General Network Interworking Scenarios are described in GSM 09.01.
When two dissimilar networks are required to interwork in order to support a communication between two subscribers, one on each network, a number of functions performing the interworking are required to support the communication. Some of these are related to the differences in signalling and are dealt with in GSM 09.03.
Examples of other aspects of interworking are:
i) the need of specific packet functions;
ii) the need of modem pools and network‑based rate adaptation.
From the mobile subscriber’s point of view, there have been defined two types of services within GSM PLMNs:
‑ basic packet mode services:
These services provide access either to the Packet handler(PH) and/or the PSPDN Port to which the individual subscriber is subscribed to.
‑ dedicated packet mode services:
These services provide access to the PH provided by the visited GSM PLMN, i.e. the "real packet world" is entered as soon as possible.
Packet services are provided in the context of the present document by means of the "circuit" transfer mode.
For the purposes of determining the required IWF functionality, it is necessary, however, to consider separately each type of service i.e. the basic and the dedicated packet mode service.