[Asterisk-Users] Need an explanation about different protocols

[Larry Keyes]

>>Is there someody who can explain me the meaning of these sentence.
>>"Sip is philosophically horizontal and H.323/MGCP are vertical"


Hi, Ignace, 

I think this refers to the fact that SIP is "lightweight", relative to
H.323. My favorite reason, though, is that you can easily "see" what is
going on with SIP, when doing debugging and tracing, (in Ethereal, say,)
whereas H.323 seems to me to a lot more work. Also, the RFC's related to SIP
are a model of clear explanation...there can be understood by mere mortals.
Since the H.323 stuff is IETF, you have to even go through hoops just to get
to the standards documents. 


Although H.323 and SIP provide many of the same services, they evolved from
different protocol families. The roots of H.323 are in the circuit-switched
world, while SIP evolved from newer standards such as HTTP and SMTP. In
their comparison of the two protocols, Schulzrinne and Rosenberg (link
below)	who were instrumental in developing SIP highlight several
differences:   	
 
.	H.323 is considerably more complex than SIP. The specifications for
H.323 run to 736 pages for the base definition. The base SIP specification
is 128 pages. Part of the reason for the complexity is that H.323 remains
backward compatible with early versions of the specification, while SIP is
designed to jettison older or obsolete features when new versions of the SIP
specification are issued. 

.	SIP has specific support for certain call control features common in
business telephone systems, including call transfer, hold, and park. 

.	SIP is extensible. In particular it allows for the addition of media
codecs via SDP. Codecs are limited in H.323 to ITU approved codecs, of which
several are proprietary and require licensing.  

Despite these limitations, H.323 and its companion protocols are still very
much in current use with videoconferencing systems and IP telephony
equipment. H.323 support is considered important enough to be included in
both Asterisk and VOCAL, and H.323 is the only protocol offered in some
newer devices such as the D-Link DVC-1000 home videoconferencing  units.   

Reference: 
Shulzrinne, H. and Rosenberg, J. (1998)  A Comparison of SIP and H.323 for
Internet Telephony  available:
http://www.cs.columbia.edu/~hgs/papers/Schu9807_Comparison.pdf

Rosenberg, J, Schulzrinne H., et. al. (2002) SIP: Session Initiation
Protocol RFC 3261 Standards Track Available:
http://www.faqs.org/rfcs/rfc3261.html <-- the original SIP RFC. 

-- Larry