Voice over IP (VoIP) stands for voice communication in computer networks based on the Internet protocol (IP). The core idea is to provide the functions of traditional telephony via cost-effective and wide-spread networking structures such as the Internet. VoIP itself is not a standard, rather it is a collective term for the various technologies (equipment, protocols, voice encoding, etc.) which make voice communications in IP networks possible.
A variety of terminology is used to describe telephony over a network (LAN or Internet). The terms "Voice over IP" or "IP telephony" are used as synonyms, although in actual fact they have different meanings.
- Strictly speaking, "Voice over IP" is merely a term for the technology of transmitting calls across data networks in real-time using the IP protocol (Internet protocol). The term is also used when the technology is implemented only in the provider's core networks, in what is known as the backbone
- The term "IP telephony" is used when the VoIP technology is also used in the terminal equipment, so that the call subscriber uses the IP network for telephony.
- "Internet telephony" is also used to describe telephony using VoIP over the Internet in general.
In the following, "Voice over IP" is usually used even to refer to IP telephony in accordance with general custom.
There are four basic types of terminal equipment that can be used for VoIP telephony:
- With software running on the PC, known as a "softphone".
- With an IP or VoIP telephone that is connected directly to the local network.
- With a conventional telephone that is connected to the local network by an adapter (analog telephone adapter, ATA).
- Via a VoIP gateway that converts telephone calls from telephones (analog and ISDN) to VoIP and can then communicate between the two "telephone worlds" like a PBX.
There is a basic difference between a VoIP connection being established between two pieces of terminal equipment that are connected directly to the data network (PC or IP telephone) and the situation where a subscriber in the land-line or mobile telephone network requires the translation of the signaling, numbers and voice data. To differentiate the various connection variants, a device in the LAN has become known as a "PC", and a device in the land-line network has become known as a "phone".
- PC-to-PC communication
- With this application, the terminal equipment has to be integrated directly into the user's LAN. Examples are a PC, an IP telephone or a telephone that is connected to the LAN using an ATA. Different software solutions are available for the PC, known as "softphones". Note that some of these programs can only communicate with users of the same software and not with softphones from other manufacturers. Communication is usually free of charge within the Internet. A current example is Skype, which uses its own protocol.
- PC-to-phone and phone-to-PC communication
- In this case, the call data has to be transmitted from the Internet to the landline network, usually using what are known as VoIP gateways. In general, these gateways are provided by providers and are subject to a fee. VoIP routers offer another option that can switch VoIP calls to an ISDN line. Examples are different LANCOM VoIP router types with a SIP gateway and ISDN interfaces. When the calls are transferred to the landline network, the usual telephone operator fees are charged. So that the subscriber can even be called on a PC, he or she needs a VoIP telephone number that is usually provided by a provider. VoIP providers usually only provide individual numbers and not complete number ranges with a switchboard number and extension numbers. This is why the numbers that are provided by public providers are not attractive to many business customers. When the LANCOM VoIP router is used with a SIP gateway, previously-used numbers can be maintained; the functions of VoIP telephony can also be used.
