ETSI TS 123 053 defines the stage 2 service description for Tandem Free Operation (TFO) in and between GSM and UMTS. Tandem Free Operation applies only to speech calls.

NOTE: The TFO principles are built so that they could be used as well by other systems other than GSM and UMTS.

In analogy with ITU-T Recommendations I.130 [2] and with reference to ITU-T Recommendations Q.65 VI.1 (Stage 2 of the method for the characterisation of services supported by an ISDN), the second stage of the following three-level structure is derived from a stage 1 service description.

• Stage 1 is an overall service description, from the service subscriber's and user's standpoints, that views the network as a single entity which provides services to the user.
• Stage 2 identifies the functional capabilities and information flows needed to support the service described in stage 1. Furthermore, it identifies various possible physical locations for the functional capabilities. The output of Stage 2, which is signalling system independent, is used as an input to Stage 3, the design of signalling system and switching Recommendations.
• Stage 3 defines the signalling system protocols and switching functions needed to implement the service described in stage 2.