GPRS Interfaces

Different network components of the GPRS are connected together by well defined interfaces. Some new interfaces to GSM have been added in GPRS to support packet switched data mainly between GGSNs, SGSNs and other network components. The following interfaces have been defined:

• Um interface between MS and BTS is very similar to GSM and defines the modulation type, error correction/detection technique, power control information etc.
• A interface between BTS and BSC defines the channel allocation, power measurement information etc.
• Gb interface connects BSCs to SGSN.
• Gn interface is used between GSNs of same PLMN to exchange user profile when the user moves from one SGSN to another.
• Gp interface is defined between two GSNs of different PLMN for exchanging the user profile and other signaling information between a SGSN and GGSN of another area.
• Gf interface is used between SGSN and EIR to query the IMEI information if a MS tries to register with the network.
• Gr interface between SGSN and HLR is used to get the user profile, the current SGSN address and the PDP address(es) for each user in PLMN.
• Gc interface between GGSN and HLR is used by GGSN to query user's location and profile to update its location register.
• Gi interface connects GGSN to external PDN (e.g. X.25 or IP).
• Gs interface between SGSN and MSC/VLR is used to perform paging request of circuit switched GSM call for combined attachment procedure.
• Gd interface between SMS-Gateway (SMS-GMSC) and SGSN is used to exchange short message service (SMS) messages.

All GSNs are connected over a GPRS backbone network over IP. Within this backbone the GSNs encapsulate and transmit PDN packets by using GPRS Tunneling Protocol (GTP). This backbone network is of two types:

• Intra-PLMN backbone connects the GSNs of same PLMN and are therefore private IP based networks of the service provider.
• Inter-PLMN backbone connects the GSNs of different PLMNs if there is a roaming agreement is between them. The gateways between these PLMNS are called border gateways and perform security functions apart from regular functions to protect the private information against unauthorized usage.