Development of a satellite network simulator tool and simulation of AX.25, FX.25 and a hybrid protocol for nano-satellite communications

[摘要] ENGLISH ABSTRACT: Nano-satellites are mostly used in lower earth orbit applications, where communicationintervals are limited, often to a combined total of less than one hour per day. With these typeof inherent limitations of lower earth orbits, there are also the physical size and equipmentrestriction of nano-satellites to consider, especially those of the CubeSat specification. It isof critical importance to use the limited time and communication resources as effectively aspossible.The network protocol has a huge influence on reliability and throughput of a satellite network.An important requisite for designing, comparing and improving network protocols isa network protocol simulator, that is able to envisage the design results. Simulation canfacilitate rapid development and unforeseen discoveries. Very little information is currentlyavailable regarding communication protocols used in nano-satellites. This thesis aims toexplore and improve the current status of nano-satellite network simulation, as well as todemonstrate the development of an improved communication protocol strategy.It was found that there is a lack of proper simulation tools for satellite networks, which ledto the development of SatSim. SatSim is a discrete event network simulation tool, developedin Python, which can be used to develop and analyse network protocols. SatSim was verifiedby comparing simulation results with other published results, which made use of differentsoftware tools and theoretical throughput calculations.AX.25 is one of the most commonly used network protocols in the nano-satellite industry.It was implemented in SatSim and verified with theoretical throughput calculations, asno other simulation data on AX.25 was available. AX.25 was used as a baseline protocolto improve upon. FX.25 was developed by the Stensat Group in an attempt to improveAX.25. FX.25 adds forward error correction to AX.25, by wrapping additional data aroundthe AX.25 frames. This method maintains backward compatibility with AX.25. FX.25 wasimplemented in SatSim and the simulation results proved that FX.25 was a more reliableprotocol than AX.25, as it can communicate at lower elevations and over noisier communicationchannels. However, the drawback of the additional forward error correction is theincreased overhead, which reduces the overall payload data throughput.A modular AX/FX.25 protocol was then implemented in SatSim, to exploit the strengths ofboth protocols. This hybrid protocol yielded significant improvements to data throughputand can enable future software defined radio or hardware developments.

[发布机构] Stellenbosch University