Data-Acquisition and Transport – Looking Forward to 2010 and Beyond

1. Data-Acquisition and Transport –Looking Forward to 2010 and Beyond Hans Hinteregger, Haystack ObservatoryTetsuro Kondo, CRLYasuhiro Koyama, CRLAlan Whitney, Haystack Observatory • 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

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.