Security, Reliability and Performance Issues in Wireless Networks
There are trade-offs between security, reliability and performance. In this dissertation, we consider these aspects of wireless operational networks in different settings. Specifically, with respect to security we look at replay attacks and functional reliability, with respect to reliability and performance we look at video transmission and visible light communications.
Packet replay attack is a type of denial-of-service(DOS) attack, wherein an attacker replays overheard packets in the wireless network. Our experiments indicate that even a single attacker can degrade the route throughput by up to 61%. We design a lightweight detection and prevention system, COPS (for Copycat Online Prevention System), that intelligently uses a combination of digital signatures and Bloom filters to cope with the attack. We implement COPS on real hardware and perform experiments on our 42 node wireless testbed. Our measurements indicate that COPS achieves its objective; it can efficiently contain the effects of replayed packets to a local neighborhood without incurring high resource consumption penalties.
Later, we look into the functional reliability (FR) of the nodes in wireless networks. The FR is typically assessed based on evidence collected by nodes with regards to other nodes in the network. However, such evidence is often affected by factors such as channel induced effects and interference. We design a framework for collaborative assessment of the FR of nodes, with respect to different types of functions; our framework accounts for the above factors that influence evidence collection. We also design a module that drastically reduces the overhead at the expense of slightly increased uncertainty in the assessed FR values. We implement our framework on an indoor/outdoor wireless testbed. We show that with our framework, each node is able to determine the FR for every other node in the network with high accuracy.
Next, we research on the performance of video transmission in wireless networks. The end-user experience in viewing a video depends on the distortion; however, also of importance is the delay experienced by the packets of the video flow since it impacts the timeliness of the information contained and the playback rate at the receiver. Unfortunately, these performance metrics are in conflict with each other in a wireless network. We investigate this trade-off between distortion and delay for video. We validate our analysis via extensive simulations. Surprisingly, we find that the trade-off depends on the specific features of the video flow: it is better to trade-off high delay for low distortion with fast motion video, but not with slow motion video. Our simulation results further quantify the trade-offs in various scenarios.
Finally, we look into a visible light system in two rooms with a door open in between. Two emitters tx1 and tx2 are located in room 1 and room 2 respectively. We use BPPM, vary pulse width within the slot to provide different dimming levels. We propose a modfied ray-tracing algorithm to calculate the channel impulse response. We also provide a BER analysis considering different combinations of system parameters. Our results show that if tx2 is just illuminating, it does not impact the performance of communications in room 1. However, if both tx1 and tx2 are transmitting, the performance in room 1 is degraded to different levels depending on the position of the receivers. To improve the performance of communications in room 1 in this case, we can increase the dimming level of tx1. Moreover, when dimming level is limited, our results show that reducing the bit rate of tx1 improves the performance in room 1 dramatically. For example, when the bit rate of tx2 is 8Mb/s, reducing the bit rate of tx1 from 8Mb/s to 4Mb/s makes the BER drop from 10^-3 to 10^-13 .