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Primary Rate Interface (PRI) is a telecommunications interface standard primarily used in Integrated Services Digital Networks (ISDNs) and is basically a service provided for larger enterprise users. PRI lines are a high-capacity service carried on T1 or E1 trunk lines, depending on the country, between the telecommunications provider's central service station and the customers' end.
The T1 trunk line used in a PRI service is divided into 24 channels of 64 Kbps capacity each. Twenty-three of these channels are called bearer channels (B channels), equivalent to having 23 telephone lines, whereas the 24th channel is called a delta channel (D channel), which is used to carry control signals and information like caller ID and information services. In contrast, an E1 trunk line has 32 channels, 30 of which are used as B channels and 2 as D channels. T1 is used by countries such as the USA, Canada and Japan, whereas most European countries use E1 lines.
Advantages of PRI lines: