Within the scope of CCITT I.500 series of recommendations the term interworking is used to express interactions between networks, between end systems, or between parts thereof, with the aim of providing an end to end communication. The interactions required rely on functions and on the means to select these functions which include the conversion of physical and electrical states and the mapping of protocols. These functions are referred to as Interworking Functions (IWFs). An IWF may be implemented in the PLMN, Integrated Services Digital Network (ISDN), in the other types of network, at the user's premises, through a third party service provider, or in some combination of these. The IWFs needed are a result of service requirements for interworking, contained in the GSM 02.xx series and in the CCITT I.200 series of recommendations.

Network interworking is required whenever a PLMN and a non PLMN together are involved to provide an end to end connection and may be required in instance of a PLMN to PLMN connection (TS 29.007). Although the GSM PLMN is not an integrated part of the ISDN network it is the intention to provide ISDN similar services to its subscribers, as defined in TS 22.001. Those services imply interworking requirements to following networks:

• Between GSM PLMN and Public Switched Telephone Network (PSTN/ISDN);
• Between GSM PLMN and Circuit Switched Public Data Network (CSPDN);
• Between GSM PLMN and Packet Switched Public Data Network (PSPDN);
• Between GSM PLMNs.

Service interworking is required when the Teleservices at the calling and called terminals are different. No service interworking has been identified as a requirement of the GSM system for PSTN/ISDN network based services (GSM 02.xx series).

The supplementary services are described in TS 22.004 and in GSM 02.8x and 02.9x series. The way of providing supplementary service interworking between PLMNs and other CEPT networks is treated in TS 23.011 and in GSM 03.8x and 03.9x series, where each supplementary service IWF is described.

In TS 29.002 the configuration of the GSM PLMN is treated, defining the entities of the GSM system, and the interconnection between PLMNs is treated. The entities of the PLMN are listed below:

• Home Location Register (HLR);
• Visitor Location Register (VLR);
• Mobile services Switching Centre (MSC);
• Base Station System (BSS);
• Gateway MSC;
• Equipment Identity Register (EIR).

3GPP TS 29.002 describes the requirements for the signalling system and the procedures needed at the application level in order to fulfil these signalling needs.

The signalling aspects of interworking between ISDN/PSTN and GSM PLMN are treated in GSM 09.03. The general signalling requirements are split into requirements for the mobile network and requirements for the fixed network. Interworking with PSTN for call set up is treated, i.e. interworking with Telephone User Part (TUP) of SS7.

The IWFs are identified and the requirements to support interworking between a GSM PLMN and a CSPDN are treated. 3GPP TS 29.004 [9] covers two methods of interworking:

• PLMN to CSPDN direct;
• PLMN to CSPDN via ISDN.

The IWFs are identified and the requirements to support interworking between GSM PLMN and PSTN for PAD access are treated. PAD access is divided into Basic PAD access and Dedicated PAD access. As regards Basic PAD access two types of network configuration (related to the location of the user) are defined, Home PAD access and Visited PAD access. Dedicated PAD access is defined as shortest possible connection to a PAD from a PLMN. Dedicated PAD access treats two types of location of the PAD:

• PAD external to the PLMN;
• PAD internal to the PLMN.

A common set of profiles for the Dedicated PAD access is listed in annexes of 3GPP TS 29.005 [10].

3GPP TS 29.006 [11] identifies the interworking functions and requirements in the interworking between a GSM PLMN and a PSPDN/ISDN for the support of Packet Switched data transmission (bearer) services. Two types of services can be supported by a GSM PLMN:

• basic packet mode services;
• dedicated packet mode services.

3GPP TS 29.007 deals with the requirements to support interworking between GSM PLMN and PSTN and between GSM PLMN and ISDN, i.e. identifying the necessary IWFs. The interworking is split into:

• interworking with PSTN for speech calls;
• interworking with PSTN for data calls;
• interworking with PSTN for dual series calls;
• interworking with ISDN for speech calls;
• interworking with ISDN for data calls;
• interworking with ISDN for dual series calls.

In GSM 09.09 the signalling interworking between messages defined in TS 24.008, 08.08 and 09.02 is treated. Interworking with the fixed network is described using TUP or ISUP of SS7. In the MSC the signalling messages of the mentioned functions are handled:

• Call establishment;
• Call release;
• Location registration;
• Handover;
• Authentication.

Diagrams show the layer 3 messages between MSC MS, MSC BSS, MSC fixed network, and MAP messages.

3GPP TS 29.010 gives guidance to information element mapping between information elements of layer 3 messages sent on MS BSS interface and information elements of MAP messages and treats additional interworking in the case that the MSC also processes the information. The signalling mapping cases are divided into transparent and non transparent operations The format of the mapping and the illustration of mapping principles in MSC is shown in the subsection dealing with interworking in the MSC.

3GPP TS 29.011 is providing a detailed specification for interworking between the A interface protocol and the Mobile Application Part for handling of supplementary services. Call related supplementary services interworking and call independent supplementary services interworking are treated separately.

GSM 09.90 clarifies how interworking can be obtained between phase 2 mobile stations and phase 1 infrastructure. The objective is to obtain this without changing the consolidated set of phase 1 specifications. GSM 09.90 specifies the necessary amendments to the phase 1 infrastructure so that an acceptable service is offered to mobile stations of phase 2, guaranteeing that a phase 2 mobile station obtains all phase 1 services. The necessary changes are outlined as well as the necessary controls and clarifications with regard to phase 1 implementation, which should be performed to the different interfaces in phase 1 infrastructure before phase 2 mobiles are offered service.