What is PRI & How Does It Work? Definition, Types, & Alternatives

Primary Rate Interface (PRI) is the communication technology that connects the traditional phone system or PBX to the PSTN (Public Switch Telephone Network).

It digitalizes phone connectivity using the Integrated Services Digital Network (ISDN), enabling traditional lines to transmit multiple data and voices simultaneously. Businesses with large operations and high call volumes use this technology, such as offices, call centers, hospitals, etc.

The PRI transmits voice and data over an ISDN line, offering many B channels (for data transmission) and one D channel (for signal control) to give users highly reliable and clear communication.

Step 1: When one of the devices in the PRI network wants to dial a number, the request travels via the D channel, sending signaling information to the network provider, indicating on which B channel the call will be connected.

Step 2: After step 1, the telecommunication service provider assigns the desired B channel from the available B channels. In this case, one of the 23 T1 or 30 E1 B channels will carry the data.

Step 3: Once the B channel is achieved, the data transmits through this channel. Since these channels are separated, many calls or data sessions can occur simultaneously without interfering with each other.

Step 4: During the call process, the D channel transports the control signals about the call progress and termination with other management tasks (like sending caller ID) related to the session.

Step 5: When the call is over, the signaling information is sent again on the D channel to release the currently used B channel back to the pools for other calls or transmissions.
 

Note:
 

• You can add a second PRI to double the call capacity if all the B channels are utilized.
• The signal speed of 64 kbps will remain consistent as there will be no bandwidth sharing between multiple channels.

There are mainly two types of PRI:
 

• T1 Trunk Line: Have a total of 24 channels. (23 B channels + 1 D channel, used in America and Japan)
• E1 Trunk Line: Have a total of 32 channels. (30 B channels + 1 D channel + 1 framing channel, used in Europe and other parts of the world)
   

Where B channels carry data/voice, and D channels provide signal.

T1 Trunk Line is used in America and Japan. It consists of 24 channels, where 23 are bearer (B) channels for data/voice transmission and 1 data (D) channel for signaling. Each B channel operates at 64 kbps, giving a total bandwidth of 1.544 Mbps.

The E1 PRI is used in Europe, Australia, and other parts of the world. It consists of 32 channels in total, where 30 are Bearer (B) channels for transmission of voice/data and the other 2 channels for “1 data (D) signaling and 1 framing/maintenance channel”. In this also, each B channel operates at 64 kbps and has a total bandwidth of 2.048 Mbps.

While PRI gives you advantages like security, high-quality voice service, and supporting PBX systems, it comes with limitations too, like being expensive, lacking advanced features (for instance, video conferencing), and scalability issues.
 

• It supports extended caller ID for both incoming and outgoing calls.
• The best thing about this technology is that it supports the traditional PBX system.
• PRI becomes a reliable option for businesses that deal with sensitive information as it works on a circuit-switch system.
• PRI provides high-quality voice service since it uses dedicated wire lines, minimizing the chances of jitter or packet loss.
• Using PRI, organizations can assign different numbers to each employee without requiring separate physical lines for every number due to the presence of multiple B channels.

• The primary rate interface system is costlier because it needs physical lines. Hence, installation and regular maintenance make it expensive for small-scale businesses.
• It has scaling problems, as you need to add an entirely new PRI line, which gives you another 23 channels when you only need a few.
• It lacks advanced features like video conferencing (having video calls with more than two people), unified communication (calls, texts, voicemails, etc, managed in a single place), etc.
• Primary rate interface systems have flexibility issues as these are tied to one specific location. Hence, remote work and accessing communication services are challenging when you move to different locations.

Though PRI is still being used, advancements in communication technologies have made the availability of hosted PBX systems, VoIP and SIP trunking, which works over IP (Internet Protocol) technology.

VoIP stands for Voice over Internet Protocol and is a technology that enables communications over the Internet. It doesn’t depend on copper wires or cellular networks for call transmission. Instead, it converts voice signals into digital packets and transmits via broadband connections. This makes it efficient and flexible for businesses of all sizes.

Benefits of VoIP over PRI:
 

• Since it works over the Internet, it provides to cost-efficiency, saving you from expensive telephone lines.
• Unlike PRI, VoIP allows one to make and receive calls from any location; it is an excellent choice for businesses supporting remote work.

SIP trunking, which stands for Session Initiation Protocol Trunking, enables voice and multimedia communications over the Internet rather than traditional phone lines. “Trunking” refers to reducing numerous communication channels into a single, logical connection or “trunk.”It acts as a bridge between your PBX phone system and broadband connection.

Advantages of using SIP trunking over primary rate interface:
 

• Like VoIP, SIP trunking allows you to make calls over the Internet, significantly reducing the cost of long-distance and international calls.
• Unlike the PRI system, SIP trunks support unlimited connection channels, whereas PRI only supports a maximum of 23 channels in one line.
• It supports more than voice calls, such as video conferencing and messages, making SIP a more integrated communication system.

The PBX system, also known as Private Branch Exchange, is a private telephone network used internally within an organization. With PBX, internal users share several external phone lines while availing of features like call forwarding, voicemail, conference calls, etc.

There are two types of PBX systems: on-premise and cloud-based or hosted PBX. On-premise PBX is located physically inside your businesses, whereas a third-party provider hosts cloud-based PBX.

If integrated with VoIP or SIP trunking, there are advantages to having a PBX system instead of PRI, which are as follows.
 

• Scaling doesn’t require additional hardware. Only contacting the service provider will do.
• No experienced technical staff are needed to maintain, manage, and upgrade the system.

Primary rate interface (PRI) offers a reliable, high-quality, and secure solution for communication for businesses that rely on legacy phone systems. PRI remains a trusted choice for businesses seeking steadiness and proven communication solutions. On the other hand, high cost, limited scalability, and older technology make it less attractive when there are newer options like VoIP, SIP trunking, and hosted PBX systems.

If you are searching for new and modern means of communication, cloud-based VoIP systems are an excellent choice. Calilio is a cloud-based phone system that is tailored to match your every business communication needs. Unlike PRI, Calilio works over the internet, which is helpful in remote work, and also provides features like call forwarding, call analytics, call routing, etc, at an affordable cost.

PRI is the interface line that connects the traditional phone system to the PSTN (Public Switch Telephone Network). It works by transmitting the voice and other data through wires from one location to another.

PRI ( Primary Rate Interface) and BRI (Basic Rate Interface) are both telephone lines, where PRI has 23 channels, and BRI has only 2 channels.

We use PRI to transmit data and voice signals over an ISDN (Integrated Services Digital Network).

There are two different types of PRI. They are (i) T1 Trunk line and (ii) E1 Trunk line, where T1 is used in America and Japan, and E1 is used in Europe and other countries.

The main difference between SIP and PRI is that SIP uses a Session Initiation Protocol for transmitting data and can have unlimited voice/data channels. In contrast, PRI uses circuit-switched connections and supports only 23 channels in one line.