Hi Safar,
Wen an NPDU has been sent to GGSN for connecting to the GPRS nEtwork, The GGSN forwards the same to SGSN through Inter PLMN IP backbone Network. Now, the SGSN compresses and partitions the npdu to LLC frames and sent to BSS using BSS GPRS protocol (BSSGP). The BSC will partition it again to RLC blocks.
I hope u r clear.