ExOR: Opportunistic Multihop routing for Wireless
Networks
by;
1
Sanjit Biswas and Robert Morris, MIT
Presented by;
Mahanth K Gowda
Some pictures/graphs adopted from authors’ slides
OVERVIEW
Traditional Routing
 ExOR: key intuitions and ideas
 ExOR: Realization
 Evaluation

2
TRADITIONAL ROUTING

Links are abstracted as wires.
B
C
E
A
Destination
Path
E
ADE
C
ADC
D
3
LINK TRANSMISSION IS A BROADCAST

Probability of reception decreases with distance

However, there is always a chance that data travels longer
10%
B
90%
C
60%
E
A
D
4
EXOR EXPLOITS BROADCAST
src
N1
N2
N3
N4
N5
dst
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
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EXOR EXPLOITS BROADCAST
N1
N2
src
dst
N3
N4

Traditional routing: 1/0.25 + 1 = 5 tx

ExOR: 1/(1 – (1 – 0.25)4) + 1 = 2.5 transmissions

Assumes independent losses
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EXOR REALIZATION: BATCHING

Packets are queued and sent in Batches

A list of forwarders prioritized by their ETX values is included

In the below topology ---> Source: A, Destination: E

Priority order : E C D B A



In other words, if E C D B A receive packets, they should forward
in that order
Other nodes listen
They forward packets only if a higher priority node has failed to
do so
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AN EXAMPLE

A has transmitted a batch of 10 packets 1-10

E receives packets 1, 2

C receives 1 3 4 10

D receives 1 2 5 9 10

B receives 1 2 3 4 5 6 7 8 9 10

E received 1,2

Now C forwards 3, 4,10 1 2 3 4 5 6 7 8 9 10

D forwards 5,9

B forwards 6, 7, 8
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
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BATCHING




A batch map indicates highest priority node that received
each packet in the batch
The map is updated and sent over along with data
Gossip mechanism: updated batch map propagates from high
priority nodes to low priority nodes and ultimately to source
When the source receives the updated batch map, it restarts
transmission if all packets haven’t got through
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EVALUATION

Comparison between traditional 802.11 is done with ExOR

Throughput between 65 randomly selected node pairs evaluated

1 mega-byte file exchanged

Batch size is 100

Data rate 1 megabit/second
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Cumulative Fraction of Node Pairs
EXOR: 2X OVERALL IMPROVEMENT
1.0
0.8
0.6
0.4
0.2
ExOR
Traditional
0
0

200
400
600
Throughput (Kbits/sec)
Median throughputs:
800
240 Kbits/sec for ExOR,
121 Kbits/sec for Traditional
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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
12
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
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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
1000
Distance (meters)
 ExOR
average: 422 meters/transmission
 Traditional Routing average: 205 meters/tx
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SUMMARY

ExOR opportunistically exploits wireless broadcast


long distance transmission
Avoids retransmission by allowing a low priority node to forward
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ISSUES

Periodic link state flooding

Queuing for batching causes delay for interactive applications

Uses constant data rate for evaluation
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THANK YOU

Questions ?
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