Performance evaluation of Licklider Transmission Protocol over Free Space Optical communication testbeds

Optical communication systems can reach transmission data rates of several Gbps but in near-Earth scenarios they are also affected by link intermittency and disruption, so that the use of the Delay-/Disruption-Tolerant Networking architecture and related protocols, such as the Licklider Transmission Protocol (LTP), seems particularly promising. For this reason, the German Aerospace Center (DLR), in accordance with Unibo, decided to devote this thesis to this topic. First, the Institute of Communication and Navigation of DLR provided us with tracks representing the dynamic state of an FSO channel between a Low Earth Orbit satellite and a ground station. Starting from these, we applied a Reed Solomon (255, 223) error correcting code and further processing to obtain “erasure vectors” (EV) representing the binary state (on/off) of the channel at a sample rate of 10 kHz. To emulate the channel in accordance with the EV traces, we developed “detemu”. Then, by using DTNperf, detemu and DTNME (the bundle protocol implementation by NASA MSFC), we performed a series of tests with LTP “red” (reliable). We developed a Python program called “LTP performance analyser” to automatically compute statistics about session durations, retransmission cycles and penalization times. Preliminary results showed that the session duration was significantly decreased by setting the retransmission time out time to 100ms instead of 1.1s, with a corresponding very significant goodput increase for a given session parallelism. We also found that by increasing the parallelism above a threshold, depending on channel characteristics and other factors, there is no advantage in terms of goodput, even if the full channel utilization is not reached, as explained in the thesis. Summarizing, LTP proved to be extremely robust to high loss ratios and an excellent match to FSO links in near-Earth scenarios.