## 2 Selfish Configuration in Unauthorized Wi-Fi Tethering
### Selfish Configuration in Wi-Fi Tethering
Table 1 plots the collision probability, and the percentage of successful receptions at the receiver in the simulation with \(B_{cell}=10\,\)Mbps corresponding to the  
Figure 4: AP Interference graph of two simulated topologies. Figure 5: Impact of selfish carrier sense on throughput of transport-layer protocols over various cellular backhaul link capacities \(B_{cl}\) for tethering in two multi-AP topologies. results that are shown in Figure 5a,c, respectively. We observe that, despite the high collision ratios (71.5% and 91.8% for Topology 1 and 2, respectively), the tethered receiver can capture the signal of interest (SoI), thanks to PHY capture effect and MIM. A majority of the successful receptions are due to MIM, i.e., 53.6% and 63.5% for Topology 1 and 2, respectively. Note that the link distances between host and guest nodes are very short, which makes the received signal at the guest node sufficiently stronger than the sum of interferences. These results clearly demonstrate that the selfish behavior using CCA manipulation abuses the short link distance property of Wi-Fi tethering and, thus, makes it possible to exploit the benefits of MIM. Consequently, the selfish tethering can achieve an unfair throughput gain, regardless of the network condition surrounding the tethering. **Impact of Tethering Channel**: Next, we study the impact of the CCA manipulation on performance when tethering is launched on a channel partially overlapping with nearby APs. As mentioned above, the tethered Wi-Fi hotspot can potentially set up the network with an arbitrary channel number, which may cause serious interference to nearby well-planned APs. We used two simple scenarios in [16] and the channel model thta is presented in [17] for our simulation. Figure 6a illustrates a simple case that the channel selected by the tethering is partially overlapping with a nearby AP. Because two overlapping channels are sensed by each other by the CCA mechanism of 802.11, its effective spectrum usage is only 20 MHz [16]. Figure 6b depicts the case when the tethering shares the spectrum with two adjacent orthogonal channels. Note that, in this case, the tethering link may suffer from channel starvation [16]; the tethering link can only transmit when both \(WLAN_{1}\) and \(WLAN_{2}\) are idle, but the probability of the two outer channels being idle at the same time is very low, because the channel activities on the two outer channels are asynchronous and may overlap randomly. We performed simulations with following setting in order to evaluate the impact of the selfish behavior using CCA manipulation in these scenarios. Each Wi-Fi consists of two client nodes where the traffic on each flow is generated with 10 Mbps downlink CBR over UDP. The capacity \(B_{cel}\) for tethering is configured to 20 Mbps (with 20 Mbps CBR/UDP traffic). Initially, the tethering link is set to be legitimate. The CCA manipulation is activated at 20 s.  
Figure 6c,d plot the resulting throughput showing the effect of selfish behavior while using CCA manipulation. When the tethering is legitimate, it achieves a fair share of shared medium with two flows in \(WLAN_{1}\) in the first scenario. In the second case shown in Figure 6b, the tethering is almost starved due to the channel starvation problem [16]. After the CCA value is selfishly configured, despite the same unfair channel condition, the tethering link achieves a significant throughput gain at the cost of significant reduction in throughput of other flows. In the same scenario that is depicted in Figure 6b, we also compare the impact of CCA manipulation with different types of selfish behavior manipulating other MAC parameters, in particular the manipulation of the backff mechanism using smaller values of \(CW_{min}\), where the manipulation of backoff mechanism is widely adopted by selfish users [18]. Figure 7 shows the simulation results with different values of \(CW_{min}\) = 31, 15, 7, and 3.