Besides emulating PBX features, SIP also can emulate AIN services found in
the PSTN. Both capability sets CS-1 and CS-2 defined by the ITU are discussed
and illustrated for SIP in Lennox, et al. [10]. AIN or advanced features in the
PSTN often take one of the following forms:
■■ Interactive voice response (IVR) system—These “voice menu” or “auto-
prompt” systems allow an automated attendant to answer a call, play
prompts, collect information using either spoken words or DTMF dig-
its, and then route the call to its final destination. This typically is
accomplished using a third-party control mechanism discussed later in
this chapter.
■■ Specialized routing services—Call routing is performed based on time of
day, origin number, traffic load, and other factors. This type of routing
decision is routinely made in proxy servers in a SIP network.
■■ Database query and information retrieval services—These services are
extremely primitive in the PSTN because of the separation of the PSTN
from the databases in which the information is stored. For a SIP-
enabled network, retrieval and return of information from the biggest
database of all, the Internet, is trivial using HTTP, FTP, and so on. Sim-
ple query services can be built using a SIP redirect server.Call Control Services and Third-Party Call Control
Call control services and third-party control are important topics in telephony,
because they enable many advanced services and features. For example, auto-
mated dialers and IVR systems can be built using SIP third-party control.
Problem Statement
Circuit-switched telephony services in the PSTN historically have been aug-
mented by advanced services in private voice networks by innovative PBX
vendors. Where even PBX technology failed to provide adequate solutions,
such as in PC-phone interaction and especially for call center applications, the
computer telephony industry (CTI) has come to the rescue. Advanced services,
however, were created at significant cost and had the drawbacks of: local reach
only because of lack of global standards, tremendous complexity that trans-
lated into very high cost of ownership, and long time-to-market. Finally, new
telephony applications and other communications have emerged with the
advent of the Internet.
SIP Telephony 199