The telephony resource is one of the most important resources of the voice portal. The telephony resource provides the interface between the portal and the telephony network. Typically, voice portal platform developers interface their VoiceXML browser/interpreters to the telephony resources using either proprietary APIs or more "standard" APIs such as Microsoft's TAPI or Sun Microsystems' JTAPI. The telephony network, from the perspective of the voice portal, begins at the network switch.
The telephony resource establishes the voice connection between the network switch and the voice portal after a series of protocol messages are exchanged between the two. For some architectures, this resource provides echo-cancellation, dual-tone multifrequency (DTMF) tone detection, and audio prompt playback/record. Echo cancellation removes much of the background voice echo from a speech channel, aiding in the clarity of the speech signal that the user hears and the speech processors receive. DTMF tones are the "touch tones" with which most telephone users are familiar.
Some architectures rely on tone detection through the telephony cards, whereas others may rely on detection through support in the speech-processing resource. Audio prompts are prerecorded prompts that can be played back to the end user. Alternatively, prompts can be recorded. Because of the limited bandwidth of telephony channels and the capabilities of the speech-delivery channel, the audio prompt playback to the end user will have different levels of perceived quality. The prompt playback and recording can also be implemented in the telephony cards or the speech processor card.