Opportunistic Routing in
Multi-hop Wireless Networks
Shunyuan Ye
12/16/05
Paper
• "EXOR: Opportunistic Multi-Hop Routing for
Wireless Networks“, Sanjit Biswas and Robert
Morris, In Proc. of ACM/SIGCOMM 2005
Outline
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Introduction
Basic idea
Protocol design details
Measurements
Conclusion
Traditional routing
• Identify a route, forward over links
• Abstract radio to look like a wired link
Radios aren’t wires
• Packet is broadcast
• Reception is probabilistic
Opportunistic routing
• Motivation
– Exploiting probabilistic broadcast
• Goal
– High throughput and network capacity
• ExOR
– Extremely opportunistic routing
Outline
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Introduction
Basic idea
Protocol design details
Measurements
Conclusion
Why ExOR good?
• Assumes independent losses
• Traditional routing: 1/0.25 + 1 = 5 tx
• ExOR: 1/(1-(1-0.25)^4) + 1 = 2.5 tx
Basic idea
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Probability falls off gradually with distance
Traditional route through N2, N4
EXOR exploits lucky long receptions
Node closest to the dst has highest priority
Outline
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Introduction
Basic idea
Protocol design details
Measurements
Conclusion
Protocol Design
• Source’s behavior
• Intermediate nodes’ behavior
• Destination behavior
Source’s 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 overhead of every packet
Priority ordering
• Goal: nodes “closest” to the destination
send first
• Higher delivery probability, closer to the
destination
Priority ordering (2)
• ETX=1/(delivery probability)
• Sort by ETX metric to dst
• Nodes periodically flood ETX “link state”
measurement
Protocol Design
• Source’s behavior
• Intermediate nodes’ behavior
• Destination behavior
Forwarders’ behavior (1)
• How can a node know whether it is one
of the forwarders or not?
• 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
Forwarders’ behavior (2)
• How can a node know whether the packet
it receives has also been received by a
node with higher priority or not?
• ExOR designs a “batch map” to record,
for every packet in the batch, the
highest-priority node known to have
received that packet.
Batch map example
• src generates a batch map before tx
• src inserts the batch map into the
overhead of every packet in this batch
Batch map example
• When N1 first receives a packet from
src, it keeps the batch map in local
• N1 updates its batch map when receiving
new packets of this batch
Batch map example
• N2 receives packets from src, so it
doesn’t know that N1 has received
packet 1 and 2
Batch map example
• After scr finished transmission, N2
begins to broadcast packets it received
• N1 also received these packets and then
update its local batch map
Batch map example
• src updates its local map and gets into
know that all the packets in the batch
has been received by dst or
intermediate nodes, no acknowledge is
needed.
Forwarders’ behavior (3)
• How can a node know when is its turn to
transmit?
• 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
Fragment example
• N2’s fragment size is 2 and fragment no.
is 1 for packet 3 and 2 for packet 2
Forwarding estimate
• When N1 receives packet 3 from N2, it
knows that there is still one packet N2
has to send, then N1 can estimate the
time to forwarding its fragment
Protocol Design
• Source’s behavior
• Intermediate nodes’ behavior
• Destination behavior
Destination’s behavior
• Actually destination is the last
intermediate node and has the highest
priority.
• After the finish of src’s transmission.
Destination sends out ten packets only
including the batch map, to inform other
nodes about the packets it has received
Outline
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Introduction
Basic idea
Protocol design details
Measurements
Conclusion
65 Roofnet node pairs
Exor: 2x overall improvement
• Median throughputs:
240Kbit/s for ExOR
121Kbit/s for Traditional
25 highest throughput pairs
25 lowest throughput pairs
Conclusion
• ExOR achieves 2x throughput
improvement
• Future work will focus on:
– Choosing best 802.11 bit rate
– Cooperation between simultaneous flows