What is IP telephony?
IP telephony, also known as Voice over IP(VoIP), is the technology of telephoning via the Internet, which is why it is sometimes referred to as Internet telephony. IP telephony has evolved from analog telephony via duplex lines to digital telephony via ISDN to become a new standard in the digital world. While the old telecommunications networks were line-based, connection setup, voice transmission and any encryption are transmitted via a data network. VoIP-capable telephones, smartphones or computers serve as end devices. According to the Federal Network Agency, there were over 37 million connections for VoIP telephony at the end of 2019.
How does VoIP work?
The challenge of VoIP telephony is that most users do not use a fixed IP address, but a dynamically generated one that is constantly changing. With VoIP, it is therefore not possible to dial a telephone number. An Internet protocol called SIP (Session Initiation Protocol) is used here, via which the end devices of the call participants register with a SIP server. This gives the devices their own SIP address in the form of "sip:98402918@beispiel.de", where the number combination is the user ID and the domain corresponds to the registrar server. With this SIP address, the user can log in from any location, provided there is an Internet connection.
When a call is made, the caller's device sends data including the caller's phone number to its SIP server, which sends this data to the caller's SIP server. The caller's phone then rings. The called party's SIP server then sends data back in the same way to the caller's end device, which receives a tone signal.
The call transmission takes place directly between the end devices; the SIP servers are no longer used for this data transmission. During voice transmission, the (analog) voice is converted into digital data packets via an AD converter (analog-to-digital) and transmitted via the Internet to the other end device, which converts the data into audible voice signals for the user via a DA converter (digital-to-analog).
To end the connection, the end devices send the corresponding information to their SIP servers.
What are the requirements for VoIP?
In order to use IP telephony, you need a sufficiently fast Internet connection due to the large number of data packets that are sent in both directions. You also need a suitable end device, i.e. an IP telephone, although a computer is sufficient. You also need a VoIP provider.
Further information:
https://www.placetel.de/ratgeber/ip-telefonie-voip-telefonie
https://de.wikipedia.org/wiki/IP-Telefonie
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