H.323 is a series of protocols that provide audio-visual communication sessions on any packet network. It is currently implemented by various Internet real-time applications such as NetMeeting.
SIP is the main challenger. It is still early days, but if SIP development continues apace, it is likely that there will be very little new H.323 development. For example, Microsoft's LCS and IBM's Lotus Workplace offering are SIP only. Similarly, while telecomms manufacturers will continue to sell H.323 based controllers (for example, Avaya's Communications Manager), all new products are currently SIP based (for example, Avaya's Converged Communications Server).
Author: Nick Shelness