Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 11.1 Gateways, SIP, and PSTN networks

To date, neither TRIP nor TGREP has seen any significant deployment or
usage by service providers.

SIP/PSTN Protocol Interworking

SIP and PSTN protocol interworking has two levels: the media and the signal-
ing levels.
The media interworking is quite straightforward. The PSTN generally uses
a 64-kb/s pulse-coded modulation (PCM) encoded Time Division Multiplex
(TDM) channel known as a trunkto carry the voice media. If Integrated Ser-
vices Digital Network (ISDN) is used, the B channel contains the 64 kb/s PCM
media stream. SIP-enabled devices generally have audio capabilities in the
form of Real-Time Transport Protocol (RTP) packets. The media interworking

SIP Phones

SIP Servers

Media: RTP

Media: TDM PCM

Signaling: SIP

Gateways

Signaling: ISUP, Q.931, CAS, etc.

SIP Servers

SIP Enabled

Devices

Telephones

Telephones

PBX

IP Network

PSTN

SIP Telephony 187