VLBI Standard Interface (VSI)

August 2000


             The purpose of VSI is to define a standard interface to and from a VLBI Data Transmission System (DTS) that allows heterogeneous DTS’s to be interfaced to both data-acquisition and correlator systems with a minimum of effort.

             VSI is defined to be compatible with tradition recording/playback systems, network data transmission  and even direct-connect systems.

             VSI is designed to completely hide the detailed characteristics of the DTS and allow the data to be transferred from acquisition to correlator in a transparent manner.

             VSI is not intended to be completely ‘plug and play’ at the first level of implementation, but instead should help to relieve many of the existing incompatibilities that new exist between various VLBI data systems.

             The VSI specification is being developed as a joint effort between the geodesy and astronomy communities, spearheaded by IVS and GVWG.

The VSI Process and Schedule


              Draft proposal created  Feb 1999 as a result of informal meetings held at Jan 99 GEMSTONE meeting.

              VSI committee selected with wide international representation from both geodetic and astronomy communities

              Decision to separate hardware (VSI-H) and software (VSI-S) specifications; do VSI-H first.

              Primary communications via e-mail.

              Numerous draft iterations of VSI-H spec have been circulated

              VSI-H meeting held at Haystack end of January 2000 with representatives from Japan, Canada, Europe, Australia and U.S.

              Review of final VSI-H specification in progress.

              Submission to IVS and GVWG for final approval (expected soon)

              Work to begin on VSI-S specification

VSI Committee


             Wayne Cannon                  York University               Canada

             Brent Carlson                    DRAO                            Canada

             Dick Ferris                        ATNF                             Australia

             Dave Graham                    MPI                                Germany

             Tetsuro Kondo                  CRL                                Japan

             Nori Kawaguchi                 NAO                               Japan

             Misha Popov                     ASC                                Russia

             Sergei Pogrebenko             JIVE                                Netherlands

             Jon Romney                      NRAO                            U.S.

             Ralph Spencer                   Jodrell                             England

             Alan Whitney                     Haystack                         U.S.

             Rick Wietfeldt                   JPL                                 U.S.

VSI-H Assumptions


1.     The VSI Data Transmission System (DTS) is fundamentally a receiver and transmitter of parallel bit streams between a Data-Acquisition System (DAS) and a Data Processing System (DPS).

2.     The meaning of individual bit streams is not specified; normally, a bit-stream will be a stream of sign or magnitude bits associated with particular samples, but the actual meaning is to be mutually agreed upon between the DAS and DPS.

3.     The received and transmitted bit-stream clock rates may be different (e.g. the playback rate into the DPS may be speeded-up or slowed-down), however all bit-stream clock rates on acquisition must be the same, and all bit-stream clock rates on transmit must be the same.

4.     A single time-tag applies to all parallel bit streams. The DAS time-tag of every bit in every bit-stream must be fully recoverable at the output of the DTS.

VSI-H Features


-       1 Gbit/sec ‘Quantum Channel’ defined

§        32 parallel bits streams

§        32 Mbps/bit-stream (extension to 64, 128 MHz for 2, 4 Gbit/sec ‘quantum channel’)

-       One standard 80-pin connector per ‘quantum channel’

-       Standardized electrical and timing specifications

-       Signal interface is entirely LVDS

-       Method of time-tagging data is totally internal to DTS and not specified by VSI-H.

-       Built-in Test-Vector Generator/Receiver capability

-       Model-delay capability to simplify direct connection to correlator

-       Two levels of compliance defined to ease transition to new systems

-       Easy media translation (i.e. tape copying)

-       Canadian S3 group, Japanese 1-Gbps group and Haystack COTS groups are already committed to VSI-H compliance!

Coming Next: VSI-S


-       Standardized software interface to DTS

-       Will focus on those functions independent of DTS technology

-       DTS-specific commands will still be necessary to some degree, depending on technology and implementation

-       Goal is to complete within next year – stay tuned!