The solution back in the day was an interface called PRI, which is short for Primary Rate Interface. PRI allowed companies to transmit up to 23 calls simultaneously over a single T1 line.
This was a big step up from previous options that required separate lines for each call.
For a period of time, PRI was the definitive way for businesses to set up their phone systems — often called trunks, for years.
Then, Voice over Internet Protocol ( VoIP ) changed everything.
While PRI is now considered legacy technology, it still has some niche use cases for companies running vintage equipment. In this post, we’ll dive into the history of PRI, how it works, and why VoIP made it obsolete for most modern business phone systems.
We’ll also cover VoIP and other alternatives later in the article.
What Is PRI?
PRI stands for Primary Rate Interface. It’s a type of digital telephone line that connects business telephone systems to the public switched telephone network ( PSTN ).
PRI carries multiple voice and data transmissions between two physical locations. It uses the Integrated Services Digital Network ( ISDN ) protocol to provide digital connectivity rather than analog landlines.
Business phone systems that need to support high volumes of concurrent calls and reliable voice/data services, like call centers, offices, hotels, and hospitals primarily use PRI.
With a PRI, businesses benefit from increased call capacity, fast data transmission, integrated voice or data, and lower cost compared to multiple analog lines.
Features of a PRI Line
To better understand how a PRI actually works, let’s first look at some of its key features:
A PRI line is made up of two pairs of copper wires connecting the service provider (network) to the customer (business end-user).
PRI is technically one line, but it can simultaneously transmit 23 separate communications (voice, data, or video.) This means up to 23 people in your company can be on the phone (or sending an IM or having a video chat) simultaneously.
Each of a PRI’s channels has a 64 kbps capacity for data transmissions.
PRI lines can work with an analog or mixed EPABX setup as well as an IP PBX system.
A PRI can join two private branch exchange ( PBX ) systems together, increasing the number of available communication channels.
How PRI Works
The PRI circuit — the actual cable that physically connects your telecom system — is available in two flavors: T1 and E1.
The main difference between the two PRI circuits has a lot to do with geographical location, though there are some technological differences to be aware of:
E1 T1 Used in Europe and Australia Used in North America and Asia Provides 64 Kpbs for data transmission Provides 64 Kpbs for data transmission Transmit and receive a rate of 2.048 Mbps Transmit and receive rate of 1.544 Mbps Physically contains two pairs of copper wires Physically contains two pairs of copper wires 32 total channels 24 total channels 30 channels used for voice, data, video 23 channels used for voice, data, video 2 channels are used for signaling 1 channel used for signaling
PRI allows for direct inward dialing ( DID ), meaning someone from outside the company can dial an employee’s direct number and reach them directly. A company can have many individual direct phone numbers for employees, while only needing a limited number of physical phone circuits.
For example, a single PRI circuit with 23 available channels could potentially support up to 50 employees with direct numbers. This works because it’s unlikely all employees will be on calls at the same time.
As a company grows, additional PRI circuits can be added to expand capacity.
So if the initial PRI circuit is fully utilized with 23 simultaneous calls, a second PRI can be added to double the capacity to 46 calls. This modular expandability makes PRI scalable and cost-effective compared to legacy analog phone systems.
PRI provides an efficient and flexible digital telephone solution for businesses.
The shared channels enable many employees to direct numbers while minimizing the physical circuits required. Capacity can be easily expanded as needed by adding supplemental PRI circuits.
Benefits of PRI
PRI offers substantial benefits. Let’s look at some reasons why many businesses still use this tech.
Cost Savings – Many employees share a small number of physical circuits, reducing costs compared to analog phone lines. Fewer physical lines mean lower monthly charges.
Scalability – Adding PRI circuits is an easy way to expand capacity as a business grows.
Call Quality – PRI provides high-quality digital voice quality compared to analog phone lines. The all-digital interface eliminates signal degradation.
Flexibility – Supports not just voice calls, but also data and video. Channels can be dynamically allocated to different media types as needed.
Productivity – Features like call forwarding , conferencing, caller ID, and more help employees be more productive. PRI supports advanced capabilities not possible on basic analog lines.
Reliability – Provides excellent reliability and uptime. Redundant circuits can be added to eliminate single points of failure. PRI is less prone to issues like line noise.
Management – Offers enhanced management capabilities, allowing full control over the telephone system from one centralized point. Adds, moves, and changes are software-defined.
Future-Proofing – PRI sets a business up with a modern voice infrastructure that can evolve as needs change. It’s a digital solution rather than outdated analog technology.
