Ebro Observatory, October 1st, 2013

1. Internet Failure and Physical Layer Architecture Ebro Observatory, October 1st, 2013 With the support of the Prevention, Preparedness and Consequence Management of Terrorism and other Security-related Risks Programme European Commission - Directorate-General Home Affairs

2. Internet Failure • Description of the early warning alert scenario • Supervision of the internet links • Reactivation of the internet links

3. Early warning alert • Description of the scenario. Most Reasonable Scenario Worst Case Analysis

4. Supervision of the Internet links NMS= NETWORK MANAGEMENT SYSTEM Agents (managed devices) Management application (central station) the reference scenario for each ECI/CGA site

5. Supervision of the Internet links • - NMSs hierarchically structured in two levels: • Level 0: CGA NMSs • Level 1: ECI NMSs • Nagios (a powerful monitoring system that enables organizations to identify and resolve IT infrastructure problems before they affect critical business processes) deployment could be quite straightforward in the SWING scenario thanks to its adaptability to distributed system case

6. Supervision of the Internet links NCSA= Nagios Service Check Acceptor OCSP= obsessive compulsive service processor NAGIOS Architecture

7. Reactivation of the Internet links. • The reactivation of the traditional Internet links and the consequent interruption of the HF links cannot be directly operated by any ECI, but they should be controlled by the interested CGA • A CGA that experiences a resurgence of its Internet connection just needs to transmits a message to the other CGAs, notifying that its normal operational status has been restored and to switch from the HF link to the restored broadband Internet connection.

8. PHY layer architectureof the SWING system 1) Selection of the modulation technology 2) System design for voice transmission 3) System design for data transmission

9. Selection of the modulation technology • Most military HF standards employ a serial-tone waveform with a powerful FEC code and temporal interleaving to exploit the time-diversity of the HF channel • The use of a temporal interleaver with an interleaving depth greater than the HF channel coherence time poses a serious problem in terms of overall link latency • The alternative approach to increase the system reliability is to exploit the frequency diversity offered by the multipath phenomenon

10. Advantages of the OFDM technology • The channel distortion appears as a multiplicative factor which can be compensated for through a bank of complex multipliers • Increased spectral efficiency due to partially overlapping subbands in the frequency domain • Simple digital implementation by means of DFT/IDFT operations • Increased resilience against narrowband interference, which only hits a small portion of the signal spectrum • Possibility of adaptively selecting the constellation size on each subband (autobaud capability)

11. Requirements of the digital voice link It will support interactive voice communications. Interactivity is a basic design constraint The maximum accepted delay is around 120 ms so as to guarantee a whole delay observed by the user below the subjective limit of 250 ms Temporal interleaving cannot be used due to the strict requirement in terms of overall delay In order for the system to be applicable to commercial vocoders, the bit rate should be 2400 bps with a BER lower than 10-2 A fixed 4-QAM constellation is used (no autobaud capability)

12. Guidelines for the design of the digital voice link • The signal bandwidth B must exceed the channel coherence bandwidth so as to capture most of the frequency diversity offered by the HF channel • The subcarrier spacing f must be much smaller than the channel coherence bandwidth Bcoh so as to make the channel response nearly flat over each subcarrier and much larger than the Doppler spread in order to avoid significant channel variations over one OFDM block

13. Design of the main system parameters • The sampling frequency fs is fixed to 14.4 kHz, which seems reasonable for implementation on commercial HW platforms • The IDFT/DFT size is fixed to N=256. This value results into a subcarrier distance f =56.25 Hz • The number of modulated subcarriers is Nu=171, while the number of null subcarriers placed at the spectrum edges is Nv=N-Nu=85 • The signal bandwidth is B=Nu f =9600 Hz

14. Transmitter structure for the voice link • FEC is accomplished by means of the industry-standard convolutional encoder with rate 1/2 and constraint length 7 • Bit interleaving is accomplished by means of a block interleaver matrix • Interleaved bits are mapped onto 4-QAM symbols without any autobaud capability

15. Requirements of the data link The data link provides non-delay sensitive services, meaning that we can relax the interactivity constraint Channel coding is necessary to provide sufficiently low packet error rate The signal bandwidth is chosen large enough so as to provide the system with the desired frequency diversity CRC and ARQ are requested for error-free packet delivery An autobaud capability is employed to adaptively select the most appropriate constellation

16. Main system parameters

17. Transmitter structure for the data link • A 16-bit CRC is appended to each data packet • FEC and bit interleaving as in the voice link • The overall bandwidth is divided into 8 subbands. A different constellation size can be used on different subbands (autobaud) • The interleaved bits are mapped onto 4QAM, 16QAM or 64QAM constellation symbols, which are transmitted within one single subband.

18. Data link waveforms

19. HF Channel Model • Coherence bandwidth can range from 500 Hz to some kHz • Coherence time can range from 1 second to more than 10 seconds

20. Data link with moderate channel condition