Real Time Data Transfer for Very Long Baseline Interferometry

This paper examines the use of the Internet in real time very long baseline interferometry (VLBI) high resolution observations in radio astronomy. Owing to the long distances between telescopes, VLBI data have traditionally been recorded to magnetic tape, and now onto hard disks. With the emergence of high data rate network links the VLBI community is beginning to transfer astronomical data via the academic internet. The current system uses TCP/IP and we report on the way TCP transports the constant bit-rate VLBI data We also give details of a new, UDP based VLBI data transfer system, VLBI_UDP, and results of recent tests, including multiple simultaneous line-rate UDP data flows lasting several hours running over the academic internet and lightpaths. Finally, we give examples of successful real-time astronomical observations using e-VLBI with data transfer rates of up to 256 Mbit/s.