Primary Rate Interface (PRI)

The PRI is a level of service assigned by the ISDN, sometimes referred to as an ISDN PRI.  The PRI provides businesses with digital access to the PSTN.

While ISDN PRI is usually associated with only Voice transmission, it is capable of transmitting Data, Video, and Faxes as well.

Transmission on the PRI

Now that we can generally answer the question “What Is PRI?”, let’s talk about how the PRI is deployed.

The PRI is typically associated with a T1 line in the United States, also called a PRI Line or a PRI Circuit.  The PRI Circuit is a single cable (usually a T1) coming into your telecom room that can carry voice and data transmissions.

A T1 line consists of 24 channels. A PRI uses 23 of these channels (called b channels) to carry the voice calls and 1 channel (called a d channel) for signaling, or communicating, between a PBX and the voice network.  This is commonly referred to as a (23b + 1d).

Each of the 23 channels can hold one phone call at a time,
for a maximum of 23 simultaneous phone calls.

The important aspect of PRI is that it is a circuit switched system.  This means that there is always an actual physical and dedicated channel for the communication to travel on.  This is different from Packet Switched system – which is the model that the internet and SIP Trunking is based on.

Use of a PRI Circuit

So we can better understand what PRI is, we also need to understand how it’s used and how customers are able to connect to businesses.

People are able to call businesses by dialing their DID (Direct Inward Dialing) number. The DID telephony number is just the actual phone number you dial, for example our main DID at OneVoice is 703-880-2500.

It’s not uncommon for businesses to have 50 DID numbers on 1 PRI Circuit. In other words, your business can have 50 different phone numbers, 1 for each employee of the company, all coming across a single PRI Circuit. As long as you don’t have more than 23 people on a call at the same time, this is a perfectly viable option.

In the event that your company continues to expand and you do require more than 23 simultaneous phone calls, your only option for expanding your existing PRI circuit is to purchase an additional PRI circuit. This purchase will provide 23 more channels for communication transmission.

Alternatives to PRI

Since PRIs provide blocks of 23 channels, there isn’t much room for flexible scalability. In addition, because PRIs are Circuit Switched, it’s sometimes a hassle once your company grows to a certain point; gaining multiple locations or a large number of employees. PRI circuits also don’t make sense for very small companies with 1 to 5 employees, since you’re purchasing 23 call channels for only 5 people.

The main alternatives to PRI Circuits are:

  • Analog POTS Lines: these are just like the old land-line you would get at your house, 1 number for 1 dial tone and 1 phone call
  • Dynamic Integrated Services: although not as commonly used, since data requirements are much higher now, some providers are able to take an existing PRI circuit over a T1 and transmit data with it as well.
  • SIP Trunking: This is a newer method of voice transmission that uses your existing data connection, and SIP capable PBX, to send and receive phone calls
  • Hosted VoIP: This is very similar to SIP Trunking, except that you don’t use a P

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Is a PRI Circuit Right For Us?

If you already own a PBX or are able to obtain a PBX at a low cost, the PRI circuit is a very viable solution for you. It’s typically recommended for medium to large businesses that can make use of 23 simultaneous phone calls, otherwise you have empty channels that you are paying for and not using. The PBX itself offers a lot of features that you see in medium to large businesses (auto attendants, rings groups, hold music, etc).

It may not be right for you if you’re a very small business or startup with only a few employees. It may not be right for you if you’re a particularly large organization with multiple locations. An MPLS with SIP Trunking may be a better solution.

We can help you determine if a PRI is in fact right for you. We’ll talk with you to discover what your business goals are, and what technological needs will satisfy those goals.

Not Quite Ready? Have Some Questions?

That’s okay too – We’re here to answer any questions you might have.  We’re ready when you are.

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