Primary Rate Interface (PRI)

The Primary Rate Interface (PRI) defines an ISDN connection with a high number of bearer channels. It is often used to link telephone systems of companies with the ISDN network, as it allows significantly more parallel calls compared to an ISDN base connection.

The channel structure of E1 Primary Rate Interface

In Europe, typically the E1 Primary Rate Interface is provided by telecommunication service providers. It has a gross data transmission rate of 2.048 megabits per second and is divided into 32 channels with 64 kilobit per second each. Out of these 32 channels, 30 channels are so-called B-channels, which are available as bearer channels. The data transmission rate of all bearer channels together comes to 1920 kilobit per second. The two remaining channels are divided into a 64 kilobit per second signalling channel (D-channel) and a 64 kilobit per second synchronisation channel.

Line interface of the Primary Rate Interface

The electrical S2M interface is mostly used as the line interface between the telephone system and the network. It can be installed in the direction of transmission on the basis of a twin wire or a coaxial transmission line. Generally, two twin wires with shielding are provided in the direction of the end device. Since a ternary HDB3 code with three electrical states is used for the line coding, the data transmission rate of 2048 kilobit per second is reached despite a bandwidth of only one megahertz.

Primary Rate Interface and telephone number allocation

Primary Rate Interface is set up as a point-to-point connection with a telephone number block. This consists of a main telephone number and a telephone number block for the phone extensions. Thus, the end devices have to be dialled directly. The telephone system handles the task of assigning the extension numbers to the end devices. In addition to a telephone number block with up to 99 numbers, it is possible to apply for a three-digit telephone number block with the provider and the Federal Network Agency.

Primary Rate Interface and VoIP telephony

With the extensive introduction of VoIP telephone, the Primary Rate Interface is becoming less and less significant. IP telephone systems can, in principle, be operated via any internet connection. The maximum number of calls that can be made in parallel depends on the bandwidth and the quality of the internet connection.

Further information

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