153Any advanced network must be flexible enough to take into consideration the
preferences and desires of users. In this chapter, we will show how SIP can use
the preferences of both the caller and the called party in call routing, features,
and services.
Introduction
Telephony services based on the intelligent network architecture for public
networks and private circuit-switched networks (PBXs) give the users, in gen-
eral, little or no control over the preferences of how calls should be handled.
Whatever call features are possible can only be subscribed to, but cannot be
exercised as individual preferences on a call-by-call basis. There are many
reasons, one of them being the frugal user interface of user devices (called
terminals in ITU standards language) and the general concept of user devices
not being the location for intelligence. Another reason is scalability. It is more
difficult to store a page full of user preferences for millions of users in central
servers of the Intelligent Network (IN) in the PSTN, and also have the data
changed by users on a dynamic basis, as compared to having such data and
access to it handled at the periphery of the network.