1. Estimation of Link Interference in Static Multi-hop Wireless Networks
Jitendra Padhye, Sharad Agarwal,
Venkata N. Padmanabhan, Lili Qiu, Ananth Rao, Brian Zill

2. Problem
Interference between links in a static wireless mesh has been observed to be a major cause of packet loss and poor performance
No empirical studies have been performed to determine the extent of interference
Many heuristic methods exist in the literature, or the interference relationship is assumed “a priori”

3. What is interference?
A decrease in aggregate throughput in a pair of links when both are active simultaneously
Link interference ratio
Caused by carrier sense and packet collisions

4. LIR Measurement Methodology
Examine 75 random links from the 152 high-quality link pairs
Send unicast packets for each link individually for 30 seconds
Send unicast packets for each link simultaneously for 30 seconds
Apply LIR formula to the measured loss rates
10 hours for 75 link pairs

5. BIR Measurement Methodology
Each node broadcasts independently for a short period and the receive rate is measured at all nodes
Each pair of nodes broadcasts simultaneously and again the receive rate is measured at all nodes
Calculate a broadcast interference ratio based on these measurements

6. LIR vs BIR

7. Heuristic Methods M1 – All links on a path interfere
(i.e. all links interfere)
M2 – Links interfere if and only if they share an endpoint
M3 – Links interfere if any pair of nodes are within 2x the communication range
Assumes 2-hop interference in evaluation

8. Evaluation of M1 and M2
24 of the 75 randomly chosen pairs have LIR = 1 and 29 have LIR > 0.9
Implies that 29 link pairs do not interfere much, if at all
M1 is too pessimistic
None of the chosen links share an endpoint, yet 46 pairs show interference
M2 is too optimistic

9. Evaluation of M3
M3 predicts that 56 of the 75 link pairs will show interference
Interference appears in 49 of these pairs
Where M3 predicted no interference, none was found

10. Is BIR a good metric?
The experiment was run calculating BIR and LIR back-to-back

11. Why do links interfere? Calculate the carrier sense ratio:
CSR indicates whether a node reduces its sending rate due to carrier sense of another node
CSR may be between 0.5 and 1.0
Of the interfering links, most had CSR of 0.5
Most interference is caused by carrier sense threshold

12. Factors ignored Unicast retransmission
Backoff can create short-term unfairness issues
RTS/CTS
Hidden terminal problems
Data/Ack collision
Autorate

13. Questions What about asymmetric interference relationships?
Intra- and inter-flow interference?
How does interference or LIR/BIR affect network throughput or packet loss statistics?