hi deepak
having multiple NSE for 1 BSC allows to have a load sharing.
Each NSE *should* represent a PCU (packet control unit), therefore having multiple NSE means your BSC is connected to several PCU.
Each NSE is connected to the SGSN with its own lot of interfaces (bearer channels in FR, IP endpoints in IP)
I don’t know if 3GPP defines this, I don’t think so.
regards,
pix