09.613GPPGeneral Packet Radio Service (GPRS)Interworking between the Public Land Mobile Network (PLMN) supporting GPRS and Packet Data Networks (PDN)TS
5.1 Service Interworking
Service interworking is required when the Teleservice at the calling and called terminals are different. For GPRS, service interworking is not applicable at the Gi reference point.
5.2 Network Interworking
Network interworking is required whenever a PLMN is involved in communications with another network to provide end-to-end communications. The PLMN shall interconnect in a manner consistent with that of a normal Packet Data Network (type defined by the requirements e.g. IP, PSDN X.75). Interworking appears exactly like that of Packet Data Networks.
5.3 Numbering and Addressing
See 3GPP TS 03.03  and the relevant sections for X.25 and IP addressing below.