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The traditional circuit-based PSTN (public-switched telephone network) is giving way to the packet-switched model of the Internet. In a few years, people will make voice calls over the Internet with ease and with a high quality of service. In the meantime, both the Internet and the PSTN will exist. There are many instances when Internet users need to access the PSTN and PSTN users need to access Internet telephony services. In the former, an Internet telephony user needs to call a telephone on the PSTN. In the latter, a PSTN telephone user needs to connect with an IP-based telephony system.

The IETF PINT Working Group has addressed the arrangement through which Internet applications can request PSTN services. The IETF SPIRITS (Service in the PSTN/IN Requesting InTernet Service) Working Group has addressed the opposite arrangement in which PSTN users request services that require an interaction between the PSTN and the Internet. Some examples covered by SPIRITS include Internet call waiting, Internet caller-ID delivery, and Internet call forwarding. See "SPIRITS (Service in the PSTN/IN Requesting InTernet Service)" for more information.

PINT-defined services handles situations where Internet users request a telephone call to a PSTN terminal such as a telephone or fax machine. Services that PINT addresses often appear on Web pages, such as click-to-dial, click-to-fax, click-to-fax-back, and voice callback services. For example, a user visiting a Web shopping site may need more information about a product they want to buy. A click-to-fax-back button initiates a sequence that sends information to the user's fax machine through the PSTN.

The original motivation for PINT-defined services was the desire to invoke the following three telephone network services from within an IP network:

• Request to call    An IP host makes a phone call to the PSTN.


• Request to fax content    An IP host causes a fax to be sent to a PSTN fax.


• Request to speak/send/play content    Causes content to be spoken out. For example, a Web page has a button that, when pressed, causes a PINT request to be passed to the PSTN, resulting in the content of the page (or other details) being spoken to the person.

The typical scenario involves an Internet-to-PSTN gateway. An IP host sends a request to a PSTN gateway, which in turn relays the request into a telephone network, upon which the telephone network performs the requested call service. PINT Services uses both SIP (Session Initiation Protocol) and SDP (Session Description Protocol). SIP is used to invite a remote server into a session. The invitation contains an SDP description of the media session that the user would like to take place.

See RFC 2458 (Toward the PSTN/Internet Inter-Networking-Pre-PINT Implementations, November 1998) and RFC 2848 (The PINT Service Protocol: Extensions to SIP and SDP for IP Access to Telephone Call Services, June 2000).

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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