EXOR: OPPORTUNISTIC MULTI-HOP
ROUTING FOR WIRELESS NETWORKS
BY
SANJIT BISWAS AND ROBERT MORRIS
Yin Lin
AGENDA

Motivation

Design

Evaluation

Summary
TRADITIONAL WIRELESS ROUTING = WIRED
ROUTING
ExOR: multi-hop unicast wireless routing protocol
 Traditional: fixed routing sequence

Choose best sequence
 Always forward through that sequence

D
B
E
A
F
C
Dst
NH
B
B
Dst
NH
Dst
NH
C
C
A
A
A
C
D
B
B
A
B
D
E
C
…
…
…
…
F
C
F
E
F
F
Routing Table
Dst: Destination
NH: Next hop
RADIOS AREN’T WIRES
 Every
packet is broadcast
 Reception is probabilistic
A
1 2 33 4455
56
66
B
src
dst
C
PROBLEM:
UNNECESSARY RETRANSMISSION
B
C
A
D
E
F
G
PROBLEM: UNNECESSARY HOPS
 Only
reach N1: result in retransmission
 Luckily reach N3: distance wasted
src
N1
N2
N3
N4
N5
dst
BASIC IDEA : ROUTE AFTER RECEPTION
Buffer received packets
 Node closest to dst forward packet

B
A
2.28
1.38
F
E
C
1.17
1.43
D
FORWARDER LIST: SHOULD I FORWARD?
Distance metric: ETX to dst
 Updated dynamically
 Sort nodes w.r.t. distance

Ethernet Header
… FL
BM
Data
ETX=2.28
B
C
ETX=1.17
ETX=2.85
A
E
ETX=0
D
ETX=1.43
E
C
D
Forwarder List
…
BATCH MAP: WHO HAS RECEIVED WHAT?
ExOR operates on batch
 Highest-priority node that received packet

Ethernet Header
… FL
BM
…
C
C
D
E
C
A
A
C
C
A
E
E
A
C
B
B
C
D
C
A
Data
B
2.28
C
1.17
2.85
E>C>D>B>A
A
E
D
1.43
0
3-PACKET BATCH EXAMPLE
123
123
C
B
E
D
A
B
E
C
E
A
B
E
C
A
E
1.17
2.28
B
D
A
E
C
E
A
2.85
A
C
E
A
D
A
E
A
E
E
A
D
123
A
E
C
A
123
D
E
D
1.43
D
A
E
C
A
E
A
E
123
0
EXOR: 2X OVERALL IMPROVEMENT
Median throughputs:


240 Kbits/sec for ExOR
121 Kbits/sec for Traditional
Cumulative Fraction of Node Pairs

1.0
0.8
0.6
0.4
0.2
ExOR
Traditional
0
0
200
400
600
Throughput (Kbits/sec)
800
25 HIGHEST THROUGHPUT PAIRS
Throughput (Kbits/sec)
3 Traditional Hops
2.3x
1000
800
2 Traditional Hops 1 Traditional Hop
1.7x
1.14x
ExOR
Traditional Routing
600
400
200
0
Node Pair
Throughput (Kbits/sec)
25 LOWEST THROUGHPUT PAIRS
1000
800
ExOR
Traditional Routing
4 Traditional Hops
3.3x
600
400
200
0
Node Pair
Longer Routes
EXOR USES LINKS IN PARALLEL
Traditional Routing
3 forwarders
4 links
ExOR
7 forwarders
18 links
EXOR MOVES PACKETS FARTHER
Fraction of Transmissions
58% of Traditional Routing transmissions
0.6
ExOR
Traditional Routing
0.2
25% of ExOR transmissions
0.1
0
0
100
200
300
400
500
600
700
800
900
Distance (meters)
 ExOR
average: 422 meters/transmission
 Traditional Routing average: 205 meters/tx
1000
COST OF EXOR

Memory cost at each node

Overhead ∝ # nodes

Choose a subnet
SUMMARY

Exploits radio properties
Send: broadcast
 Receive: probabilistic


Solves problems w/ traditional routing
Unnecessary retransmission
 Extra hops


Sol: Buffer packets
Sol: Choose “best” forwarder
Dynamic forwarder choice
Forwarder list – order distances to dst
 Batch map – packet sent exactly once


Performance
2x throughput
 Longer & fewer hops
