1. Data-Acquisition and Transport – Looking Forward to 2010 and Beyond Definition of ‘Data-Acquisition and Transport’ Continuum of Transport Options Limitations of Current technology -Magnetic tape -Magnetic disks e-VLBI -Advantages -Problems -Status and Prospects Questions to be answered Hans Hinteregger, Haystack Observatory Tetsuro Kondo, CRL Yasuhiro Koyama, CRL Alan Whitney, Haystack Observatory

2. Definition of ‘Data Acq and Transport’ and Options ‘Data Acqusition and Transport’ encompasses all means used to move raw VLBI data from a station to a correlator Includes several possible options –Situation: No data link to correlator °Record data, ship to correlator, playback into correlator –Situation: Nearby high-speed POP to correlator °Record data, drive to POP, transmit over network and record at correlator, playback at correlator –Situation: Slow network connection to correlator °Disk FIFO buffer, transmit over network at whatever speed is sustainable, record at correlator, playback into correlator –Situation: High-speed networks from some, but not all stations °Transmit data in real-time, record at correlator, playback into correlator –Situation: High-speed links from all stations °Transmit and correlate data in real-time (no recording)

3. Magnetic Tape Advantages -Potentially lower media cost, but this not current trend Disadvantages -Messy mechanical head-to-tape interface -Unreliable -Expensive drives -Difficult to take advantage of new technology -Relatively small market -Data rates limited to ~1 Gbps per tape

4. Magnetic Disks Advantages -Fast technology advance -Rapidly falling prices -Easy to take advantage of new technology due to standardized electrical interfaces -Very large market Disadvantages -Must use many disks to get to very high data rates -Not suited to unattended operations; shipping may be required

5. e-VLBI Advantages -Potential higher sensitivity due to higher data rates; 10-100 Gbps are realistic possibilities -Lower or eliminate media costs -Fully automated operations; reduce costs -Rapidly diagnose problems -Enables distributed correlation with commodity PC’s Disadvantages -‘Last-mile’ problem – high cost of connection to telescopes -Potential high recurring network cost

6. e-VLBI Status and Projection Installation of global high-speed (many at 10Gbps or higher) between continents and major cities is mostly complete –‘Last mile’ connection to telescopes is biggest single problem Significant excess global capacity in place; has allowed R&D networks to get extremely good deals with telecom companies for dark fibers; prices are probably as low now as they will ever be! Cost of end-point terminal equipment is falling rapidly; GigE is now commodity; 10 GigE becoming available, will drop rapidly in price Bottom line: In almost all cases, acquisition of dark fiber (or access to lambda) is much cheaper in long run than paying for telecom services

7. Questions under study Cost of last-mile connection to stations? Technology options for last-mile connections? Cost-benefit analysis of recording vs. e-VLBI, including mixed options? Extrapolation of recording technology – cost, capacity, expandability, compatibility e-VLBI options for station unlikely to be connected via fiber (satellite?)? Are current VSI (H, S, E) interface specifications adequate? Do they need to be /modified/expanded? How? Others?

8. Conclusions Data transport technology is almost certainly up to the task, but is part of a rapidly emerging landscape. e-VLBI will almost certainly be the driver in any new intergrated geophysical observatory e-VLBI looks particularly attractive, and may be the only practical technology for the next generation of VLBI systems, but cost to install and sustain is a major question. Acquisition of dark fiber is clearly preferable, but availability and policies differ widely from country to country.