Viability of ISI-Based TETRA over Satellite

Satellite communications, by its very nature of providing wide-area coverage, can play an important role in interconnecting TETRA networks. A satellite link may be used to support any of the interfaces in TETRA network. However, the selection of particular interface applies to specific scenarios and provides different advantages and disadvantages. The viability of a satellite link for the interconnection of two TETRA networks using the Inter-System Interface (ISI) is studied with respect to desired functionality and QoS. ISI-based TETRA over satellite can be used in emergency situations when no terrestrial infrastructure exists between the two networks, as well as in normal operations as an alternative to ground based interconnection infrastructure. The response time of different services in a given network architecture is of primary concern because large propagation delay is the main source of service degradation. A modified version of critical path analysis, as it applies to protocols, is used to estimate total propagation delays for a set of ISI services. The appealing property of the critical path length is that it gives an accurate estimate of total propagation delay for a given service and thus represents a quantitative measure of QoS degradation as experienced by the end user. The results presented show larger degradations of QoS for the group call scenarios. The most disturbed are acknowledged group call setup and group call maintenance. A Performance Enhancing Proxy (PEP) is proposed to improve the performance of the TETRA services over the ISI if satellite links are involved. Key-Words: TETRA, satellites, Inter-System Interface, propagation delay, critical path, QoS