Mark 5C Recorder

by Gustaaf Van Moorsel last modified Dec 28, 2014 by Jonathan Romney

The VLBA's data transmission system comprises the recorder units at the stations, playback units at the correlator, and the magnetic disk modules that are shipped between those units.  The Mark 5C recording system was developed jointly by NRAO, Haystack Observatory, and Conduant Corporation.  It closely resembles the Mark 5A version used previously by the VLBA, and the Mark 5B used at some other observatories.  In particular, identical disk modules are used.  However, Mark 5C is a packet-based system, which allows a more straightforward functionality than its predecessors.   It simply records the payload of each 10G Ethernet packet received from the RDBE, without imposing any special recording format.  All formatting of the observed data — most essentially, the precision time tags — is internal to the packet payloads, which are transmitted directly from recorder to playback by the Mark 5C system.  Mark 5B formatting has been used internally, for initial compatibility with some existing correlators, but a transition to the VLBI Data Interchange Format (VDIF) is currently under way.

Each Mark 5C unit accommodates two removable modules, each in turn comprising eight commercial disk drives.  As used on the VLBA, these modules are recorded sequentially at a maximum rate of 2 Gbps, matching the current maximum RDBE output rate.  Modules of 16-TB capacity were procured with funding awarded through NSF's MRI-R2 program, sufficient to support wideband modes for approximately 40% to 50% of observing hours.

Further information on the Mark 5C system is available in the Sensitivity Upgrade memo series, and in the Haystack Mark 5 series.