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Chapter 8Final conclusions pThe major drawback of using CATV networks for satellite communications, as seen inFigure 8.2a, is the long propagation link delays, which can result in serious performanceimplications.This is because very large propagation delays (in the order of hundreds of milliseconds)will results in equally large scheduler look-ahead delays. The propagation delay to theheadend and back in such systems will be twice the roundtrip delay to the satellite,which in the case of Geo-stationary Earth Orbit (GEO) satellites placed at about 22.3miles above the Earth would be of ≈ 500 ms [9]. Therefore the explicit CRGC cycle willresult in large intervals of network idle time due to the number of contention slots the schedulerwill have to subscribe.For delay sensitive applications, such as voice conversations, the propagation delay overthe two-way path between conversants influences the perceived quality of service. Thisproblem can be ameliorated with LEO satellites placed at ≈ 500 – 1000 miles above theEarth. This relatively short distance reduces transmissions delays significantly to ≈ 50ms [9]. The drawback is that a large number of satellites, arrayed on a constellation ofmultiple satellites in a polar orbit is required to maintain path continuity.The long propagation delays of satellite systems undoubtedly will reduce the systemefficiency of a CATV MAC protocol. An interesting issue requiring further research isto analyse to what extent long propagation delay can degrade the quality of service ofGEOBACKBONECity xPSTNCity yLEOFDDIDQDBMobile NodeISPMobile NodeHeadendHeadendMobile NodeMobile Nodea) Satellite Mobile/Fixed b) Digital CATV b) Terrestrial Mobile/ Fixed WirelessFigure 8.2 – Similarities of CATV with fixed/mobile terrestrial and satellite wirelessarchitecture.8-21