The term VoI Prefers to the family of protocols that enable voice communication over IP based computer networks. VoIP systems use session control protocols for established the calls and audio codecs for encoding and decoding the audio data.
Nowadays, most mobile devices have access to IP-based data services (GPRS, 3G, WiFi). Thus, VoIP solutions can also be implemented on mobile devices. VoIP on mobile devices is seen as an alternative to the voice services offers by the mobile providers because it represents a more cost effective way of voice communication (especially for long distance calls).
Most VoIP solutions are implemented using a centralized SIP  server that handles both the session control and the user registration. This solution can have scalability problems when the number of users increases. Also, the centralized SIP server represents a single point of failure. In order to solve this issues, the centralized location service can be replaced by a P2P protocol. The clients will use a Distributed Hash Table (DHT) for registering and looking for communication partners
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