[ITU-T V.152] describes the voiceband data (VBD) operation of voice-over-Internet protocol (VoIP) gateways, which may be abbreviated as VBDoIP. [ITU-T T.38] describes the facsimile packet relay operation of voice-over-Internet protocol (VoIP) gateways, which may be abbreviated as FoIP. Both these gateway models consider explicitly interworking between an IP-based packetswitched network on one side and a circuit-switched network on the other side.

This Recommendation describes procedures for direct service interworking (according to the definition in clause 3.2 of [ITU-T Y.1251]) of two IP domains, whereby one IP domain uses V.152-based VBDoIP transport and the other IP domain uses T.38-based FoIP transport.

The term voiceband data is an umbrella term for all kinds of teleservices that use a "data-oriented transport" in the frequency band of the narrow-band voice spectrum (which is a 3.1-kHz-band). The data-oriented transport is realized by modem protocols (the definition for modem can be found in clause 3.13 of [ITU-T V.152]), as defined, e.g., within the ITU-T V-series Recommendations. Teleservices may be categorized into three major applications areas: facsimile, text-based communication and general data services. The three VBD application areas may be summarized as (by using notation "application/transport"):

– Facsimile/modem: Gateway technologies for PSTN-to-IP interworking, see, e.g., [ITU-T V.152] for pass-through mode and [ITU-T T.38] for packet relay mode.

– Text/modem: Gateway technologies for PSTN-to-IP interworking, see, e.g., [ITU-T V.152] for pass-through mode and [b-ITU-T V.151] for packet relay mode.

– Data/modem: Gateway technologies for PSTN-to-IP interworking, see, e.g., [ITU-T V.152] for pass-through mode and [b-ITU-T V.150.1] for packet relay mode.

This Recommendation only considers facsimile/modem.