Recommendation ITU-T V.153 defines service interworking between two IP domains, whereby one IP domain is using voiceband data over IP (VBDoIP) transport according to Recommendation ITU-T V.152 and the other IP domain is using facsimile relay over IP (FoIP) transport according to Recommendation ITU-T T.38. Two modes of operation are specified for the T.38-to-V.152 interworking function: native versus emulated interworking.