03.603GPPGeneral Packet Radio Service (GPRS)Release 1998Service descriptionStage 2TS
Network interworking is required whenever a PLMN supporting GPRS and any other network are involved in the execution of a GPRS service request. With reference to Figure 1, interworking takes place through the Gi reference point and the Gp interface.
The GPRS internal mechanism for conveying the PDP PDU through the GSM PLMN is managed by the GSM GPRS network operator and is not apparent to the data user. The use of this GSM data service may have an impact on and increase the transfer time normally found for a message when communicated through a fixed packet data network.
5.2.1 PSPDN Interworking
GPRS shall support interworking with PSPDN networks. The interworking may be either direct or through a transit network (e.g., ISDN). GPRS shall support both X.121  and E.164  addresses.
GPRS shall provide support for X.25 virtual circuits and X.25 fast select. X.75  or X.75′  may be used for interworking with X.25 PDNs.
The GPRS TEs have addresses provided by the GSM PLMN GPRS service operator and belong to the GPRS service domain. The PSPDN TE sends data to the GPRS TE by use of the GSM PLMN GPRS DNIC (Data Network Identification Code) or equivalent that uniquely identifies the GPRS network.
5.2.2 Internet (IP) Interworking
GPRS shall support interworking with networks based on the internet protocol (IP). IP is defined in RFC 791 . GPRS may provide compression of the TCP/IP header when an IP-datagram is used within the context of a TCP connection.
In a similar way to the PSPDN X.25 case, the GSM PLMN GPRS service is an IP domain, and mobile terminals offered service by a GSM service provider may be globally addressable through the network operator’s addressing scheme.