"VOIP", "IP Telephony", "Internet Telephony" are used interchangeably. It is also defined as the ability to transfer voice, fax and video over IP.
1995 - VocalTec has first introduced IP Telephony software product "Internet Phone" in 1995. This year is called the year of the Hobbyist.
1996 The year of Gateway.
1998 The year of Gatekeeper
1999 The year of the application
Gateways: These are used to adapt traditional telephony to the Intenet.
Gatekeepers: Responsible for address resolution and call routing.
Codec's: Coders are used for efficient bandwidth utilization. The coder-decoder compression schemes (CODEC's) are enabled for both ends of connection and the conservations proceeds using Real-time Transport Protocol (RTP)/ User Datagram Protocol (UDP)/Internet Protocol (IP) as the protocol stack.
In VOIP systems, analog voice signals are digitized and transmitted as a stream of data packets over a digital data network (called packet switched networks).
In public switched telephone networks or PSTN (called Circuit-switched networks), a telephone call is reserved an end-to-end physical circuit between the origin and the destination for the duration of the call. Hence, for this duration of the call, the circuit is fully available to that call and is not available to any other network users. Whereas in packet switched networks, instead of reserving a circuit between endpoints, messages or files are broken up into many small packets and each packet might be taking different route from origin to destination. Thus circuit switched networks have poor utilization of network resources whereas this is eliminated in packet switched networks.
Add-on services and unified messaging
Merging of Voice and data
Quality of Service (QoS)
Quality of Voice (QoV)
Standards and Interoperability