Senarist,
The SDCCH Drop occurs during the SDCCH pahse, which is the phase when the MS and the MSC communicates transparently about ciphering, authentification, etc…
So, the SDCCH phase ends when the MSC sends the “ASSIGNMENT REQUEST” to the BSC (it is not a transparent message anymore).
Then starts the TCH ASSIGNMENT phase, until the message ASSIGNMENT COMPLETE is sent from MS to MSC.
Regards,