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.
The HLR/HSS contains GPRS and EPS subscription data and routeing information. The HLR/HSS is accessible from the Gn/Gp SGSN via the Gr interface, from the S4-SGSN via the S6d interface and from the GGSN via the Gc interface. For roaming MSs, the HLR/HSS may be in a different PLMN than the current SGSN.
An A/Gb mode MS operates in one of three modes of operation. The mode of operation depends on the network domains that the MS is attached to, i.e. only PS or both PS and CS domain, and upon the MS's capabilities to operate PS and CS domain services simultaneously.
Class A mode of operation: The MS is attached to both PS and CS domain, and the MS supports simultaneous operation of PS and CS domain services.
Class B mode of operation: The MS is attached to both PS and CS domain, but the MS can only operate one set of services, PS or CS services, at a time.
Class C mode of operation: The MS is exclusively attached to the PS domain.
The three modes of operation are defined in TS 22.060.
An Iu mode MS operates in one of three modes of operation. However, these operation modes are different from the ones of an A/Gb mode MS due to the capabilities of an Iu mode RAN to multiplex CS and PS connections, due to paging co-ordination for PS services and CS services that are offered by the CN or the UTRAN/GERAN-Iu, etc. The different Iu mode MS operation modes are defined as follows:
CS/PS mode of operation: The MS is attached to both the PS domain and CS domain, and the MS is capable of simultaneously signalling with the PS and CS core network domains. This mode of operation is comparable to the class A mode of operation defined for A/Gb mode. The ability to operate CS and PS services simultaneously depends on the MS capabilities (for example an A/Gb mode MS of class B, which can not operate simultaneously CS and PS services, may have the same limitations when changing to Iu mode and CS/PS mode of operation).
PS mode of operation: The MS is attached to the PS domain only and may only operate services of the PS domain. However, this does not prevent CS-like services to be offered over the PS domain (e.g. VoIP). This mode of operation is equivalent to the A/Gb mode GPRS class C mode of operation.
CS mode of operation: The MS is attached to the CS domain only and may only operate services of the CS domain. However, this does not prevent PS-like service to be offered over the CS domain. The CS mode of operation is outside the scope of this specification.
All combinations of different operation modes as described for A/Gb mode and Iu mode MSs shall be allowed for GERAN and UTRAN multisystem terminals.