Karthik,
If CSU is transcoder unit – then answer is different for different equipment vendors.
TC, never have signalling point, so it never have SS7 connection to BSC.
In Nokia DX200 BSS – transcoder is connected to BSC via LAP-D only for O&M linkage.
In Ericsson AXE 810 there is no separate transcoder, usually it is combiled node BSC/TRX, so it have internal connection via DL bus.