NEXPReS technological objectives. The next step — to be made in NEXPReS — in the development of e-VLBI is the deployment of a transparent caching system that allows all scientific VLBI experiments to benefit from the increased sensitivity, flexibility and robustness of real-time VLBI. The ideal situation is an array that works continuously with real-time connections, something that applies to LOFAR in the near term and that is anticipated for SKA. But even for the radio telescopes of the future a caching mechanism between digitization and transport, or between transport and correlation, can be anticipated. LOFAR operations, for example, include a mode in which a short data segment can be frozen in order to re-focus the telescope elements on a transient event. This technique will access unknown territories in astrophysics, and there are both scientific and operational motivations to extend this capability to other interferometers. For the VLBI array of the near-future it is clear that a caching mechanism can overcome all of the limitations that currently prevent one from deploying e-VLBI for all scientific experiments. We intend to build an operational system over the next 3 years in which all telescopes in all experiments will stream their data to JIVE in real-time. During the experiment, the real-time result is inspected at the correlator and made available to the astronomer, and at the same time the data is also buffered at the correlator and at the telescope. If the infrastructure has insufficient capacity at the time of the experiment, the transport can be completed later, either through direct connectivity or by traditional VLBI practices. Ideally, the end of the observation completes the correlation, the processing of the incoming signals. But there could be a variety of reasons why the data buffered at the telescope or correlator would be needed for further processing; technical failure of the correlator or transport layers could be an obvious reason. Another reason could be that the astronomer wants to reprocess the data to obtain higher spectral resolution or a different phase centre for his observations. Finally, other scientific reasons for such reprocessing might exist, for example the availability of better source or geodynamic parameters. In the case of very special once-off events it may be desirable to keep a copy of the data for a much longer time, for example when monitoring a spacecraft landing on a solar system body.
