Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Peer-to-Peer Third-Party Call Control

SIP can be used for complex call control applications in the peer-to-peer con-
trol model. We will discuss an example where a dialer application on a desk-
top computer of a secretary can control his or her own phone, and also can be
used to set up calls between two other phones (such as between the boss and a
customer, as shown in Figure 11.6). In this application, SIP for presence is used
to display on the computer the state of the secretary’s own phone. For sim-
plicity, it is assumed that the presence publisher for the phone is located in the
phone itself. This is quite doable with intelligent SIP phones and does not
require more complex message exchanges with a dedicated presence server.
The framework for peer-to-peer call control with SIP is detailed in [12].

Figure 11.6 Example of peer-to-peer third-party call control

Secretary PC Secretary Phone Boss Customer

REFER/200 OK

NOTIFY/200 OK

INVITE/200/ACK

INVITE/200/ACK

INVITE/200/ACK

INVITE/200/ACK

INVITE/200/ACK

RTP audio

BYE/200 OK

BYE/200 OK

REFER/200 OK

NOTIFY/200 OK

REFER/200 OK

NOTIFY/200 OK

REFER/200 OK

NOTIFY/200 OK

REFER/200 OK

NOTIFY/200 OK

REFER/200 OK

NOTIFY/200 OK

REFER/200 OK

NOTIFY/200 OK

1. Setup call to Boss


2. Put Boss on hold


3. Setup call to
   Customer


4. Put Customer
   on hold


5. Setup Boss to call
   Customer


6. Send BYE to Boss


7. Send BYE to Customer


8. Boss and
   Customer talk

SIP Telephony 205