CBF to SDP Interface for Early Array Releases

It has been realised for some time that with so few baselines the data rate for AA0.5 and AA1.0 is very low. It has also been realised that the HEAP structure of the SPEAD protocol as currently written for the CBF to SDP interfaces implies that only transmitting a single channel per HEAP is inefficient.

As a result there have been some operating assumptions made, particularly by Team YANDA, who developed the CBF emulator and receive pipeline, and subsequently MID-CBF that mitigate this. The following has been suggested:

The simplest modification to the ICD is to update the 'shape' of Correlator Output Data (item 0x600A) from [n_bls] to [n_chan][n_bls]. When n_chan is 1 this collapses to the original single channel per heap. 

This is implemented in the emulator since the earliest releases. And has recently been incorporated into the firmware of the MID CBF-SDP interface:

from W.Kamp

Update: Just finished implementing the firmware for this. Slight change in strategy. For AA0.5 and AA1, will have 20 channels per SPEAD heap. This means there will be 744 SPEAD streams per 200 MHz of bandwidth. A heap will therefore be ~25kB.

The purpose of this ADR is to raise awareness of this issue, and ensure that all groups are aware of this assumption. Perhaps a lightweight addendum to the CBF-SDP ICD can be generated to incorporate these changes.