the L1 needs both a PDU (in TX_data.req) as well instructions for
the actual transmission (in DL_TTI.req) to encode a PDSCH message. In
nFAPI, it can happen that a DL_TTI.request message has been received
(configuring the PDSCH transmission), without the TX_data.request having
reached the PNF. The L1 assumes to have the PDU.
To avoid problems, ensure that only the pair of DL_TTI.req/TX_data.req
is delivered. Otherwise, drop either message.