EXOR

1
EXOR

• Opportunistic Multi-Hop Routing for Wireless
  Networks
• Sanjit Biswas and Robert Morris
• M.I.T. Computer Science and Artificial
  Intelligence Lab

2
Outline

• Introduction
• Motivation
• Background
• Basic Idea of how ExOR works
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

3
What is ExOR ?

• 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

4
Outline

• Introduction
• Motivation
• Background
• Basic Idea of how ExOR works
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

5
Motivation

• Why the need for a new routing protocol ?

6
Motivation

• Why the need for a new routing protocol ?
• Traditional routing protocols were designed for
  wired networks.
• These protocols dont take into account
  underlying wireless dynamics at MAC and PHY
  layer.

7
Motivation
Reference 802.11 Wireless Networks The
Definitive Guide O'Reilly Publications . Mathew
Gast. April 2005
8
Motivation

• 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 doesnt arrive , packet is
  retransmitted.

Reference 802.11 Wireless Networks The
Definitive Guide O'Reilly Publications . Mathew
Gast. April 2005
9
Motivation

• 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
10
Outline

• Introduction
• Motivation
• Background
• Basic Idea of how ExOR works
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

11
Approaches to Routing
Routing Protocols
Traditional Routing Protocols Choose Best
Sequence of nodes Between source and Destination
Cooperative Diversity Protocols Uses broadcast
transmission to Send info through multiple
relays
12
Cooperative Diversity In Wireless Networks

• 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.

13
Outline

• Introduction
• Motivation
• Background
• ExOR
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

14
How is ExOR different ?

• 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.

15
Source
Source
Destination
Destination
Traditional Routing
ExOR Routing
16
Source
Source
Destination
Destination
Traditional Routing
ExOR Routing
17
Source
Source
Destination
Destination
Traditional Routing
ExOR Routing
18
Source
Source
Destination
Destination
Traditional Routing
ExOR Routing
19
Source
Source
Destination
Destination
Traditional Routing
ExOR Routing
20
Source
Source
Destination
Destination
Traditional Routing
ExOR Routing
21
Outline

• Introduction
• Motivation
• Background
• ExOR
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

22
Why ExOR might increase throughput (1)
src
dst
N1
N2
N3
N4
N5
75
50
25

• 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

Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
23
Why ExOR might increase throughput (2)
N1
25
100
25
100
N2
100
25
src
dst
N3
100
25
N4

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

Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
24
Outline

• Introduction
• Motivation
• Background
• Basic Idea of how ExOR works
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

25
ExOR design challenges

• 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.

26
ExOR design challenges

• Agreement amongst the nodes which received each
  packet.
• Agreement protocol should have low overhead.
• Solution -- Batch Forwarding and Node State

27
ExOR batching
tx 0
N4
N2
tx 57 -23 ? 24
tx 100
tx ? 9
src
dst
N3
N1
tx ? 8
tx 23

• Challenge finding the closest node to have rxd
• 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

Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
28
Node State

• Each node maintains this for each batch of
  packets its operating on.
• Packet Buffer
• Local Forwarder List
• Forwarding timer
• Transmission Tracker
• Batch Map

29
Packet Format
30
Forwarder List
N2
N4
src
dst
N1
N3

• 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 (Dijkstras
  algorithm)
• Source sorts, includes list in ExOR header

Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
31
Scheduling Transmissions
32
Outline

• Introduction
• Motivation
• Background
• Basic Idea of how ExOR works
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

33
Evaluation Details

• 65 Node pairs
• 1.0MByte file transfer
• 1 Mbit/s 802.11 bit rate
• 1 KByte packets

Traditional Routing ExOR
802.11 unicast with link-level retransmissions Hop-by-hop batching UDP, sending as MAC allows 802.11 broadcasts 100 packet batch size
Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
34
ExOR 2x overall improvement
1.0
0.8
0.6
Cumulative Fraction of Node Pairs
0.4
0.2
ExOR
Traditional
0
0
200
400
600
800
Throughput (Kbits/sec)

• Median throughputs 240 Kbits/sec for ExOR,
• 121 Kbits/sec for Traditional

Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
35
25 Lowest throughput pairs
1000
ExOR
4 Traditional Hops 3.3x
Traditional Routing
800
600
Throughput (Kbits/sec)
400
200
0
Node Pair
Longer Routes
Reference pdos.csail.mit.edu/biswas/exor-sigcom
m.ppt
36
Outline

• Introduction
• Motivation
• Background
• Basic Idea of how ExOR works
• Why ExOR works ?
• Design Challenges
• Results and Evaluation
• Conclusion

37
Conclusions

• Positives
• Works well on long range links
• Works well on one hop links
• Weaknesses
• Doesnt integrate well with TCP
• Working with multiple radios