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 (Section 6.9) 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 used internally for compatibility with some existing correlators. An eventual transition is planned to the VLBI Data Interchange Format (VDIF), specified at http://vlbi.org/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 initial 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-R
program. Unfortunately, commissioning tests of the
2-Gbps capability have encountered unacceptably high failure rates
in these modules. The resulting inability to record wideband
observations reliably is currently the primary obstacle to wideband
scientific observations, and the focus of commissioning efforts.
Further information on the Mark 5C system is available in the Sensitivity Upgrade memo series at http://www.vlba.nrao.edu/memos/sensi/ , and in the Haystack Mark 5 series at http://www.haystack.mit.edu/tech/vlbi/mark5/index.html .