A telephone line is an infrastructure access point through which a landline telephone can operate. If the telephone is connected directly to an external telephone line and the network of the respective provider, an internal telephone line extension via a PBX (telephone system) is connected as an intermediary. Internal extension telephones can call each other locally without using the provider's network. For identification and to establish a connection every extension telephone gets a unique internal or external phone number. The technology and the transmission protocol used may vary depending on the respective telephone connection. There are analogue, digital ISDN and Voice-over-IP (VoIP) connections. The network termination to the provider is in the form of the connection socket or telephone socket. In addition to telephones for voice communication, and independent of the technology of any particular terminal, fax machines and other telecommunication equipment can operate via a telephone line.
The analogue telephone connection
The basic principle of an analogue telephone line has hardly changed over the last one hundred years. Voice transmission takes place via an analogue system in a defined frequency range. For signalling and call set-up a special pulse or multi-frequency signals are used. Devices such as fax machines or modems also use sound for transmission of data, using a defined frequency and sequence.
The digital ISDN telephone line
An ISDN telephone connection is a digital landline. The network termination occurs via the so-called NTBA. A basic ISDN connection consists of two B-channels and one D-channel. It can run two conversations at the same time, with a data rate of 64 kilobits per second. The signalling data is transmitted via the D-channel. For direct connection to an ISDN bus you need an ISDN telephone. Analogue phones can be operated via a digital/analogue converter or through intermediary telephone systems at the ISDN connection.
The Voice-over-IP (VoIP) telephone line
More and more network operators are switching their telephone networks to operate via Voice over IP telephony. VoIP transports the voice data in the IP data stream. The telephone connection is provided via an internet connection. A separate analogue or ISDN connection is no longer needed. A single Internet connection is all that is needed for voice and data. In addition to regular telephones that are connected via a router or adapter, telephone functions can be performed by a PC Softphone. Furthermore, the telephone system can be operated as a cloud solution. This makes having your own physical telephone systems unnecessary and ensures high flexibility.