Mark 5C Recorder

by Gustaaf Van Moorsel last modified Jul 08, 2012

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 new Mark 5C 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 functionally more straightforward 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.  Initially, Mark 5B format is being used internally, for compatibility with some existing correlators.  An eventual transition is planned to the VLBI Data Interchange Format (VDIF).

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, intended to suffice for recording a majority of VLBA observations at the 2-Gbps data rate, were procured with funding awarded through NSF's MRI-R2 program.  Unfortunately, commissioning tests of the 2-Gbps capability encountered unacceptably high failure rates in these modules, which limit the throughput currently achievable in wideband observations.

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