Tolerable delay for Satellite System

Tolerable delay

Delay tolerated by a system is application dependent. An end to end delay of more than 400 ms is quite disturbing in a conversation, but delays of minutes or hours are acceptable for e mail delivery. The tolerable time delay influences several features of a mobile satellite communication system. A non real time system can tolerate a break in the communication link by data recovery techniques, whereas in a circuit mode service, a continuous end to end connection must be maintained.

Non real time systems use packet switching where packets and messages are stored in buffers at intermediate nodes of a virtual link and transferred when conditions are favourable. Inmarsat C and EUTELTRACS geostationary systems use such an architecture. The ORBCOM system represents a LEO store and forward system.

For a real time geostationary system, ignoring RE link blockage due to obstructions, satellite visibility remains uninterrupted as users operate within static footprints. The boundaries of the spot beam coverage are fuzzy, extending several tens of kilometres which leads to a graceful degradation in signal quality for slow moving vehicles. Thus, slow moving mobiles need not handover a call to the next beam as the users spend a considerable period in this fuzzy zone where signals degrade gradually. However, handover is necessary for fast moving mobiles such as aircraft. In non geostationary satellite systems, satellite visibility is variable due to dynamic footprints. Therefore, a user is likely to communicate through different beams during a call, making handover essential for the network.