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Generally, SIP-adopting companies can expect a greater likeli-
hood of interoperability among proxies, phones, and gateways.
This integration is quicker and easier because these devices
are modeled after traditional telephony devices with more
predictable features and logic.
Interoperability for other services such as firewalls, Interactive
Voice Response systems (IVRs), and conferencing servers is
less proven because these applications aren’t as rigorously
tested at interoperability events. Because these types of
products tend to have more advanced and proprietary fea-
tures than familiar voice-centric products, interoperability
with these types of applications will be more complicated.
In such cases, the application-level logic built into these sys-
tems is much more complex than simply setting up a session.
Important supporting protocols, such as those being devel-
oped within the XCON working group for centralized confer-
encing, are being developed in the IETF.
External from the Enterprise
Interoperability with the PSTN is achieved through gateways
and through SIP-T, a protocol used to carry ISDN signaling
within SIP messages.
Connecting carrier SIP networks to enterprises is possible
today, as described in Part 4. However, because so many
implementation options in carrier and enterprise devices
exist, several interoperability hurdles remain.
Some enterprise communications vendors, including Avaya,
are actively promoting programs that encourage interoper-
ability with carrier networks. Avaya cofounded SIP Connect
(now part of SIP Forum) to define rigorous standards for the
carrier-to-enterprise interface. Through its SIP certification
program for service providers, Avaya has tested and certified
interoperability with a number of service providers.
SIP Interoperability Efforts
SIP is an open standard — and the up-and-coming stan-
dard for VoIP and other forms of peer-to-peer and group
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