1. ExOR: Opportunistic Multi-hop routing for Wireless Networks
by;
Sanjit Biswas and Robert Morris, MIT

2. Traditional Routing Links are abstracted as wires. B C E A D
Destination
Path E
A D E C
A D C

3. Link transmission is a Broadcast
Probability of reception decreases with distance
However, there is always a chance that data travels longer
10% B C
60%
90% 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

5. ExOR Exploits BroadcastN1
25%
100% N2
25%
100%
src
dst
25%
100% N3
25%
100% N4
Traditional routing: 1/ = 5 tx
ExOR: 1/(1 – (1 – 0.25)4) + 1 = 2.5 transmissions

6. ExOR Realization: Batching
A list of forwarders are prioritized by their ETX values
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
dst
src
src
dst

7. An example A has transmitted a batch of 10 packets 1-10
E receives packets 1, 2
C receives
D receives
B receives
E received 1,
Now C forwards 3, 4,
D forwards 5,
B forwards 6, 7,
Priority order : E C D B A

8. ExOR: 2x overall improvement
1.0
0.8
0.6
Cumulative Fraction of Node Pairs
0.4
0.2
ExOR
Traditional
200
400
600
800
Throughput (Kbits/sec)
Median throughputs: Kbits/sec for ExOR,
121 Kbits/sec for Traditional

9. Summary ExOR opportunistically exploits wireless broadcast
long distance transmission
Avoids retransmission by allowing a low priority node to forward

10. Issues Periodic link state flooding
Queuing for batching causes delay for interactive applications
Broadcast property will result in errors in case of optimal bit rate encoding

11. Thank You
Questions ?