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How ISDN PRI Relates To VoIP
The Primary Rate Interface works for both old school phone systems and VoIP telephony.

By: John Shepler

We tend to think of VoIP as a “new generation” phone service based on IP technology unrelated to the historical public switched telephone network. Well there’s some truth in that, but no matter what you implement you’ll still be dealing with the good old PSTN. The bigger your company or organization, the more “outside” phone lines you’ll need. You should take a close look at ISDN PRI to make the connection to all the phones that are not part of your enterprise VoIP telephone system.

What is a PRI?
It stands for Primary Rate Interface. That’s telephony talk for a particular standard related to another telephone industry term, ISDN. ISDN is the Integrated Services Digital Network. There’s a history to the development of ISDN that relates to the once lofty ambition of replacing all analog home and business phone lines with digital versions. You may not have heard of this, as it fizzled when broadband Internet came along and usurped the data role intended for ISDN. Even so, the business version persists and is more popular now than ever. That’s what’s called PRI or ISDN PRI or even T1 PRI.

Why T1 PRI?
It’s because ISDN PRI is physically transported on a T1 line. T1 is another telephone industry standard widely used in business for dedicated Internet and point to point data connections. T1 lines have the advantages of being provisioned on copper wires so you don’t need fiber optics installed. They can be extended to nearly any distance,are highly reliable and reasonably priced. The pricing has gotten really attractive lately due to all the competition in the industry. With all this going for it, T1 makes a great choice to carry PRI.

What’s different between a regular T1 line and a T1 PRI line?
The line speeds and voltages are the same. What differentiates a PRI is how it is organized. Instead of a big chunk of bandwidth, ISDN PRI is set up as 23 channels that each represent one digital telephone line. A 24th channel is used for signaling and data such as Caller ID. It’s an upgrade to an older T1 telephone standard that was set up as 24 phone channels and no Caller ID.

What's In a Trunk
The beauty of bringing in an ISDN PRI line or “trunk”, as it is called, is that you get 23 outside phone lines coming in on a single digital line. It’s a lot more compact and is almost always cheaper, usually a lot cheaper, than dealing with 24 individual analog business lines.

Interfacing Considerations
The interface is also generally very easy. Most PBX and IP PBX telephone systems either come with ISDN PRI capability or offer it through a plug-in module you can buy. All the circuitry to convert between 23 individual phone lines and the PRI digital trunk are implement on that circuit board. You simply plug-in one or more ISDN PRI lines and you have outside phone lines and a lot of them.

Why Not Pure VoIP?
But why would you connect a T1 PRI to a VoIP phone system rather than keep it 100% VoIP? The simple explanation is that the rest of the world isn’t serviced by VoIP or at least the same VoIP provider. The only thing universal is the PSTN, where you can dial any number and get connected to any telephone in the world. Your enterprise VoIP phone system can save you a bundle on internal calls that stay on your own network. You can also buy a SIP Trunk that connects you to a telephone service provider completely in IP or Internet Protocol. But, guess what happens at that service provider when you want to call an outside number? That’s right. You’ll be switched through to the ISDN PRI lines that connect them to the public phone network.

Enterprise VoIP Services
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