PDF of this tutorialTelephone companies offload voice calls from public switched telephone networks (PSTN) to voice-over–Internet Protocol (VoIP) networks because it is cheaper to carry voice traffic over Internet protocol (IP) networks than over switched-circuit networks. In the future, IP telephone networks are expected to enable innovative new multimedia services while working seamlessly with legacy telephone networks.
A VoIP network carries voice traffic cheaper than a switched-circuit telephone network because IP–telephony networks make better use of available bandwidth. In a PSTN, for example, a dedicated 64 kilobits per second (kbps) end-to-end circuit is allocated for each call. In a VoIP network, digitized voice data is highly compressed and carried in packets over IP networks. Using the same bandwidth, a VoIP network can carry many times the number of voice calls as a switched circuit network with better voice quality. The savings realized in using VoIP networks are often passed onto users in the form of lower costs.
In 2001, U.S. telephone companies are expected to offload between 15 and 20 percent of overseas voice calls to VoIP network operators such as iBasis, and the percentage is rising each year as VoIP network infrastructure is rolled out. Other countries, such as China, carry an even higher percentage of voice traffic over domestic and international VoIP networks.
In addition to voice data, signaling data is exchanged between switched-circuit telephone networks and VoIP networks. Signaling information is used to setup, manage, and release voice calls, and support telephony services such as caller ID, toll-free calling, and mobile roaming and authentication.
The remainder of this tutorial introduces the topic of interworking the PSTN and next-generation VoIP networks to support voice calls and telephony services.
