Being an official standard recommendation from the
ITU-T, the H.323 protocol
suite includes many standars and technologies coming from the telecommunication industry.
Compared to SIP, H.323 is more seen in the "professional" videoconferencing world.
Ranging from software-only solutions (like XMeeting) to expensive conference-room,
installations, there exist many different solutions using H.323 on the market.
Free solutions for other platforms than Mac OS X are Microsoft NetMeeting (Windows), Ekiga (Linux) or PacPhone (Windows). Companies like Polycom or Tandberg offer various non-free H.323 solutions.
Multipoint conferences, thus conferences with more than one remote party can be established by using a so-called Multipoint Conferencing Unit (MCU). Again, there exist high-speed MCU servers for the professional market as well as software-only solutions like OpenMCU (LINK)
A Gatekeeper provides the directory-server functionality (mentioned on the previous page) for H.323.
The SIP is published by the IETF and is available for free as an RFC. The design of SIP closely follows the design of other popular Internet Standards (e.g. HTTP). SIP can be used to establish any sort of session between users, not necessarily limited to audio- and videoconferences. Compared to H.323, SIP is more found in the user consumer market and is widely used to offer VoIP solutions.
A Registrar Server or short "Registrar", together with a SIP proxy, provide the directory-server functionality (mentioned in the previous page) for SIP.