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Opportunistic Multi-Hop Routing for Wireless NetworksSanjit Biswas and Robert Morris M.I.T. Computer Science and Artificial Intelligence Lab
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Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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An integrated routing and MAC protocol Aims to increase the throughput of large
unicast transfers in multi hop wireless networks.
Based on cooperative diversity routing
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Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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Why the need for a new routing protocol ?
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Why the need for a new routing protocol ?
Traditional routing protocols were designed for wired networks.
These protocols don’t take into account underlying wireless dynamics at MAC and PHY layer.
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Reference : 802.11 Wireless Networks: The Definitive Guide O'Reilly Publications . Mathew Gast. April 2005
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RF Link Quality – As compared to wired Ethernet, 802.11 has to deal with interference, noise and multipath fading in unlicensed bands which results in loss of packets.
Solution – Positive Acknowledgments When ACK doesn’t arrive , packet is
retransmitted.
Reference : 802.11 Wireless Networks: The Definitive Guide O'Reilly Publications . Mathew Gast. April 2005
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ExOR tries to avoid these retransmissions by relying on cooperative diversity scheme.
Reference : 802.11 Wireless Networks: The Definitive Guide O'Reilly Publications . Mathew Gast. April 2005
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Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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Traditional Routing Protocols
Choose Best Sequence of nodes Between source and Destination
Cooperative Diversity Protocols
Uses broadcast transmission to Send info through multiple relays
Routing Protocols
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Classic network architectures employ point to point transmissions.
Cooperative transmission takes advantage of broadcast transmission to send information through multiple relays concurrently.
Originally meant to avoid multipath fading.
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Introduction Motivation Background ExOR Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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ExOR broadcasts each packet, choosing a receiver to forward only after learning the set of nodes that actually received the packet.
By delaying this decision until after reception, multiple long but radio lossy links are given a trial.
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Traditional Routing ExOR Routing
Source
Destination
Source
Destination
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Traditional Routing ExOR Routing
Source
Destination
Source
Destination
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Traditional Routing ExOR Routing
Source
Destination
Source
Destination
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Traditional Routing ExOR Routing
Source
Destination
Source
Destination
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Traditional Routing ExOR Routing
Source
Destination
Source
Destination
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Traditional Routing ExOR Routing
Source
Destination
Source
Destination
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Introduction Motivation Background ExOR Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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Best traditional route over 50% hops: 3(1/0.5) = 6 tx
Throughput 1/# transmissions
ExOR exploits lucky long receptions: 4 transmissions
Assumes probability falls off gradually with distance
src dstN1 N2 N3 N4
75%50%
N5
25%
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Traditional routing: 1/0.25 + 1 = 5 tx ExOR: 1/(1 – (1 – 0.25)4) + 1 = 2.5 transmissions Assumes independent losses
N1
src dst
N2
N3
N4
25%
25%
25%
25%
100%
100%
100%
100%
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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Agreement amongst the nodes which received each packet.
Need for a metric which decides the node which is closest to the destination.
Not too many nodes should be potential forwarders.
Minimize collisions.
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Agreement amongst the nodes which received each packet.
Agreement protocol should have low overhead.
Solution -- Batch Forwarding and Node State
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Challenge: finding the closest node to have rx’d
Send batches of packets for efficiency Node closest to the dst sends first
Other nodes listen, send remaining packets in turn
Repeat schedule until dst has whole batch
src
N3
dst
N4
tx: 23
tx: 57 -23 24
tx: 8
tx: 100
rx: 23
rx: 57
rx: 88
rx: 0
rx: 0tx: 0
tx: 9
N1
N2
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Each node maintains this for each batch of packets its operating on.
Packet Buffer Local Forwarder List Forwarding timer Transmission Tracker Batch Map
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Goal: nodes “closest” to the destination send first
Sort by ETX metric to dst Nodes periodically flood ETX “link state”
measurements Path ETX is weighted shortest path (Dijkstra’s
algorithm) Source sorts, includes list in ExOR header
src
N1
N2
N3
dst
N4
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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65 Node pairs 1.0MByte file transfer 1 Mbit/s 802.11 bit rate 1 KByte packets
Traditional Routing ExOR802.11 unicast with link-level retransmissionsHop-by-hop batchingUDP, sending as MAC allows
802.11 broadcasts100 packet batch size
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Median throughputs: 240 Kbits/sec for ExOR, 121 Kbits/sec for
Traditional
Throughput (Kbits/sec)
1.0
0.8
0.6
0.4
0.2
00 200 400 600 800C
um
ula
tive F
ract
ion o
f N
ode P
air
s
ExORTraditional
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Node Pair
4 Traditional Hops3.3x
Longer Routes
Thro
ughput
(Kbit
s/se
c)
0
200
400
600
800
1000 ExORTraditional Routing
Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt
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Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion
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Positives Works well on long range links Works well on one hop links
Weaknesses Doesn’t integrate well with TCP Working with multiple radios