Enhanced Telephony Services
One remarkable promise of SIP-enhanced telephony services is that they can
be implemented across the open Internet environment, working effectively
across service provider boundaries and between equipment and software
from many vendors. It remains to be seen to what extent this promise of
extending PBX-like rich call features across the Internet will be fulfilled.
Enhanced services in telephony come in three possible forms:
■■ PBX or Centrex features
■■ Custom Local Area Signaling Services (CLASS) features
■■ Advanced Intelligent Network (AIN) servicesFeatures have very specific names and definitions in the PSTN and PBX
world. However, to discuss their analog in SIP, we will use generic names,
which may or may not exactly map to the PSTN or PBX features. Although the
IETF does not standardize features or services, many of these services imple-
mented using SIP are described in Johnston [7].
Standardizing the key PBX functions across the Internet may herald a sig-
nificant disruption in the PBX market, where all products are vendor-propri-
etary and interoperability (as with the ITU QSIG standard) is difficult to
achieve. In addition, PBX phones from different vendors are not interchange-
able. IP PBXs based on SIP have the potential of basic standards-based inter-
operability and also the potential of interchangeable SIP phones for baseline
PBX features. We will take a closer look at baseline PBX features.
PBX or CLASS features generally include the following:
■■ Call transfer—There are three types of call transfer services (blind, unat-
tended, and attended) that can be implemented using the REFER
method [8]. In a blind transfer, the transferor sends a REFERand then
immediately sends a BYEand terminates the existing session without
waiting for the outcome of the transfer. In it is an unattended transfer,
the transferor may keep the transferee on hold pending the outcome of
the REFERrequest. Once the transferor receives notification that the
transfer has succeeded, a BYEis sent to tear down the existing session.
Finally, the attended transfer involves a temporary conference call
between the three parties, in which the transferor knows the exact
progress of the transfer. Once the transfer is complete, the transferor can
then drop out of the call. The types of call transfer are described in
Table 11.3 and in [9].
■■ Call waiting—This is a service implemented on single-line telephones.
Since there is no such thing as a “line” in a SIP network, this feature
does not have an exact analog. However, a SIP phone that offers multiple196 Chapter 11