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How ISDN PRI Connections Work
ISDN PRI telephone trunks cut costs with digital bundled phone lines for PBX systems.

By: John Shepler

Traditional analog business phone lines are useful in very small retail and office locations and any business where just a few outside lines are all you need. Once you accumulate more than half a dozen, it’s time to look at a telephone trunk like option like ISDN PRI.

What is ISDN PRI?
ISDN PRI stands for Integrated Services Digital Network Primary Rate Interface. Now you know why they abbreviate it. ISDN is a set of standards for how to transmit phone calls digitally on ordinary telephone wire. PRI is the larger of two major options. The other, BRI or Basic Rate Interface, provides two telephone lines, one telephone and one 64 Kbps data line, or a 128 Kbps data line. It’s mostly used for low speed data connections or to transmit high quality audio for broadcast and production.

Where is PRI Used?
PRI is the business telephone trunk service that is highly popular for use with PBX phone systems. PRI is transmitted on a T1 line using two twisted copper pair. Chances are you are already familiar with T1 lines for dedicated Internet access at 1.5 Mbps. ISDN PRI, also called T1 PRI, uses the same T1 line technology. In fact, T1 lines were originally designed to transport telephone conversations in bulk between telco switching offices.

PRI Phone Channels
What do you get with ISDN PRI service? A single trunk can support up to 23 separate phone conversations, each in its own time division multiplexed channel. Think of these as the equivalent of 23 outside lines. Each line can be set up for local, long distance, incoming only, outgoing only, toll free or some combination. There are actually 24 channels on the T1 line. The last one is reserved for switching signals to support the other 23 plus data such as Caller ID.

Connecting With PRI Trunks
ISDN PRI lines almost always plug into an interface card made especially for this purpose. There’s at least one in most PBX phone systems. Some have four or more ISDN PRI connectors so that you can add dozens of outside lines if your operation is large enough to need them. Call centers are big users of ISDN PRI, as are large business offices.

Cost Trends For PRI
PRI pricing has been getting better over the years, following the declining cost of T1 lines. A few years ago, PRI became more cost effective when you exceeded 10 or 12 outside phone lines. Today, that number is more like 6 or 10 lines, depending on where you are located.

SIP Trunking vs PRI
Other than analog phone lines, the direct competitor to ISDN PRI is SIP Trunking. PRI is a traditional telephone technology that connects your PBX system to the public switched telephone network. SIP Trunking is a packet based technology originally designed to support enterprise VoIP telephone systems. SIP Trunks connect directly to converged networks for enterprise VoIP support and can supply both phone and Internet service for smaller companies. SIP Trunks can also be ordered with analog, IP or PRI handoffs to connect to any PBX telephone system.

Telephone Trunks For Your Phone System
What type of telephone trunk line is best for your particular situation? Consider the interfaces you have available and get competitive prices and availability for business telephone lines and trunks to make a good decision.

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