Opportunistic Multihop Routing for Wireless Networks

1
Opportunistic Multi-hop Routing for Wireless
Networks

• EXOR Opportunistic Multi-Hop Routing for
  Wireless Networks, Sanjit Biswas and Robert
  Morris, of ACM/SIGCOMM 2005
• Presented by Peng Guan

2
Outline

• Background
• Basic idea
• Protocol design details
• Measurements
• Conclusion

3
Background

• Wireless networks

Wireless mesh network
Wireless ad hoc network
4
Traditional routing
packet
packet
packet
A
B
src
dst
packet
packet
C

• Identify a route, packets forward over fixed path
• Retried on failures

5
Wireless different from wire
A
B
No such thing as a LINK
src
dst
1
2
3
4
5
6
1
2
3
6
3
5
4
2
3
4
5
6
1
2
4
5
6
1
C

• Packet is broadcast
• Reception is probabilistic

6
Outline

• Background
• Basic idea
• Protocol design details
• Measurements
• Conclusion

7
Basic idea
A
B
src
dst
packet
packet
packet
packet
packet
C
packet
packet
packet
packet

• Figure out which nodes rxd broadcast
• Node closest to destination forwards

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Basic idea (2)
1
1
2
2
1
s
3
batch
d
1
2
1
2
2
1
4
5
1

• The source broadcasts a batch to a sub-set of
  nodes
• The node closest to the destination forwards
  (broadcast) fragment

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ExOR might providemore throughput

• Assumes independent losses
• Traditional routing 1/0.25 1 5 tx
• ExOR 1/(1-(1-0.25)4) 1 2.5 tx

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ExOR might providemore throughput (contd)

• Probability falls off gradually with distance
• Traditional route through N2, N4
• Take advantage of transmissions that reach
  unexpectedly far, or fall unexpectedly short
• Node closest to the dst has highest priority

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Assumptions and Goals

• Assumptions
• Many receivers hear every broadcast
• Gradual distance-vs-reception tradeoff
• Receiver losses are uncorrelated

• Goals
• Exploiting probabilistic broadcast
• High throughput and network capacity

12
Outline

• Background
• Basic idea
• Protocol design details
• Measurements
• Conclusion

13
ExOR Packet header format
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Protocol Design

• Sources behavior
• Intermediate nodes behavior
• Destination behavior

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Sources behavior

• Collects enough packets of the same destination
  to form a batch
• ExOR operates on batches of packets for
  efficiency
• Selects a set of nodes to be candidate
  forwarders, and includes the prioritized list in
  the header of every packet

16
Priority ordering

• Goal nodes closest to the destination send
  first
• Higher delivery probability, closer to the
  destination

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Priority ordering (2)

• ETX1/(delivery prob)
• Sort by ETX metric to dst

Forwadlist ECDBA Broadcast in this order
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Protocol Design

• Sources behavior
• Intermediate nodes behavior
• Destination behavior

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Forwarders behavior

• Q How can a node know whether it is one of the
  forwarders or not?
• A Check the forwarder list in the overhead of
  the received packet
• If the node finds itself in the list, buffer the
  packet and keep state of this batch
• If no, discard the packet

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Forwarders behavior (2)

• Q How can a node know whether the packet it
  receives has also been received by a node with
  higher priority or not?
• A ExOR designs a batch map to record, for
  every packet in the batch, the highest-priority
  node known to have received that packet.

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Forwarders behavior (3)

• Q How can a node know which packet it should
  transmit?
• A ExOR add a fragment size and fragment
  number in the overhead of every packet.
• Fragment size number of packets the node has to
  send
• Fragment no. the index of the sending packet in
  the fragment

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Forwarders behavior (4)
B 11SSS
1
2
1
D
S
3
1
4
2
B SSDSS
2
1
2
3
4
5
B SSSSS
B 2SD2S
1
3
4
2
B 2S22S
B 2SD2S
Sending Packets 1,2,3,4,5 Forwarder List
D,2,1,S
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Protocol Design

• Sources behavior
• Intermediate nodes behavior
• Destination behavior

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Destinations behavior

• Actually destination is the last intermediate
  node and has the highest priority.
• After the finish of srcs transmission.
  Destination sends out ten packets only including
  the batch map, to inform other nodes about the
  packets it has received

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Outline

• Background
• Basic idea
• Protocol design details
• Measurements
• Conclusion

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65 Roofnet node pairs
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ExOR 2x overall improvement

• Median throughputs
• 1. 240Kbit/s for ExOR
• 2. 121Kbit/s for Traditl

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25 highest throughput pairs
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25 lowest throughput pairs
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Conclusion

• ExOR achieves 2x throughput improvement
• Future work will focus on
• Choosing best 802.11 bit rate
• Cooperation between simultaneous flows

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THANKS

• QA?