This diagram shows the simplified structure of a GPRS Core Network along with the names of some of the interfaces / reference points between systems.

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• Gi - the reference point at which the GPRS core network connects to the internet. Alternatively, corporate customers may have a direct connection to this point for higher security. This reference point is normally just an IP network, though a tunnelling protocol such as GRE, IPinIP or IPsec may be used instead.
• Gr - the reference point between the SGSN and the HLR used to allow the SGSN to retrieve subscriber information from the HLR. The communication here is based on SS7
• Gb - the interface between the SGSN and the PCU which is the point of connection of the GPRS core network to the GSM radio network (BSS). This carries both signalling and user data. The communication here is based on Frame Relay.
• IuPS - the interface between the SGSN and the RNC which is the point of connection of the GPRS core network to the UMTS Radio Access Network UTRAN. The communication here is abased on Broadband SS7 over ATM for signalling and GTP-U over IP for user data.
• Gs - the connection between the SGSN and the MSC. This is optional and is used primarily in GSM networks to improve signalling efficiency, especially on the radio interface. In UMTS almost the same effect is achieved simply through the action of the RNC. This interface uses the BSSAP+ Protocol.

This diagram is an export from an Open Office drawing. The original is also available for editing.