There are two kinds of backbone networks. These are called:
intra-PLMN backbone network; and
inter-PLMN backbone network.
The intra-PLMN backbone network is the IP network interconnecting SGSNs, GGSNs, Serving GWs and PDN GWs within the same PLMN and it interconnects GGSNs and Serving GWs with RNCs if Direct Tunnel functionality is supported.
The inter-PLMN backbone network is the IP network interconnecting SGSNs, GGSNs, Serving GWs and PDN GWs with intra-PLMN backbone networks in different PLMNs.
Every intra-PLMN backbone network is a private IP network intended for GPRS packet domain data and signalling only. A private IP network is an IP network to which some access control mechanism is applied in order to achieve a required level of security. Two intra-PLMN backbone networks are connected via the Gp interface using Border Gateways (BGs) and an inter-PLMN backbone network. The inter-PLMN backbone network is selected by a roaming agreement that includes the BG security functionality. The BG is not defined within the scope of GPRS. The inter-PLMN backbone can be a Packet Data Network, e.g. the public Internet or a leased line.