Integrated Services Digital Network Primary Rate Interface (ISDN PRI)

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 23 voice channels and one data channel for signaling over a single line.

While many companies today are upgrading their private branch exchange (PBX) telephone network with internet protocol (IP) telephony or voice over internet protocol (VoIP) , 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 relies on 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 multiline telecommunications circuit that connects a business site directly to a central office switch. Multiple channels are multiplexed 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, which comprises 23 voice and data channels, and one signaling channel. In Europe and Australia, digital signals are transmitted over an E1 line, which comprises 30 voice and data channels, and one signaling channel. 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 a specific number of channels (23 voice channels in the US) on one PRI circuit. The cost of one PRI line can be less than 23 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, because businesses can assign up to 23 telephone numbers to each PRI circuit.

In addition to being able to assign individual channels to a specific user or purpose, a business can have one main number that’s available on all 23 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

One drawback to ISDN PRI is that because it relies on copper phone cabling, it can be expensive to implement and upgrade. Copper cabling can be damaged by a number of issues, including inclement weather, excavating contractors or vandalism. It also tends to degrade as it ages. Capacity that can only be purchased in 23-line units means that it can take longer to scale and modify infrastructure when compared to SIP trunking.

SIP trunking, the technology that supports 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 using Verizon SIP trunking can increase or decrease call capacity quickly through a self-service portal, 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. 

Both ISDN PRI and SIP trunking have advantages and disadvantages. 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.

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