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Integrated Services Digital Network Primary Rate Interface 

Use a reliable digital connection to transmit voice, video and data. 

Integrated Services Digital Network Primary Rate Interface (ISDN PRI) is a telecommunications standard that creates a clear digital connection between your business and a public switched telephone network (PSTN).

ISDN was developed in the 1980s as a digital alternative to the analog telephone network. ISDN services are delivered in one of two ways—with a Basic Rate Interface (BRI), which was commonly used for high-speed dial-up connections, or with a Primary Rate Interface (PRI), which in the US supports voice and data signaling over a single line. 

While many companies today are upgrading their private branch exchange (PBX) telephone network with internet protocol (IP) telephony, ISDN PRI is beneficial for companies that want a high-quality digital connection for high-bandwidth applications with relatively fast call set-up.

What is ISDN PRI?

ISDN PRI is a local exchange telecommunications access service that enables traditional phone lines to carry voice, data and video traffic. It’s ideal for businesses whose applications need data integrity, secured bandwidth and reliable speeds. As with traditional telephone services, PRI historically used copper cable to transmit voice signals. However, ISDN PRI improves basic phone service by providing data, image and video services—in addition to voice—on a single circuit that has multiple channels. 

How does ISDN PRI work?

ISDN PRI is a multi-line telecommunications circuit that connects a business site directly to a central office switch. Multiple channels are split over a single line to enable the transmission of digital voice, video and data signals in increments of 1.54 megabits per second (Mbps). In the US, Canada and Japan, digital signals are transmitted over a T1 line. In Europe and Australia, digital signals are transmitted over an E1 line. The signaling channel provides call data, such as the caller’s telephone number and name (caller ID).

How can ISDN PRI help my business?

Businesses typically look to ISDN PRI to handle higher call volumes at a lower cost than traditional business phone service. Establishing a direct connection from a PBX to the PRI phone trunk allows companies to put several channels on one PRI circuit. The cost of one PRI line can be less than separate analog trunks.

ISDN also helps businesses simplify their telecommunications system. With the direct inward dialing (DID) feature, also called direct dial-in (DDI) in Europe, one PRI connection can handle local, long distance, voice, data, video conferencing and faxing, and all of those functions can be done simultaneously because they can be assigned to different channels. DID also allows outsiders to call an extension directly instead of having to go through a PBX auto attendant.

In addition to being able to assign individual channels to a specific user or purpose, a business can have one main number for several channels. If multiple people call at the same time, the calls can land in any channel that is free, instead of requiring the user to dial another number manually when one line is busy.

ISDN PRI is also ideal for businesses that want to:

  • Connect legacy PBX systems that are not yet IP-enabled
  • Use dial-up modems, such as for credit card terminals, alarm panels, payroll systems and postage meters, that are not compatible with session internet protocol (SIP) trunking solutions
  • Send or receive a large quantity of faxes, such as at a law office
  • Avoid relying on data bandwidth to support voice

SIP trunking, the technology that supports voice over internet protocol (VoIP), may be a better choice for businesses that are looking to upgrade their traditional phone infrastructure or scale rapidly. SIP trunking is sold on demand. Businesses can increase or decrease call capacity often the same day a change is requested, so companies can pay for the capacity they need, when they need it. 

SIP trunking is also a good choice for businesses looking to integrate their telephony systems with a multimedia unified communications solution to improve collaboration and productivity. Because SIP trunking works over an internet connection, it’s a cost effective alternative to ISDN PRI and is ideal for businesses with multiple sites and remote workers.

However, with SIP trunking, you need a switch, internet connection, customer PBX and firewall to mitigate risks of a cyberattack. 

To choose between ISDN PRI and SIP trunking, businesses need to evaluate how they want to use the telecommunications technology, how fast they will need to grow and what approach their infrastructure will support. They should weigh their desire for collaboration, call quality and reliability, as well as their need for security, before choosing one technology over the other.