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IP telephony concepts

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2, Windows Server 2008, Windows Server 2008 R2, Windows Server 8 Beta

IP telephony concepts

IP telephony refers to telephone communications over TCP/IP networks. In contrast to the PSTN, which consists of analog and digital signals over a circuit-switched network, IP telephony is packet switched. All information to be transmitted over the network is separated into data packets. Each packet has a header containing its source and destination, a sequence number, a block of data content, and an error-checking code. Routers and servers direct these packets over the network until they arrive at their destination. When the packets arrive, the sequence number is used to reassemble the packets in their original order. Unlike PSTN telephony, which dedicates a circuit for a telephone call, the data packets share a circuit with other transmissions.

How IP telephony works

IP telephony blends voice, video, and data by using TCP/IP as a common transport, effectively collapsing these separate channels into one. Clients using IP telephony employ either existing multimedia hardware or a phone plugged into a PSTN adapter.

IP telephony supports voice communication, voice and video mail, and video on demand. IP telephony also enables voice and video conferencing over the Internet and existing IP-based LANs and WANs.

Benefits of IP telephony

IP telephony provides the following benefits:

  • IP telephony moves multimedia traffic over any network that uses IP. This offers users both flexibility in physical media (PSTN lines, xDSL, ISDN, leased lines, coaxial cable, satellite, and twisted pair) and flexibility of physical location.

  • IP telephony lowers costs of existing services, such as voice and broadcast video, and broadens means of communication to include audio and video conferencing.

TAPI support for IP telephony

Two telephony service providers provide IP telephony support in the Windows Server 2003 family: the H.323 TAPI Service Provider and the Multicast Conference TAPI Service Provider. These service providers support the H.323 and IP multicast video conferencing standards and enable different IP telephony programs to interoperate with each other. Organizations can now take full advantage of the global reach of the Internet or the immediacy of a corporate intranet for real-time communication.

Quality of Service for IP telephony

Packet-switched networks such as the Internet do not dedicate a connection between parties, so they cannot guarantee Quality of Service (QoS) as the PSTN does. Historically, they have been used for programs, such as e-mail and file transfer, where variable QoS is acceptable.

Organizations can now design private networks to provide adequate bandwidth and redundant paths between switching points for high quality IP telephony. New software and hardware have improved QoS, and voice over IP on a WAN can be as good as standard PSTN calls, even if the network is congested. For more information about QoS, see QoS overview.