ABSTRACT -- IP Telephony is bringing new types of application such as IP-PBX, IP-Based Call Center, Voice Mail, and Unified Messaging. To make voice communication on the packet-switched network, we need some kind of signaling protocols comparable to SS7 in circuit-switched network like PSTN. Two signaling protocol standards, H.323 and Session Initiation Protocol (SIP), are being developed to serve this purpose. These two standards are competing to dominate IP Telephony market. H.323, as developed earlier by ITU-T, is currently dominating the market. SIP, which is developed by Internet Engineering Task Force (IETF), is expected to replace H.323 in the future. In this paper, we give the overview of H.323 and SIP and later discuss the strange and the weakness of both standards.

ABSTRACT -- H.323, SIP, IP Telephony, Call Center, VOIP, Signaling, Call Control, CTI