Voice over IP (VoIP)

Definition of Voice over IP (VoIP) in The Network Encyclopedia.

What is Voice over IP (VoIP)?

Voice Over IP, or VoIP, is an umbrella term for a set of technologies that allow voice traffic to be carried over Internet Protocol (IP) internetworks such as the Internet. The term “Voice over IP” was coined by the VoIP Forum, a consortium of companies dedicated to the development and extension of IP telephony technologies. Voice over IP (VoIP) is the prime example of “convergence” in the networking and telecommunications industry because it enables telephony and computer networking traffic to be combined into a single data stream of IP packets.

Various vendors have offered proprietary VoIP solutions, but most have recently moved toward standards-based solutions that implement the H.323 and T.120 communication protocols developed by the International Telecommunication Union (ITU). These include voice-encoding industry standards such as G.723 and G.729. One goal of these standards is to enable integration between IP telephony and global cellular telephony standards such as the Global System for Mobile Communications (GSM). However, the move toward standards has been slow, and only a few vendors offer end-to-end carrier-class VoIP services with acceptable levels of communication latency.

Vendors tout the following advantages of using VoIP technologies:

  • Lower long-distance costs:
    VoIP enables users to avoid long-distance charges by placing local calls to their Internet service provider (ISP) and having the encapsulated voice traffic move free of charge over the Internet. The problem with this approach is that transmission over the Internet is unpredictable and often has unacceptable levels of latency. One solution is for vendors to construct their own private IP internetworks for VoIP services, but this is an expensive prospect even for the largest carriers and has contributed to the slow progress in voice/data convergence. Some companies offer their own VoIP services to employees using the internal company network, but this approach is still costly and requires expertise to maintain.


  • New solutions for remote access by mobile users:
    VoIP solutions allow users to communicate with remote stations using both voice and data integrated into a single IP data stream. For example, a user can browse a Web site and talk to customer support at the site simultaneously by using a single modem connection.