Figure 2.6 Desktop SIP phones: (a) Business SIP phone; (b) Consumer SIP videophone
(a) Courtesy: snom Technology; AG (b) Courtesy Grandstream Networks, Inc.
What SIP Does Not Do
The preceding list of communications services that can be provided by SIP
should not leave the impression that SIP is a “miracle protocol” that can solve
all communications problems [33].
As will be discussed in Chapter 6, “SIP Overview,” SIP is a very powerful,
yet simple and general protocol for establishing interactive communication
sessions across the Internet. SIP is a protocol for initiating, modifying, and ter-
minating interactive sessions. This process involves the discovery of a user,
wherever he or she may be located, so that a description of the session can be
delivered to the user. There are quite a number of features and services that SIP
was notdesigned to support, such as the following:
■■ SIP is not meant to replace all known telephony features and services
from circuit-switched networks with identical services. There are many
telephony services that have their rationale because of the limitations of
circuit-switched technology and in legacy telecommunications regula-
tion, rather than in objective needs for communication. The majority of
the countless Class 5 telephone switch features make no sense on the
Internet. Local telephone number portability is another example of a
service that makes no sense on the Internet. While SIP can support localabInternet Communications Enabled by SIP 33