SIP Communications For Dummies, Avaya Custom Edition
34
Working Together: SIP and
the PSTN
Clearly the telco world with its country codes, area codes,
city codes, and telephone numbers will continue to serve
many people for some time to come. So, how do you call SIP
users with URIs from old push-button phones, and how do
you call landline users from SIP-enabled devices?
Fortunately, the mapping between SIP and telephony proto-
cols has been defined. Gateways that link the Internet with the
PSTN are widely deployed and used by VoIP users every day.
SIP URIs can also be used to carry telephone numbers. For
example, sip:+12025551212@example.com;user=phone
contains the phone number for directory assistance in
Washington, D.C.
By porting a PSTN telephone number to a SIP/PSTN gateway,
incoming telephone calls can be routed to SIP phones, call
managers, and PBXs.
Service providers have already adopted SIP for their internal
PSTN telephone call routing. With SIP-to-PSTN interworking
(providing connectivity between these two systems through
a defined interface standard) now in place, carriers such as
Global Crossing, AGN Networks, and others are working with
equipment vendors like Avaya to offer exciting new SIP trunk
services to the enterprise market.
Trunking with SIP
Trunking refers to the means used to transport inbound and
outbound calls between the enterprise and external entities
(including branch offices and other remotely located parties
such as business partners, customers, and suppliers). In this
section we describe the differences between traditional and
SIP trunks, as well as some of the characteristics of SIP
trunks that are attractive to businesses.
08_041498 ch04.qxp 3/16/06 5:52 PM Page 34
