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PRI systems have been in use for more than three decades and they use channels that are carried on T-carrier systems. T-carrier systems are an entirely digital voice transmission system that uses pulse code modulation and time-division multiplexing for fast transmissions speeds.


What is PRI?

A primary rate interface or PRI is a provider-free system of communication that allows organizations to use a copper wire connection to transmit data, video, or files using a network architecture.

PRI systems allow for 23 B-channels on a single phone line. With a PRI-enabled communications solution, multiple users can have extensions and numbers on a single connection. Physically, PRI lines are comprised of two pairs of copper wires that provide duplex capability. Duplex capability allows secure data transmission from two of these wires to be received by the other two.

PRIs are integrated services digital networks (ISDN). There are two types of these, which include:

  • Basic rate interfaces (BRI) solutions are intended for home and small business use. BRI systems have both a B- and D-channel, but unlike PRI lines, the B-channels can only house a single communication session.
  • Primary rate interfaces are a more enterprise-ready solution thanks to the number of concurrent lines on the B-channel.


PRI Provides Enterprise-level Service

The versatility of PRI is based on the capability for 23 separate communications to be made concurrently n limited to calls. With a PRI line, 15 of the lines can be used for outbound calling, and the remaining eight communications lines are usable for file-sharing or video conferencing.

Each of the channels associated with a PRI connection has a 64 kbps data transfer rate, and the entire line has a combined speed of 1.544 Mbps over a T-1 connection. With this kind of connection, analog signals have a sample rate of 8,000 samples per second.

pri vs bri

Since PRI uses dedicated lines with a preset speed, call quality is high for both voice and video calls. While PRI lines aren’t as fast as using high-speed gigabit internet to hit the VoIP minimum speed, the stability is still a draw for organizations that want an on-premise, hardware-based solution.

In a business environment, PRI systems are also very convenient because it is a single line that houses 23 communication channels. In addition to these channels, which exist on the B side, there is also a D-channel on PRI lines that can be used for control and signaling info. The D-channel is often used to transmit call data like caller ID.


PRI Interfaces with VoIP Systems

Enterprise clients may opt to have a private branch exchange (PBX) system that uses VoIP to power the communications processes of a company. These PBX systems have a server architecture and allow for some high-end customization when coupled with existing IT processes. These IP-PBX systems can be linked by PRI lines, which is a method of increasing the number of communications channels for a business significantly.


Primary Rate Interface is Scalable

For a small business, 23 channels per the B-channel can be beneficial, but enterprise business needs require a higher number of concurrent communications options. Adding onto the dialing capacity of a business is easy since it only takes an additional PRI circuit to open up inbound and outbound calling on another 23 channels – creating a dialing and communications capacity of 56 concurrent instances.


Steady Internet Bandwidth isn’t Required

Unlike traditional VoIP business solutions, PRI systems are not reliant on a company’s internet connection. These are dedicated, copper wire-based lines that rely on conventional telecom, so they rely on the physical cabling and hardware that are used in public switched telephone network (PSTN) systems. Those businesses that have limited bandwidth thanks to geographical location or other factors can quickly hook up a PRI system to a building's copper line connection.


E1 versus T1

E1 lines that are used in Australia and Europe have an additional eight lines per B-channel. These actually use an E-carrier system, which is an improved version of T-carrier. Despite having more concurrent connections, E1 PRI lines are only capable of the same 64 kbps file transfer rates as T1 lines. In total, E1 has a higher file transfer rate of 2.048 Mbps versus T1’s 1.544 Mbps, but this differential isn’t noticeable since the extra bandwidth is distributed over the lines.


How Does PRI Work?

PRI networks allow for 23 lines to quickly be added onto a company’s dialing configuration.  Unlike hosted VoIP solutions, a business can implement upgrades with a capable IT team, allowing teams to scale quickly and easily.

With a preset number channels, any upgrades will have to be done with a full understanding that there’s no in-between. This means that any upgrade will include 23 (or 30 with E1) channels, which may or may not go unused depending on dialing needs.

