A Study Of Channel-Rate Asymmetry In Delay/Disruption-Tolerant Network

It is widely known that deep-space?including cislunar?communications are characterized by an extremely long link propagation delay,lengthy link disruptions,asymmetric channel rates,and lossy data links.While the issues of the long delay,link disruption,and lossy links are already well studied,an extensive study for the problem of the channel-rate symmetry in deep-space communications is quickly needed.In this dissertation,a solid study is presented for the problem of asymmetric channel rate which severely affects the data transmission efficiency in deep-space exploration missions.The proposed study is conducted by adopting the recently proposed Licklider transmission protocol?LTP?of the delay-/disruption-tolerant networking?DTN?technology.DTN was proposed as an overlay-network technology for reliable data delivery in a challenging communication environment,and its typical application is for effective data delivery in deep-space communications.LTP is proposed as the main data transport protocol of DTN.The study is conducted in both a theoretical manner?i.e.,analytical modeling?and an experimental manner using a PC-based experiment testbed infrastructure.In details,for the first time,the following four issues that are related to the channel-rate symmetry and critically affected the transmission effectiveness of deep-space communications are studied:?1?Analytical modeling for the minimum number of data bundles?i.e.,a threshold number?to be aggregated within a LTP data block to avoid delay in ACK transmission.ACK delivery is delayed for LTPCL transmission over highly asymmetric space channels.An analytical model is built with the effect of full overhead taken into consideration to compute the minimum number of data bundles,as a threshold,that should be aggregated within an LTP block in order to avoid delay of an ACK transmission.?2?Analytical modeling for an optimal number of bundles to be aggregated for the best transmission efficiency of LTP.The experimental results also show that goodput performance increases roughly with an increase in the aggregated block size.A model is also built for calculating the optimal number of bundles to be aggregated,N_{Bundle-Optimal},that will result in the best transmission efficiency;Performance comparison between LTP?with bundle aggregation mechanism implemented?and TCP.The TCP performance degradation with a highly asymmetric channel ratio can be explained from the prospect of delay in ACK transmission or simply,increase in round-trip time?RTT?.?3?Analytical estimating of the round-trip time?RTT?for LTP transmission.Accurate estimate of file RTT is essential for efficient transmission control,reliable delivery,and bandwidth usage optimization of a protocol.Analytical models are built for RTT estimate of LTPCL transmissions with the effect of delay caused by space channel-rate asymmetry,channel impairment and link disruption taken into consideration.The theoretical development presented for each of the above issues is validated by conducting data transfer experiments using the experimental testbed.The validated analytical framework and the findings from the extensive experiments presented in this dissertation should be useful for the design of China's future space internetworking and inter-planetary communication systems for the recently initiated Mars exploration missions.