Cisco Voice over IP (VoIP) defines a way to carry voice calls over an IP network including the digitization and packetization of the voice streams. IP Telephony utilizes the VoIP standards to create a telephony system where higher level features such as advanced call routing, voice mail, contact centers, etc., can be utilized.

Session Initiation Protocol (SIP) is a peer-to-peer, multimedia signaling protocol developed in the IETF. SIP is ASCII-based, resembling HTTP, and reuses existing IP protocols (DNS, SDP, etc.) to provide media setup and teardown. Since its first publication in 1999, SIP has generated a high level of interest in the VoIP industry, and many people believe that SIP will become the de facto standard protocol for future voice networks.

IP Telephony is the technology for transmitting voice communications over a network using open-standards-based IP. Enabled by Cisco AVVID (Architecture for Voice, Video and Integrated Data), Cisco IP Telephony solutions use a single network infrastructure for the transmission of data, voice, and video traffic, delivering the business benefits of a converged network (increased productivity, greater business flexibility, and reduced operational costs) to enterprise-level organizations.
With Cisco IP Telephony, you have a comprehensive suite of IP-based hardware and software solutions to choose from that scale to meet your needs. In addition, this suite of solutions can integrate with existing systems to help you migrate to full IP Communications and protect your technology investments.
The Cisco IP Telephony solution has four primary components:

• Infrastructure
Includes public switched telephone network (PSTN) gateways, analog phone support, and digital signal processor (DSP) farms. Cisco IP Telephony infrastructure solutions can support multiple client types such as hardware phones, software phones, and video devices, as well as provide options for integrating traditional PBX, voice mail, and directory systems.

• IP Phones
Cisco IP phones combine the functions of a traditional telephone with an Ethernet connection and optional customizations such as access to stock quotes, employee extension numbers, and Web-based content. The Cisco IP SoftPhone is a Windows-based application for the PC that offers IP functionality in conjunction with a Cisco IP phone or as a standalone end station.

• Cisco CallManager
This software-based call processing agent extends enterprise telephony features and functions to packet telephony network devices such as IP phones, media processing devices, voice-over-IP (VoIP) gateways, and multimedia applications.

• Voice Applications
Voice applications are physically independent from the call processing and voice processing infrastructure, and they may reside anywhere within your network.