Automatically set the A / alarm bits in Tx if no signal on Rx
At least according to one of my German retronetworking books on PDH it is stated that an E1 interface must set the A[larm] bit in TS0 if no received signal is detected.
I expect we don't do that yet as we have no real concept of LOS.
Relying on the host software to insert that A bit is not really a good idea, as the host has the least idea of what's happening on the physical layer.
So I would guess, similar to the way how we automatically can generate the E bits in case of Rx CRC errors, it would be good to generate the A bits based on whether or not we see incoming Rx clock cycles.