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[Write to us](/gb/request-information "Write to us") ### Primary Rate Interface (PRI) [The channel structure of E1 Primary Rate Interface](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/primary-rate-interface-pri#section-3) [Line interface of the Primary Rate Interface](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/primary-rate-interface-pri#section-4) [Primary Rate Interface and telephone number allocation](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/primary-rate-interface-pri#section-5) [Primary Rate Interface and VoIP telephony](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/primary-rate-interface-pri#section-6) # Primary Rate Interface (PRI) The Primary Rate Interface (PRI) defines an [ISDN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) connection with a high number of bearer channels. It is often used to link [telephone systems](/gb/gb/products/cloudya) 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](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/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](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/national-international-phone-numbers) 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](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip), the Primary Rate Interface is becoming less and less significant. [IP telephone](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/internet-telephony) 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. ### Related Articles ### Voice over Internet Protocol (VoIP) **What is VoIP?** The **Voice over Internet Protocol,** also called IP telephony, is a method of communication for making calls over a broadband Internet connection as an alternative to regular calls made over an analogue phone using the [PSTN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). These VoIP calls can happen entirely over the Internet but where required, gateways also link to the PSTN. This means that VoIP users can still connect to anyone with a telephone number including local, mobile, and international numbers. VoIP services can be accessed from a computer with a so-called [softphone,](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/softphones) from an IP phone, from a smartphone or from a conventional landline phone that is connected to a VoIP adapter. 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