| The transmission of voice over packet switched networks was an active research area in the late 70' s and the early 80' s. Much of the work then focused on using packet switching for both voice and data under ARPANET. After entering the 1990s, especially in recent years, along with the development of Internet and speech coding, it obtained breakthrough progress and practical application. IP network exists everywhere at present, and IP technology is ripe day by day. However, the application of IP telephone is popularized gradually too; Inside the enterprise, the demand of IP Telephony of the LAN replacing PBX system increases day by day. The multimedia communication systems on the basis of packet switched network receive the concerns of researchers and computer manufacturers more than anytime in the past.VoIP (Voice over IP) is a technology that sends packet voice data over IP networks. IP telephone generally refers to the system that adopts VoIP technology carrying on speech. Utilizing the existing network conditions along with IP phone technology, the purpose of this thesis is not only to develop an IP telephony software under Windows' platform, but also to carry on some helpful discussions and studies of the key technology and IP telephone system of LAN.This thesis first introduces the basic principles of IP telephony and its key technology, then expatiates on the design and development of PC-to - PC's IP telephone software, E- Hello, which was developed under Windows platform based on the LAN environment. This thesis also analyzes the factors that affect the voice delay of IP telephony and has realized the technology of silence suppression. E-Hello realizes such functions as speech record, speech play, etc. As the result of experiment, E-Hello has been proved have pretty good voice effect. However, this thesis also discusses how to use the technology of IP to replace the PBX system and analyzes the benefit of LAN IP phone system relative to the PBX system.
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