What Is a PRI Phone System?


When a business wants to access the Internet over its phone line, it can use a Primary Rate Interface system. The PRI includes multiple phone channels for carrying phone calls and data. Home users and small business may do fine with Basic Rate Interface, which offers a lower level of service using the same technology.

Integrated and Digital

  • Both PRI and BRI phone system standards are part of the Integrated Services Digital Network, a set of standards for sending digital data over phone lines. With an ISDN PRI system installed, a business can hook up all its office phones, fax machines, computers and other fixed communications technology without having an individual outside line for each machine. PRI systems in the United States use T1 lines to carry data at high speeds; in Europe, they use comparable E1 lines.

Channeling Data

  • A PRI phone system T1 line has 24 channels; a BRI system has 30. Each channel operates at 64 kilobytes per second. In PRI, the first 23 channels carry voice or data while the 24th -- called the D-channel -- provides control and signal information as well as data for nonvoice functions such as caller ID. The multiple channels allow a PRI system to manage data flexibly, adjusting call loads as needed. With PRI systems, employees can make phone calls, access Web pages and videoconference over the same network at the same time.

How PRI Connects

  • A business with a PRI line usually links it to a private branch exchange located on the company's property. The PBX connects all the phones in the office to a single exchange with an outside line. With a PRI phone system, the Internet connects through the exchange as well. The exchange sends everything to the phone company over a single line. The company then breaks apart the individual channels and sends the calls or data to the right destinations.

Pros and Cons

  • PRI is flexible; if a particular usage is data heavy, the system can devote multiple channels to it. A single T1 line is cheaper than having 23 analog lines or installing an individual line for every phone. A company can double the number of channels by adding just one more T1 line, or obtain fewer channels at a reduced price if it doesn't need 24. If a company doesn't have a high enough call volume, a PRI may be a bad investment compared to analog lines. PRI is slower than a DSL Internet connection.

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