stu 1 def 1 pqr 1 vwx 1 yza 01111
Problem 1? Packets can be decoded as soon as they arrive Problem 2? In the event of a disconnection, all the packets that have been received are useful
01111 01111
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Proposed fault-tolerant mechanism
- Advantages of FT over the RT mechanism
– Packet can be decoded when they arrive – Dictionaries are not reinitialized at each block – Availability of the backward link is not mandatory
- Disadvantage
– Compression is conservative because the
dictionary is only updated at the end of each block
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Experimental setup
- We applied the RT and FT mechanism to LZW
(RT-LZW and FT-LZW)
– Real road traffic dataset (Scoot) – Block sizes of 20 and 66 packets – Varied packet loss rate on a link from 0% to 90%
- We measure
– Energy-efficiency – Delay
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Evaluation - Energy-efficiency
The energy-efficiency of RT-LZW increases with the block size
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Evaluation - Energy-efficiency
(1) The energy-efficiency of FT-LZW decreases as the block size increases and (2) small block sizes cannot be used in highly lossy environments
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Evaluation - Energy-efficiency
RT and FT mechanisms (1) degrade linearly as the packet drop rate increases, and (2) are comparable
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Evaluation - Delay
FT is 2-3 times faster than RT for all block sizes
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Conclusions
- Standard compression algorithms fail in lossy
environments
- FT is comparable to RT in terms of energy-
efficiency in static networks, and better in dynamic networks
- FT is 2 to 3 times faster than RT
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Thank you
RT FT
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Evaluation - Energy-efficiency
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Evaluation - Energy-efficiency
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Evaluation - Energy-efficiency
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