The GPRS element interfacing with the NSS is SGSN. The important interfaces to the NSS are the SGSN-HLR (Gr), SGSN-EIR (Gf), and SGSN-MSC/VLR (Gs) . The other interfaces are implemented through the intra-PLMN backbone network (Gn), the inter-PLMN backbone network (Gp), or the external networks (Gi).