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Voice over IP

IP Telephony also called Internet telephony or IP telephony, is the technology that makes it possible to have a telephone conversation where the signal is carried over the Internet or a dedicated network in Internet Protocol (IP) packets, instead of over dedicated voice transmission lines. This allows to eliminate circuit switching and the associated waste of bandwidth. Instead, packet switching is used, where IP packets with voice data are sent over the network only when data needs to be sent, i.e. when a caller is talking.

Its advantages over traditional telephony include:

Note that voice over IP traffic does not necessarily have to travel over the public Internet: it may also be deployed on private IP networks.

The protocols used to carry the signal over the IP network are commonly referred to as Voice over IP, or VoIP protocols. VoIP protocols include the heavyweight H.323, which also provides videoconferencing and data capability, MGCP (Media Gateway Control Protocol), SGCP (Simple Gateway Control Protocol), and SIP (Session Initiation Protocol), a protocol to initiate VoIP connections.

Table of contents
1 Corporate and telco use of VoIP
2 VoIP implementation challenges
3 VoIP protocols
4 Mass-market telephony over broadband Internet access
5 See also

Corporate and telco use of VoIP

Although few office environments and even fewer homes use a pure VoIP infrastructure, telecommunications providers routinely use IP telephony, often over a dedicated IP network, to connect between their switching stations, where they convert the dedicated voice signal to IP packets and back. The result is a data-abstracted digital network which the provider can easily upgrade and use for multiple purposes.

Corporate customer support centers which provide support over telephone often use IP telephony exclusively to take advantage of the data abstraction that comes with it.

The benefit of using this technology is the need for only one class of circuit connection and better use of the available bandwidth. IP telephony is commonly used to route traffic that may be originated from and terminated at conventional PSTN telephones.

VoIP is now widely deployed by carriers, especially for international telephone calls. Most commonly, users are completely unaware that their telephone call is being routed over IP infrastructure for most of its distance, instead of entirely over the circuit switched PSTN.

VoIP is also used by large companies to eliminate call charges between their offices, by using their data network to carry inter-office calls. They may also use VoIP to reduce the costs of calls outside the company, by carrying them to the nearest point on their network before handing them off to the PSTN.

There are companies which offer a gateway to the PSTN from any VoIP phone. You can simply dial a conventional telephone number and the telephone call will be routed over your internet connection to the company that operates the gateway, and they will bill you, not the local phone company. Enum makes it possible to dial traditional E.164 phone numbers, but be connected entirely over the internet if the other party uses Enum, so you do not incur any expenses other than the internet connection fees.

VoIP implementation challenges

Because IP does not by default provide any mechanism to ensure that data packets are delivered in sequential order, or provide any Quality of Service guarantees, implementations of VoIP face problems dealing with latency and possible data integrity problems.

One of the central challenges for VoIP implementers is restructuring streams of received IP packets, which can come in any order and have packets missing, to ensure that the ensuing audio stream maintains a proper time consistency. Another important challenge is keeping packet latency down to acceptable levels, so that users do not experience significant lag time between when they speak and the signal is decoded on the other end of the connection.

Solutions to these problems:

VoIP protocols

In the overwhelming majority of implementations, the RTP protocol is used to transmit VoIP traffic ("media").

For signaling, there are several alternative protocols:

Mass-market telephony over broadband Internet access

A new development has been the introduction of maas-market VoIP services over broadband Internet access services, in which subscribers make and receive calls as they would over the PSTN. This requires an analog telephone adapter (ATA) to connect a telephone to the broadband internet connection. A company in the US, called Vonage, uses IP to offer unlimited calling to the US and Canada for a flat monthly fee. One advantage of this is the ability to make and receive calls as you would at home, anywhere in the world, at no extra cost. As calls go via IP, this does not incur charges as call diversion does via the PSTN, and the called party does not have to pay for the call.

For example, somebody may call you on a number with a US area code, but you could be in London, and if you were to call another number with that area code, it would be treated as a local call, regardless of where you are in the world. However, the broadband phone is likely to complement, rather than replace a PSTN line, as it still needs a power supply, while calling the US emergency services number 911, may not automatically be routed to the nearest local emergency dispatch center, or be of any use for subscribers outside the US.

See also