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

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”

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

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

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

LIR vs BIR

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

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

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

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

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

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

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