Voice over IP

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Voice over IP

(VoIP) Any technology providing voice telephony services over IP, including CODECs, streaming protocols and session control. The major advantage of VoIP is lower cost, by avoiding dedicated voice circuits.

Currently VoIP is being deployed on internal corporate networks, and, via the Internet, for low cost (and low quality) international calls. It is also used for telephony applications such as voice and fax mail.

The ITU standard is H.323, which is a whole suite of protocols, while the IETF has developed the much simpler SIP to solve the session control problem and MGCP/Megaco to solve the gateway problem.
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IP on Everything

Vinton G. Cerf, VP and Chief Internet Evangelist at Google, has been called the "Father of the Internet." In 1992, his "IP on Everything" T-shirt caused quite a stir at an IETF conference, where he was the keynote speaker. He was right. Meant as a tongue-in-cheek forecast of the future, the IP protocol, which is the foundation of the TCP/IP protocol suite, has indeed become the world standard.

Almost Everything Runs on IP
The Internet, underscored by IP, has not only affected every business and IT department, but it changed global communications. In addition, IP is not only the protocol of the Internet; it became the default protocol for local networks in every company, as well as the home. See IP telephony, VoIP, IP Multimedia Subsystem, IP network, Internet, TCP/IP and dumb network.

Tongue-in-cheek, But Right On
"Vint" Ceft was quite the visionary as IP is indeed on everything today. More accurately, "everything is on IP," but then the T-shirt wouldn't have been fun. (Image courtesy of Boardwatch Magazine.)

IP Everywhere
Including an Apple TV set-top box, which streams content from the Internet and home network to the TV set. IP is running just about everywhere.

Even Voice
Years ago, the first data transmissions rode over voice networks. Today, a huge amount of voice traffic rides over data networks, namely IP networks. Voice evolved from FDM-based analog circuits to TDM-based digital circuits to IP packet networks. See VoIP.

IP telephony

The two-way transmission of voice over a packet-switched IP network, which is part of the TCP/IP protocol suite. The terms "IP telephony" and "voice over IP" (VoIP) are synonymous. However, the term VoIP is widely used for the actual services offered (see VoIP for details), while IP telephony often refers to the technology behind it. In addition, IP telephony is an umbrella term for all real-time applications over IP, including voice over instant messaging (IM) and videoconferencing. See TCP/IP.

Starting in the late 1990s, the Internet and its TCP/IP protocol suite began to turn the data communications and telephone industry upside down. IP became the universal transport for data and video communications worldwide, and it is increasingly becoming the infrastructure for voice traffic as well. Today, every communications carrier has built or is using an IP backbone for some or all of its voice services. In addition, large enterprises are either already using IP for some amount of internal voice traffic or have plans to do so.

Data Over Voice Became Voice Over Data
Starting in the 1960s, data was transmitted over analog telephone networks, and by the late 1980s, data routinely traveled over digital voice circuits. By the 1990s, the majority of worldwide communications traffic had changed from voice to data, and as IP networks began to flourish, the economics of using IP for voice began to emerge.

Although the backbone of the global telephone network had been converted to digital for some time, the circuit-switched nature of the public switched telephone network (PSTN) is wasteful. Even though one person talks and the other listens, both "to" and "from" channels are always dedicated. In addition, newer voice codecs cut the digital requirement from the traditional 64 Kbps (PCM) down to 8 Kbps with respectable quality. Thus, the bandwidth requirement for voice on an IP network is 1/16th that of the PSTN's dedicated, digital circuits.

Varying Quality
Starting in the mid-1990s, advertiser-supported, free telephone service from PC to PC or between phones and PCs using the Internet became popular, especially for international calls. Call quality over the Internet can be erratic because the Internet provides no guarantee of quality of service (QoS). However, when an organization has control over its network, quality can be excellent. Private enterprises with their own IP networks, as well as major telcos and IP telephony carriers that have developed IP backbones, can provide voice quality that competes with the traditional PSTN.

Transport and Signaling
IP telephony uses two protocols: one for transport and another for signaling. Transport is provided by UDP over IP for voice packets and either UDP or TCP over IP for signals. Signaling commands that establish and terminate the call as well as provide special features such as call forwarding, call waiting and conference calling are defined in a signaling protocol such as SIP, H.323, MGCP or MEGACO (see IP telephony signaling protocol).

The integration of packet-switched IP with the traditional SS7-based telephone system was a complex undertaking with numerous protocols competing for attention. See ITXC and IP on Everything.

Integrating IP Telephony with the PSTN
This shows the interaction between the traditional telco system and IP carriers, which are often one and the same. Note the difference between voice packets (blue lines) and signaling (red lines). (Illustration assistance courtesy of GNP Computers and Pulver.com.)

Not Quite IP Telephony
We can be certain that they didn't have the IP protocol in mind when they set up this telephone switchboard in 1882. It was used to switch phone calls between all the lawyers in Richmond, Virginia. (Image courtesy of AT&T.)


(Voice Over IP) A digital telephone service that uses the Internet for transport, as well as private IP networks. "IP" stands for "Internet Protocol." In order that calls can originate and terminate from regular telephones, connections to the public telephone network (PSTN) are also provided. Telephone companies, cable companies and dedicated providers offer VoIP calling for a fixed monthly fee or low per-minute charge. Customers must have Internet access.

Telephony Protocols: SIP/H.323 and Skype
VoIP uses two telephony protocols for handling connections (see SIP and H.323), and most VoIP systems support both. Skype uses its own protocol (see Skype).

Handset Based
All IP handsets plug directly into the Internet, and a large variety of native IP desk and cordless phones are available. Traditional analog phones can also be used by plugging them into an analog telephone adapter (ATA), which converts the analog signals to IP and vice versa (see analog telephone adapter).

Software Only = Softphone
VoIP may be entirely software based, which uses an app in a mobile device or a computer equipped with microphone and speakers. Typically free if both sides are on devices; calls to a regular telephone are per minute. In 1995, VocalTec Communications introduced the first VoIP service in the U.S.; entirely software based (see softphone). Skype is a very popular softphone-based VoIP service (see Skype). See SIP provider.

VoIP Features
Along with other features, voicemail, caller ID, call forwarding and a softphone option are typically part of a VoIP package. Phone numbers with area codes outside of one's own home area may also be an option (see virtual phone number). See IP telephony for more details and history of the technology.

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