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Mark 5

Mark5 VLBI Data System

The Mark 5 system is being developed at Haystack Observatory as the first high-data-rate VLBI data system based on magnetic-disc technology. Incorporating primarily low-cost PC-based components, the Mark 5 system supports data rates up to 2048 Mbps, recording to an array of 8 inexpensive removable ATA disks. A feasibility Mark 5 system demonstrated fringes in March 2001. In mid-2002, the first prototype systems, dubbed Mark 5P, were put into routine operation at Kokee, Hawaii and Wettzell, Germany in support of daily UT1 Intensive measurements with correlation on the Mark 4 correlator at Haystack Observatory. The Mark 5A system, pictured below, was developed primarily to re-packaged the disks into convenient '8-pack' modules and was placed into service in early 2003. Approximately 250 Mark 5 systems are now (mid-2009) in operation. Support for Mark 5 development was provided by BKG, EVN, KVN, MPI, NASA, NRAO and USNO.

For more details please check out the Mark 5 System Overview document

Figure 1 Mark 5 prototype chassis Full size photo Figure 2 Mark 5 '8-pack' module Full size photo

Features of the Mark 5 System
The features of the Mark 5 system are: Why Discs? Though both magnetic-disc technology and magnetic-tape technology have made great strides over the past few years, the pace of magnetic-disc development has been no less than spectacular, far exceeding even disc-industry projections. The figure below displays a comparison of disc and tape prices over the past several years, showing that disc prices (on a $/GB basis) continue downward in a still-accelerating trend. Current (mid 2009) consumer ATA disc costs are ~$0.07 US/GB and falling; current Mark4/VLBA tape prices are ~$2US/GB and remaining steady. A single Mark 5 system with sixteen 700 GB disc drives will record continuously 1024 Mbps for 24-hours unattended!

Figure 3 Disk and tape prices vs.time

Mark 5 Development Program

The Mark5 development program, which started with the Mark 5A at 1 Gbps, has now evolved to next-generation Mark 5A+/Mark 5B/Mark 5B+ systems, and was expanded in 2007 to include the Mark 5C.  Each generation has expanded and extended the capabilities, extending to 4 Gbps with the Mark 5C, and all generations are compatible with standard Mark 5 disk modules.  All of the Mark 5 systems have been jointly developed with Conduant Corporation of Longmont, CO, which provides the underlying ‘real-time’ disk-recording technology.  Conduant also makes Mark 5 systems available to the VLBI community at significant discounts from standard commercial prices.  A brief summary of each of the Mark 5 generations:

  1. Mark5A: The Mark 5A system, released in 2003, is intended as a direct replacement for a Mark 4 or VLBA magnetic-tape transport at either a station or a correlator.  It records 8, 16, 32 or 64 tracks from a Mark4/VLBA formatter, and plays back in the same Mark4/VLBA format. As such, the Mark 5A is a direct replacement for a Mark4 tape unit at 1024 Mbps and VLBA tape unit at 512 Mbps. The disks are organized into two '8-pack' modules which may be used in a ping-pong fashion for near-continuous recording and playback.
  1. Mark5B: The Mark 5B is VSI-H-compliant system with capability up to 1024 Mbps; no external formatter is necessary. The system will also support several backwards-compatibility modes with existing Mark4/VLBA correlator systems. The Mark 5B has been in service since the summer 2006.
  1. Mark 5A+: The Mark 5A+ is identical to Mark 5A in hardware, but has had its FPGA code augmented to support playback of disk modules recorded on Mark 5B.  In this special ‘backwards-compatible’ playback mode, the Mark 5B data are transformed into VLBA tape-format data streams which can be directly accepted by Mark 4 correlators.  The Mark 5A+ became operational in 2006 and is used at some Mark 4 correlators.
  1. Mark5B+:  The Mark 5B+ is a VSI-H-compliant system with recording capability extended to 2048 Mbps.  The Mark 5B+ is in all other respects identical in capability to the Mark 5B.  The Mark 5B+ has been in service since late 2006.
  1. Mark5C:  The Mark5C is a new system under development with the capability to support up to 4096Mbps from a 10 Gigabit Ethernet data source; in parallel developments, new Digital Backend systems are being developed as data sources for the Mark 5C in the U.S. and Europe.  Playback, as well, is via a 10 Gigabit Ethernet data stream, which is compatible with on-going software-correlator developments in Australia, Europe and the U.S.  The Mark 5C is fundamentally a ‘formatless’ packet recorder, though a parallel Mark 5C data-format specification defines a suggested standard VLBI data format.  In a departure from earlier Mark 5 generations, as well as almost all previous VLBI systems, the Mark 5C breaks the normal constraint of 2n frequency channels, allowing more flexibility in recording and playback.  In addition, for backwards compatibility, the Mark 5C can be configured to write data to disk in a Mark 5B-compatible data format that allows playback on standard Mark 5B/5B+ systems.  Prototype Mark 5C units are expected to be available in 2010.

Compatibility Considerations
The Mark 5 system was designed for extensive forward and backwards compatibility with existing VLBI systems. For example, data may be recorded with a VSI-compatible interface and re-played into any Mark4/VLBA correlator. Conversely, data may be recorded from a Mark4/VLBA system and re-played into any VSI-compatible correlator. In addition, it is expected that existing interfaces to S2 recorders can be easily adapted to record on Mark 5B, which can then be re-played into either a VSI-compatible or Mark4/VLBA correlator. This inter-compatibility among various systems will allow a much broader and flexible use of existing VLBI facilities throughout the world. e-VLBI Support The Mark 5 system allows easy connection of a VLBI data system to a high-speed network connection. Because the Mark 5 system is based on a standard PC platform, any standard network connection is supported. Depending on the availability of high-speed network connections, this can be accomplished in at least two ways:
  1. Direct Station to Correlator: If network connections allow, data may be transferred in real-time at up to 1 Gbps from Station to Correlator, either for immediate real-time correlation or buffering to disc at the Correlator.
  2. Station Disc to Correlator Disc: If network connections are not sufficient to allow real-time transmission of data to the Correlator for processing, data may be recorded locally to disc at the Station, then transferred to disc at the Correlator at leisure for later correlation.
Depending on the available network facilities, either entire experiments or small portions of experiments may be transmitted electronically. The latter has been particularly useful for verifying fringes in advance of important experiments.


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