Traditionally, telephone calls are carried through Public Switched Telephone Networks (PSTN), which provides high-quality voice transmission between two or more parties. On the other hand, data such as email, web browsing etc. are carried over packet-based data networks like IP, ATM and Frame Relay. In the last few years, there has been a rapid shift towards using data networks to carry both the telephone calls and the data. This so called convergence of voice and data networks is very appealing due to many considerations.

VoIP systems digitize and transmit analog voice signals as a stream of packets over a digital data network. VoIP technology insures proper reconstruction of voice signals, compensating for echoes due to the end-to-end delay, for jitter and for dropped packets and for signaling required for making telephone calls.

The IP network used to support IP telephony can be a standard LAN, a network of leased facilities or the Internet.

VoIP calls can be made or received using standard analog, digital and IP phones. VoIP gateways serve as a bridge between the PSTN and the IP network. A call can be placed over the local PSTN network to the nearest gateway server, which moves it onto the Internet for transport to a gateway at the receiving end. With the use of VoIP gateways, computer-to-telephone calls, telephone-to-computer calls and telephone-to-telephone calls can be made with ease.

Access to a local VoIP gateway for originating calls can also be supported in a variety of ways. For example, a corporate PBX (Private Branch Exchange) can be configured so that all international direct dialed calls are transparently routed to the nearest gateway. High-cost calls are automatically supported by VoIP to obtain the lowest cost.

To ensure interoperability between different VoIP manufacturers, VoIP equipment must follow agreed upon procedures for setting up and controlling the telephone calls.

H.323 is one such family of standards that define various options for voice (and video) compression and call control for VoIP.

Other call setup and control protocols being utilized, and or being standardized include SIP, MGCP, and Megaco.

IP Telephony goes beyond VoIP transport and defines several value added business and consumer applications for converged voice and data networks. Examples include Unified Messaging, Internet Call Center, Presence Management, Location Based Services etc.

For more views on the same subject, check out the above qualified links for more introductory information on VoIP.