Abstract
The purpose of this research study was to investigate solutions to degradations in QoS (Quality of Service) for Broadband Internet provision via Geostationary Satellite Systems. Analysis as well as trials using commercial equipment have shown that these can result from certain Radio Resource Management (RRM) schemes, the limited capacity per user as well as atmospheric channel impairments. The latter is being effectively addressed by Fade Mitigation Techniques, leading however in a reduction and variation of the useful capacity. In addition, internet applications and protocols have been developed for terrestrial networks and therefore their adaptation to the satellite architecture characteristics, or the opposite, is often necessary. An important challenge has also been to improve individual application performance while at the same time optimise the overall system performance and maximising the transmission capacity. The optimisation methodology adopted is cross-layer design, a concept based on the exchange of parameters between non adjacent layers of the protocol stack. Six cross-layer mechanisms are proposed as part of this research targeting VoIP (Voice over DP) as well as TCP applications (Transport Control Protocol) using connection splitting. The mechanisms employ optimisation algorithms utilising real-time information from the MAC (Medium Access Control) as well as other non adjacent layers. The performance assessment performed via software simulations demonstrated important improvements in QoS. For VoIP for example, the MOS score which can be achieved even under congestion is above or around 80 in the R factor scale, with a theoretical maximum of 93. Simulations on TCP have also indicated how a cross-layer design allows the beginning of file data transfer at the physical layer to start earlier, which is important for small files. With respect to bigger files, a “cross-layer TCP” version can reduce the transfer delay of a typical file by about 50% with respect to TCP New Reno and 30% compared to TCP Hybla. The principal conclusion from this research is that broadband satellite systems can offer comparable QoS to terrestrial networks for most popular internet applications. This is under the condition that adequate capacity is available and adaptive techniques are implemented across the network, optimising performance jointly over several layers of the protocol stack. To this purpose the cross-layer designs proposed as part of this research become an essential tool.