Why do links interfere?

Our methodology also helps determine why two links interfere with one another. Consider links $L_{AB}$ and $L_{CD}$ that interfere with one another. During the broadcast experiments done to determine $BIR$, we had nodes $A$ and $C$ broadcast alone, as well as together. Consider the ratio of their send rates, when they were broadcasting together to when they were broadcasting alone. Define carrier sense ratio:

$$\small CSR = (S_{A}^{AC} + S_{C}^{AC}) / (S_A + S_C) .$$ (3)

Note that we are using broadcast packets, so both senders send at the same data rate. If two senders are within the carrier sense range of each other, then only one of them would be able to send at a time, resulting in a $CSR$ value of 0.5. If the senders are not within each other's carrier sense range, $CSR$ will be 1. Intermediate values can result from noise, differences in sensitivity of antennas, signal strength fluctuations due to environmental factors etc.

In the baseline scenario shown in Figure 2, 46 link pairs have $LIR < 0.9$, indicating some degree of interference. Of these, 34 link pairs had a $CSR$ of 0.5. Thus, carrier sensing seems to be the major cause of interference in our testbed. We see similar results for the other three scenarios considered in Section 5.2. We believe that this is one of the reasons why $BIR$ and $LIR$ show a good match under all scenarios. We are currently investigating this issue further.