A list of call scenarios in use at present would include the following:
■■ Managing telephony applications from the desktop PC
■■ Click-to-connect
■■ Internet call waiting
■■ Instant communicationsMore complex call control models, however, are used at present in conven-
tional telephony systems. Call control, especially in private voice networks,
can be extremely complicated. Some examples are given here, with increasing
degrees of complexity:
■■ Pick up a call that was ringing someone else’s phone.
■■ Monitor a call in progress such as for call center operations.
■■ Join a conference call (whether scheduled or spontaneous).
■■ Transfer a call to another party.
■■ Receptionist and secretary model. A caller on the PSTN calls an
employee accessible via a private voice network. The call will first reach
the receptionist, who will inquire about the nature of the call and for-
ward it to the desired party. The called party may have a secretary who
may screen the call before connecting it to the boss.
■■ Call center applications. In a call center scenario, a customer call for
support to an 800 number may be routed to an enterprise call center by
a public carrier, depending on the call’s origin and time of day. The call
reaches the call center and, depending on the interaction of the caller
with an IVR, the call may be routed according to load and skill set to an
appropriate agent group. The agent taking the call may refer the caller
to a subject expert in another location and may stay online to make sure
that the call has been routed to the customer’s satisfaction. This com-
plex scenario is accomplished at present with quite expensive call soft-
ware on various carrier and private network switches, using a rather
high count of circuit switch ports both on the carrier side and within
the private voice network that owns the call center.The complexity in the preceding scenarios addresses real business require-
ments for customer, vendor, and partner relations and were addressed by
circuit-switched telephony in combination with CTI. Though Internet engi-
neers promote simplicity as an engineering design goal, it is felt SIP-based
solutions have to deal with such complexity as well.NOTEExamination of the preceding scenarios for call control shows that
protocols and standards from conventional telephony models—IN/AIN, PBXs,
and softswitch devices—control protocols cannot meet the requirements for
advanced voice services.200 Chapter 11