in a gateway involves terminating a PCM trunk on the PSTN side and bridg-
ing the media with an IP port that sends and receives RTP packets. Codec con-
version between PCM and another codec is possible in the gateway, or the
gateway may simply reuse the 64 kb/s PCM as RTP/Audio-Video Profile
(AVP) 0, a common codec supported by nearly every SIP device capable of
sending and receiving audio. The gateway refuses SIP sessions that do not
contain an audio channel, and will decline all media types that it does not
know how to map into PSTN telephony voice channels.
The signaling interworking is much more complex. While a SIP network is
“flat” in terms of not having a different ITU-style user-to-network interface
(UNI) and network-to-network interface (NNI), the PSTN uses many different
signaling protocols to complete a call. For example, a PSTN call may enter
the PSTN as a PBX trunk, in which a Circuit-Associated Signaling (CAS) pro-
tocol is used to out-pulse dialed digits as multifrequency (MF) tones. The tele-
phone switch then signals to other telephone switches using ISDN User Part
(ISUP) signaling, which is carried out-of-band in a dedicated packet switched
network known as Signaling System 7 (SS7). Alternatively, ISDN (Q.931)
D-channel signaling may be used.Types of Gateways
The PSTN signaling protocol that a SIP/PSTN gateway will use will depend
on the way it interfaces with the PSTN. We will consider two types of
gateways:
■■ Anetwork gatewayis a high-port-count gateway that is typically owned
by a PSTN carrier and interfaces with other PSTN switches using ISUP
and ISDN as its NNI. A network gateway is typically located at a PSTN
central office, where other large telephone switches are located. A net-
work gateway is considered part of the PSTN trust domain.
■■ An enterprise gateway, on the other hand, is typically a small-port-count
gateway that may be owned by a PSTN customer and interfaces with
the PSTN via UNI protocols such as CAS and ISDN. This device typi-
cally will be located on a customer’s premises or building. An enter-
prise gateway is not part of the PSTN trust domain.SIP and Early Media
A number of Internet drafts have been written to document the basic map-
ping between SIP and PSTN protocols. However, the base SIP specification
was found to be missing one key component of successful SIP/PSTN inter-
working—support of early media. In the PSTN, call progress indicators are188 Chapter 11