Mixed Internet-PSTN Services
There are a wide range of interworking modes between the PSTN- and SIP-
based IP communication networks. Early work on SIP was mainly focused on
interworking with the PSTN and with PBXs, and you might suppose this was
the price the SIP developers’ community had to pay for making SIP acceptable
to telephone companies as the signaling standard for VoIP. In retrospective,
given the other innovative services enabled by SIP and the faster-than-
expected diminishing importance of wireline telephony compared to mobile
telephony and the Internet, we don’t believe this work is now as important as
it was thought to be initially.
PSTN and INTerworking (PINT)
PSTN and Internet INTerworking(PINT) [24] is a service where an action from
the Internet (such as a click on a web page) invokes a PSTN service, such as set-
ting up a call between two phones (RFC 2848) or between two fax machines, or
connects a fax machine to an information service that can send a fax on
demand. Applications are click-to-connect, click-to-fax, click for information,
and various others.
SPIRITS
Servers in the PSTN Initiating Requests to InTernet Servers(SPIRITS) [25] is the
name of a family of IN services on the PSTN that can be implemented using
SIP. It also applies to such services as Internet call waiting, where an event
(calling a busy phone line) on the PSTN can generate an action on the Internet
(call waiting pop-up panel on the PC that is using the called line for Internet
access).
TRIP
The Telephony Routing over IP(TRIP) protocol [26] is designed to find the
desired gateway to terminate a call on the PSTN. Given the increasing number
of IP telephony gateways, it may not be practical to maintain huge SIP routing
tables. It also may be desirable to route calls to gateways that meet certain cri-
teria. The Telephony Routing Protocol is modeled after the IP Border Gateway
Protocol (BGP) routing protocol and inherits its scalability.
Figure 2.5 shows an enterprise network connected to an Internet service
provider (ISP) with SIP servers and various other SIP devices such as SIP
phones and also the gateway to the PSTN.
Internet Communications Enabled by SIP 29