Call Setup Protocols

Call setup protocols use TCP and UDP to encapsulate the call setup and takedown phases of
a telephone call. They handle functions like the mapping of phone numbers to IP addresses,
generating dial tones and busy signals, ringing the callee, and hanging up. There are two
families of call setup protocols: one set from the telephony community and the other from
the data networking community.
We get the call setup protocols H.323 and MGCP (Media Gateway Control Protocol) from
the telephony community by way of the ITU. H.323 is the most widely deployed call setup
protocol in use today; a report from Insight Research in January 2001 indicated that 89% of
VoIP calls were using H.323. H.323 is actually a family of telephony-based standards for
multimedia, including voice and videoconferencing. MGCP is the less flexible version, for
use with inexpensive devices like home telephones.
The family of H.323 protocols has been refined for many years, and as a result, it is robust
and flexible. But the cost of this robustness is that it has high overhead: a calling session
includes lots of handshakes and data exchanged for each function performed.
SIP (Session Initiation Protocol) and Megaco (another acronym for Media Gateway Control
Protocol) are lightweight protocols developed by the IETF in the data-networking
community. SIP in particular represents typical data-networking logic, which asks, Why use
a heavyweight protocol (such as H.323) when a lightweight protocol (such as SIP) will get
the job done most of the time? SIP is the current “industry darling”—it is supported by
Cisco and Nortel, and Microsoft has recently started shipping SIP client interfaces with its
Windows XP operating system.
Although the H.323 family of call setup protocols is predominantly used today, the Insight
Research report cited above predicts that the four protocols discussed here, H.323, MGCP,
Megaco, and SIP, will each be used in roughly equal proportions within the next few years