A Method of Path Sharing for Multiple VDIF Data Streams Transmission

Abstract

One unit of the VLBI digital backend, CDAS2-PFB, can deal with the signal coming from one RF band with two different polarization, such as left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP), or horizontal polarization (HPOL) and vertical polarization (VPOL). The processing results of each polarization comes out from two different 10-Gigabit Ethernet (10GE) ports and sent to different recorders. In the VLBI Global Observation System (VGOS) stations, since there are four RF bands, four CDAS2-PFB units needed to be installed and eight data streams will be produced. While the recorder named Mark6 in VGOS station, have only four 10GE ports for data transferring, eight data streams need to be connected to the recorder. A method based on time division multiplexing technology has been designed to sharing one output port with two data streams in each CDAS2-PFB unit. In the design, a dedicated format named VLBI Data Interchange Format (VDIF) has been chosen to framed the data coming from different polarization. Each data stream can be identified by the value of specific field in the VDIF frame header after recorded in disks. And a switching module has been designed by two nested state machines, one for port switching and the other for interface controlling. It will be convenient to upgrade the switching module if the number of data streams increasing. The design has been verified in FPGA and merged to the original firmware of CDAS2-PFB. Furthermore, the relationship between the data rate of the input streams and the maximum transmission capability of the output path have also been discussed in order to show the conditions that the method is suitable for the application.