Space Communication Protocol Standards-Transport Protocol (SCPS-TP) over cislunar communication links and delay impact

Researchers know that the performance (that is the efficiency) of TCP deteriorates with the increase in link delay and bit-error rate (BER ). This is because of its window-based transmission controls algorithms. In an environment, particularly a space environment, which involves large bandwidth-delay products (BDP), scientists and researchers found that the window-based mechanism is not favorable for a reliable end-to-end data transfer. It is in this kind of an environment that the potential of the rate-based Internet-type protocols to have low loss by maintaining a smooth data flow has made them attractive in space communications. However, very few studies have been done on development and evaluation of rate-based transmission control for space environment. Space Communication Protocol Standards-Transport Protocol (SCPS-TP) is a set of TCP extensions aimed at reliable transfer of information between space mission end systems. Some works have been done on throughput performance evaluation of SCPS-TP over LEO and GEO-satellite channels. In this paper, we present an experimental performance evaluation of SCPS-TP over a simulated cislunar communication link and also provide a comparative analysis of channel delay impact on the rate-based and the window-based transmission mechanisms over space-Internet links. We intend to see how effective both the window-based and the rate-based transmission control mechanisms in coping with space-channel delay from LEO satellite links, GEO satellite links, with long Cislunar link delay, especially when hybridized with a high bit-error rate (BER), and which option has throughput advantage over the other. The evaluation is based on the results of the file transfer experiments conducted using the Space-to-Ground Link Simulation (SGLS) test-bed. The experimental results reveal that the protocols have statistically significant performance difference over an error-prone link with a BER around 10-5 at which Pure Rate Control shows superior performance over the other three protocols. SCPS-Vegas performs poorly over a long-delayed, error-prone cislunar channel. Quantitatively, the throughput advantage of SCPS-Pure Rate Control is around 3000 bytes/sec over all other three protocols over both symmetrical and asymmetrical channels at BER=10-5. Moreover, even when the link delay is becoming longer, SCPS-Pure Rate Control maintains a higher throughput compared to other protocols. This verifies our prediction that rate-based protocol is the least sensitive to the change of link delays for both channels. Hence, Pure Rate Control is very suitable for a long-delayed, error-prone cislunar communication links.