Even though there’s less reliance on the internet for PRI systems, systems can still be used for file sharing, video conferencing, and chat. The steady 64 kbps per channel provides high-fidelity audio and video over T1 and E1 lines. Unlike VoIP, PRI isn’t as reliant on bandwidth, so there’s little chance of increased jitter or packet loss with a PRI. With that in mind, poor phone service can result in some loss of call quality.

One of the hallmarks of a hosted solution is redundancy; VoIP service providers that utilize a cloud server model will have multiple geographically spread out servers that can pick up calls. With this, customers can rely on high uptimes.

With PRI, this same type of redundancy is also available but with each calling solution being possible on-premise. When one PRI phone system fails, this means that the organization can quickly switch to backup until repairs are made. In most situations, PRI will provide stability, but if there’s a true catastrophe that knocks out a system, even the backups will be affected, which is where a VoIP configuration may pull ahead of PRI.

It’s also noteworthy that additional redundant PRI lines can be a bit cost-prohibitive and will require additional PSTN gateways.


PRI vs SIP Trunking

The difference between PRI vs SIP is that while PRI lines are entirely digital, they still require a physical connection and a PSTN gateway to provide a communications channel for an organization. Businesses that are unsure of what SIP trunking is should learn about how session initiation protocol systems allow for communications through virtual connections.

SIP trunks connect entirely through the internet, and thus, are an entirely VoIP-based method of contact. With SIP trunking providers, there’s hardly any hardware, and the quality of the calling solution is entirely dependent on the internet connection available in the office environment.

Additionally, with SIP, you don’t have to consider having excess lines; scaling calling capacities upwards only requires a change in the software configuration. A SIP trunk user can even add a single line to a company’s dialing capabilities, and if there’s the need for a much larger number of lines, this is easily implemented as well. SIP trunks will require an IP-PBX system. Even with this requirement, SIP tends to be a much cheaper and agile option for some businesses.

SIP is definitely a system that can benefit a business greatly, but this doesn’t mean that there isn’t an argument for PRI trunks. PRI still stands out as a viable solution for those companies without a reliable internet connection – these systems work well over traditional PSTN lines and have good call quality.


What are Pros and Cons of PRI?

Pros Cons
The system is useful for any organization with less internet bandwidth. Long-distance costs extra since PRI requires traditional copper lines.
PRI systems can link two IP-PBX systems. The building will require a copper line network.
PRI is good for setting up various lines and extensions in an office. Telecom carriers provide the contracts for PRI systems charging per-minute dialing costs.
Despite the limit of 23 communications channels, a business can have up to 50 numbers on a single PRI line. There’s just the limitation of 23 concurrent communications events. There’s no wiggle room; with a PRI line, you’ll get 23 channels to use.
Multiple PRI lines in a single office can provide a reasonable degree of redundancy. PRI lines have notoriously expensive hardware requirements.
Bolstering the communications channels with a PRI system can be quick, and each new line provides 23 new channels. With only 64 kbps per line, there’s simply less bandwidth per channel compared to some VoIP solutions.


PRI is an Enterprise Solution

While some may say that the solution is starting to show some identifiable signs of age, PRI definitely has its advantages, especially for businesses that want a versatile communications line. The 23 lines are nice and stable, the wired system is very reliable, and despite some hardware requirements, the system is also fairly easy to set up in buildings with existing copper lines.

Some businesses even opt to use a hybrid communications setup. With a configuration like this, a legacy PRI system can be used to manage the local calling of an organization, and the preexisting PSTN phones can be used this way. For unified communications activities that require higher bandwidth, a SIP trunking or hosted PBX solution can be used. This way, video conferences, webinars, file sharing, and screen sharing can be used without incurring a per-minute charge from the telecoms.

Our interactive comparison tables of business VoIP providers can offer insight on upgrading a legacy system to one that uses internet-based communications.