appropriate federations and security protections established
between them. The IP circuit continues to carry e-mail,
Internet, and other corporate traffic as it does today, and
voice is simply layered on top of the circuit as another IP
application. SIP sets up and tears down voice calls to and
from the enterprise over this IP circuit.
On-net calls traverse the carrier’s VoIP backbone (which is
typically dedicated to voice so that voice quality can be guar-
anteed). Off-net calls ride the carrier IP network until the last
mile, where a gateway converts VoIP to TDM for calls to PSTN
parties (see Figure 4-1).
Figure 4-1: SIP trunks change how you make connections to telco
carriers.
SIP trunks offer a number of benefits, including:
ߜ PSTN origination/termination: Many SIP service
providers support origination/termination services
directly to the PSTN from their SIP networks. This
practice enables the enterprise to reduce the monthly
recurring costs associated with multiple TDM circuits
by deploying a single IP pipe to the service provider
network.
ߜ DID and 800-Number Mobility: These features take
advantage of the fact that SIP is geographically agnostic.
They allow calls destined to a local or 800 number to be
Phones
SIP Proxy
Router
IP Data/Telephony
All Communications
Service
Provider
SIP Network
PSTN Long
Distance
PSTN Local
Access
L
A
N
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