This Recommendation defines the inter-operation of two PSTN to IP network gateways that facilitate the end-to-end connection of V-series DCEs over an IP network. The principal characteristics of these gateways are: a mechanism to allow the transparent transport of modem signals end-to-end, a mechanism to allow the termination of modem signals at the gateways and the transport of the data between gateways, the definition of a transport protocol, which is suitable to relay data between gateways, and procedures to allow gateways to transition between Voice-over-Internet Protocol and Modem-over-Internet Protocol operation.