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Interferometric fringes of giga-bit VLBI was successfully detected in 1998 on the off-line processing base using an 1- Gbit recorder. Though an 8-Gbps VLBI system, an 8-Gbps sampler and an 8-Gbps correlator are now available, the highest possible observing rate has been limited at 2-Gbps or slower by a limitation of an available data recording system. That is a reason the authors have started the developments of a realtime VLBI system in which 2-Gbps or higher rate of observing data is transferred to the correlator through an optical fiber link now established in the data transmission mode of STM-16. The authors and the Communications Research Laboratory, Japan (CRL) have already succeeded the realtime VLBI on STM-1, at 155-Mbps data rate, and proceeds trial manufacturing for the higher data rate than 2-Gbps by considering a fact that an 100-Gbps or higher data rate is already achieved on the most advanced optical data transmission link by using high speed devices for MUX and DEMUX logics. National Astronomical Observatory, Japan (NAO) now promotes the OLIVE project, Optical LInked VLBI Experiment, which directly connects VLBI observation stations, the Usuda 64-m and the Nobeyama 45-m radio telescopes, to a central correlator in Tokyo through an optical fiber link in realtime. Now 256-Mbps data transmission and correlations have been succeeded. The authors have an experimental plan to transmit 2-Gbps data of the Usuda 64-m telescope to the Kashima correlator about 200-km apart with each other at 2.4- Gbps data rate through an optical fiber link of a STM-16 mode. The authors are developing the ultra-high-speed ATM transmitter (TX) and receiver (RX), and the experiments of 2.4-Gbps transmission rate was made with the OLIVE network. The data transmission line is the Nobeyama-Musashino-Nobeyama with the data rate of 2 Gbps in February, 2000. After this communication successfully tested, the authors can expect great improvement on the VLBI sensitivity to observe more dark and compact object.
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