Hi qamber
I think may be u r fresher in the telecomm line first of all i would like to WELCOME to the TELECOMM WORLD.As u might be aware of the NSS(Network switching system)BSS(Base station subsystem)
NSS is related more towards communicating to other exchanges and subsystems like HLR VLR IN IWF and many more nodes also communicate with BSS.
But BSS mainely toward BTS,BSC TCU
or u may consider it to the radio side.
In short BSS towards radio side and NSS more towards diff. exchanges nodes.U may quiry further