Stephen Horan

Design of a space channel simulator using virtual instrumentation software

The ability to simulate satellite channels gives protocol designers the ability to test design options prior to their use in space communications. Here, we describe a software-based space channel simulator with variable channel error-rate, channel delay, and user bit error rate. This simulator is used to model the communications channel for testing various Internet-type protocol options for space communications.

Which transmission mechanism is best for space Internet: window-based, rate-based, or a hybrid of the two?

There is an urgent need to find the best congestion-control mechanism for space Internet. This article presents an experimental study of rate-based congestion control, window-based congestion control, and a hybrid of the two in a simulated small-satellite environment using the space-to-ground link simulation (SGLS) testbed. The study is done by examining the throughput performance and transmission behavior of pure rate-control and a variant of each of the window-based transmission control protocol (TCP) selective acknowledgment (TCP-SACK) and TCP-Vegas congestion-control mechanisms, with and without the channel-rate control being hybridized. The study reveals that the traffic-shaping mechanism of a rate-based transmission mechanism is more effective than the bursty flow of window-based mechanisms in error-prone space environments with a long link delay. Pure rate-control is preferable to other mechanisms in space in which channel resource reservation is available. The performance differences arise from their different behaviors in controlling data transmission.

Unreliable CCSDS File Delivery Protocol (CFDP) over Cislunar Communication Links

The Consultative Committee for Space Data Systems (CCSDS) file delivery protocol, also known as CFDP, is a new standard protocol designed to meet a comprehensive set of file transfer requirements in space communications, especially deep space missions. There has been theoretical and experimental evaluation on the performance of CFDP operation in low Earth orbit (LEO) and geo-stationary Earth orbit (GEO) space environments. However, very few works have evaluated the performance of CFDP in cislunar space, even though the protocol has been particularly developed for deep space communications. In this work we discuss an experimental investigation of the core file-delivery operation of unreliable CFDP, operating with reliable transmission control protocol (TCP), over a simulated cislunar communication link under various channel conditions. Our intention is to evaluate the transmission effectiveness of the core file-delivery operation in unreliable CFDP running on top of a reliable TCP/IP stack over a long-delayed cislunar link, especially when accompanied by link disruptions and a high bit error rate (BER). The performance of unreliable-CFDP/TCP is also compared with other protocol options of CFDP, such as reliable-CFDP/TCP and reliable-CFDP/UDP. The experiment was carried out through realistic file transfers by running the CFDP protocol stack over a communication test-bed incorporating the qualities that define cislunar communication channel characteristics. The research results and discussions presented in this paper should work equally well in any space mission with a round-trip time (RTT) that is comparable to that of the cislunar space.

Impact of Van Jacobson header compression on TCP/IP throughput performance over lossy space channels

The impact of Van Jacobson header compression (VJHC) on the throughput performance of Transmission Control Protocol/Internet Protocol (TCP/IP) over lossy space channels is studied in an experimental manner using a test-bed. The experimental results show that VJHC benefits the transmission at bit error rates (BERs) around 10/sup -6/ or less, but also results in performance degradation in an environment with higher BER.

Optimal acknowledgment frequency over asymmetric space-internet links

The acknowledgement (ACK) strategy of transmission control protocol (TCP) has been modified to delay the ACKs for a configurable period of time that is related to the round-trip time (RTT) to improve its performance in asymmetric links such as space communications. A general relationship between the asymmetric channel ratio and the optimal ACK frequency to achieve maximum throughput is not available. This paper studies the optimal ACK frequency with respect to the channel RTT to achieve the maximum throughput over various asymmetric, geo-stationary earth orbit (GEO)-satellite links using a test-bed. It is found that when the asymmetric channel ratio is as high as about 100/1, the optimal ACK frequency must be increased with an increase in asymmetric ratio to complement the impact of the reduction of ACK channel rate. As a rule of thumb, every two-fold increase in asymmetric ratio requires the optimal ACK frequency to be increased by one ACK per RTT. A regression model was built to reflect the relationship between the optimal ACK frequency and the asymmetric channel ratio and has been validated

Small satellite access of the Space Network

Small satellites have been perceived as having limited access to NASAs Space Network (SN). The potential for satellite access of the Space Network when the design utilizes a fixed antenna configuration and low-power, coded transmission is analyzed. From the analysis, satellites using this configuration in high-inclination orbits are shown to have a daily data throughput in the 100 to 1000 Mbit range using the multiple access communications service.

Protocol Testing of SCPS-TP over NASA's ACTS Asymmetric Links

As the transport layer protocol of the Space Communication Protocol Standard (SCPS) suite, SCPS-Transport Protocol (SCPS-TP) is actually transmission control protocol (TCP), with a set of extensions aimed at improving TCP performance in space communications. Some work has been done on the performance evaluation of SCPS-TP in comparison with TCP. However, those works have been done by testing SCPS-TP via emulation test-beds. We present the file transfer experimental work of testing the SCPS protocol stack over NASAs geostationary orbit (GEO) Advanced Communications Technology Satellite (ACTS). Considering that space communication channels in practice are frequently asymmetric in terms of channel bandwidth, our work is concentrated on the evaluation of the effectiveness of SCPS-TP in coping with channel rate asymmetry. We conclude that SCPS-TP is better suited than TCP over long-delayed, asymmetric, satellite channels with a throughput advantage of over 30 Kbyte/s for the transmission of a 10 Mbyte file. A hybrid of the channel rate control and acknowledgment frequency reduction mechanisms makes significant contributions to the performance enhancement of SCPS-TP over asymmetric space channels.

First experimental demonstration of full-duplex communication on a single laser beam

We present the first experimental demonstration of full-duplex communication on a single laser beam. The forward link beam simultaneously transmits the forward link data and serves as the carrier wave for the return link. A fraction of incident forward link beam is modulated and retro-reflected back to the receiver location. The forward link data format must be designed so that it is invisible to the return link’s data detection system. We have named this architecture the “Lightwire. In addition there was no system trade-off required by this architecture. Thus we have demonstrated full-duplex on a single optical beam without any performance penalty. We will present the experimental system and discuss the experimental results.

Internet-type protocol testing in a simulated small satellite environment

The throughput results for file transfers using file sizes ranging from 1 Kbytes through 1 Mbytes using both the standard TCP/IP and SCPS protocol stacks over a PPP link are reported. Channel properties were simulated using a space channel simulator with a range of balanced and unbalanced link speeds and channel error rates. The throughput results show the effects of link configuration and channel error rate on file transfer time. The host computer configuration options for the protocols are factored into the comparison. The throughput reporting shows the effects of header compression and selection of congestion algorithm upon the results. The TCP/IP ftp and SCPS-FP using the VJ congestion control algorithm results give similar results and better results than SCPS-FP with the Vegas congestion control algorithm in these experiments. No noticeable delay effects were noted with links delays corresponding to GEO orbits with file transfers of 1 Mbytes.

Performance of space communication protocol standards (SCPS) over ACTS satellite links

Space communication protocol standards (SCPS) has been developed for the reliable transfer of information in the future space and interplanetary Internet. Some work has been done on performance evaluation of SCPS by testing it over the low-rate channels and/or via an emulation test-bed. However, this work has been done by testing SCPS over the low rate channels and/or via an emulation test-bed. It is expected to have an evaluation of the complete SCPS protocol suite in a realistic space environment with a large bandwidth-delay product (BDP). In this paper, we present an experimental performance evaluation of SCPS suite, by comparing with standard TCP/IP, over the NASAs Advanced Communications Technology Satellite (ACTS) high speed links with data rate up to 4 Mb/s.