Drawbacks of PRI Systems
Up until the 1990s, most business phone systems relied on legacy PBX equipment. While functional, these aging PBX systems were clunky, limited, and difficult to connect to the analog telephone network.
This is where the PRI came in.
For years, PRI was the go-to solution for companies needing to connect their antiquated PBX systems to the PSTN.
But, technology marches on. VoIP emerged in the 2000s as a disruptive force in telecommunications, and PRI began to show its age. Why?
Cost – PRI lines tend to be more expensive than some other telephony options like VoIP. The hardware and installation costs can be high.
Capacity limits – Each PRI line supports 23 B channels for voice/data and 1 D channel for signaling. So a single PRI limits you to 23 concurrent calls. To scale up, you need multiple PRIs.
Complexity – PRIs involve more complex hardware and configurations than analog lines. This can make moves, adds, and changes more difficult. It also requires trained technicians to install and maintain.
Reliability – Since PRIs rely on the service provider’s network, any issues with their network can impact reliability. Problems like line cuts or switch outages can cause service disruptions.
Limited flexibility – PRIs support only the signaling standards and features offered by the telco provider. So it may not be possible to enable newer IP-based features.
Legacy technology – PRIs are tied to older time-division multiplexing (TDM) networks. As telcos move toward IP-based networks, support and evolution for PRIs will diminish over time.
Limited availability – In some geographic areas, telco providers may not offer PRI services. Availability is diminishing as providers shift focus away from TDM.
Alternatives To PRI
There’s no denying primary rate interface technology changed the way businesses could communicate with customers 30+ years ago. But since that time, advances in telecommunications technology have shined a pretty bright light on some of PRI’s shortcomings.
Businesses are looking for alternatives to PRI. There are several options out there, but not every option is a good fit for every business.
Here’s a look at some key PRI alternatives to consider.
1. Analog POTS lines
POTS is Plain Old Telephone Service. It’s the old-school phone line that connected every house and business.
Your grandparents had one. Your parents had one. You might even have POTS at your house. In the residential space, POTS is often referred to as a landline or a PSTN.
POTS provides service through a single analog circuit connecting the service provider and the user.
A POTS supports one communication at a time. This means a single phone call renders the line unavailable for anything else, like a fax or modem internet access.
At one point in history, analog POTS lines were the way people communicated by phone. And it could still work as a solution for a small business today if phone traffic is minimal.
POTS is generally reliable, but the technology has reached its apex. As other communications systems develop, POTS is slowly disappearing from the scene.
2. SIP trunking
Session Initiation Protocol ( SIP ) is the method of transmitting voice communication over a data network. A Voice over IP call is an example of a SIP session.
SIP trunking is sort of like a POTS. But, with SIP trunking no phone lines run all over the place. Instead, the ‘lines’ are virtual because the phone system is connected to the service provider through your network (your internet connection). In the past, SIP trunks have been frequently used as a replacement for POTS or PRI systems. Depending on your business size, it’s still a viable option.
3. Hosted VoIP
Today, businesses of all sizes are moving to a hosted VoIP solution .
A hosted VoIP — also known as a hosted PBX — simply means everything is hosted off-site. The provider hosts the service, deals with the servers and software, and handles maintenance, updates, and troubleshooting.
The service provider manages all of the backend work (whether it be calls, chats, video conferences, etc.) and routes them to the business’s existing phone system.
The big advantage of a host VoIP system over PRI is apparent.
The business itself doesn’t have to worry about expensive equipment, installing and maintaining a big server, or upgrading to the latest software. These things are taken care of by the service provider, saving the business a lot of money, manpower, and headaches.
Hosted VoIP vs. PRI vs. SIP Trunking
Let’s take a closer look at the most popular telecommunications solutions on the market today: VoIP, PRI, and SIP trunking . How do the three stack up?
Feature PRI SIP Trunking Hosted VoIP Functionality Traditional digital phone lines to carrier’s PSTN Uses SIP protocol for voice calls over internet Cloud-based phone system hosted offsite by a provider Number of simultaneous calls Up to 23 Variable Variable Scalability Limited Scalable Scalable Flexibility Limited Flexible Flexible Cost Typically more expensive than SIP trunking or hosted VoIP Uses SIP protocol for voice calls over the internet Can be less expensive than PRI, but may vary depending on usage Equipment & maintenance Requires on-site legacy PBX system and hardware and maintenance Can reduce PRI/PSTN line costs by sending calls over the internet No on-site equipment required. Hardware is minimal, just IP phones needed
