3. Determination of the topology of high survival radio communication network

Aim of this document is to define a physical/logical topology of a HF communication network connecting ECIs and CGAs. We have analyzed the existing techniques, focusing on those architectures considered useful for the future SWING system development. The main goal is to provide a minimum flux of essential information in case of a terrorist attack that may put out of order the Internet connection between some CGAs and their controlled ECIs. MAC requirements and physical layer requirements have been thoroughly discussed. In order to meet the MAC requirements, three different schemes have been investigated: a contention free strategy, a contention based approach and a mixed solution. Among them, the last scheme (mixed solution) seems to be the most appropriate for the SWING system. Specifically, for communications between ECIs and their home CGA the access could be managed through a contention free token protocol (HFTP) enabling the radio terminal to transmit when it holds the token. The choice of this technique is motivated by its ability to efficiently manage the access of multiple ECIs to the HF channel in case of an internet failure over a huge area. For communications between CGAs a contention based protocol (DCHF) is suggested, according to which the transmitting radio terminal must sense the channel before sending its data. Such technique is suited for light traffic networks and fits well with the inter CGAs communication scenario. Due to the specific topology of the SWING system, a dedicated network layer seems unnecessary. The data link is characterized by higher rates and less stringent delay requirements as compared to the voice link. In such a case, the ARQ protocol is used for error-free packet delivery and the autobaud capability is suggested to continuously adapt the data throughput to the prevailing channel conditions. The transmission bandwidth of nearly 100 kHz provides the system with a high degree of frequency diversity, which can be exploited to further increase the reliability of the data link. The modulation parameters provided by the preliminary system design have been employed for the link budget analysis. In doing so we have assumed a worst-case scenario for ionospheric propagation. The EIRP values are given for the HF link between the CGA in Rome and the relative ECIs. The main constraints are dictated by ionospheric conditions and by the environmental noise. Since the technological features of the antenna limit the minimum value of the operative frequency which can be used to establish the HF link, a frequency management system seems necessary for the SWING network.